Discourse-Aware Text Simplification:
From Complex Sentences to Linked Propositions

Christina Niklaus<EMAIL_ADDRESS>
Matthias Cetto<EMAIL_ADDRESS>
School of Computer Science, University of St.Gallen, St.Gallen, Switzerland
André Freitas<EMAIL_ADDRESS>
Idiap Research Institute, Martigny, Switzerland

Department of Computer Science, University of Manchester, Manchester, UK
Siegfried Handschuh<EMAIL_ADDRESS>
School of Computer Science, University of St.Gallen, St.Gallen, Switzerland

University of Passau, Passau, Germany

Sentences that present a complex syntax act as a major stumbling block for downstream Natural Language Processing applications whose predictive quality deteriorates with sentence length and complexity. The task of Text Simplification (TS) may remedy this situation. It aims to modify sentences in order to make them easier to process, using a set of rewriting operations, such as reordering, deletion, or splitting. These transformations are executed with the objective of converting the input into a simplified output while preserving its main idea and keeping it grammatically sound.

State-of-the-art syntactic TS approaches suffer from two major drawbacks: first, they follow a very conservative approach in that they tend to retain the input rather than transforming it, and second, they ignore the cohesive nature of texts, where context spread across clauses or sentences is needed to infer the true meaning of a statement. To address these problems, we present a discourse-aware TS approach that splits and rephrases complex English sentences within the semantic context in which they occur. Based on a linguistically grounded transformation stage that uses clausal and phrasal disembedding mechanisms, complex sentences are transformed into shorter utterances with a simple canonical structure that can be easily analyzed by downstream applications. With sentence splitting, we thus address a TS task that has hardly been explored so far. Moreover, we introduce the notion of minimality in this context, as we aim to decompose source sentences into a set of self-contained minimal semantic units. To avoid breaking down the input into a disjointed sequence of statements that is difficult to interpret because important contextual information is missing, we incorporate the semantic context between the split propositions in the form of hierarchical structures and semantic relationships. In that way, we generate a semantic hierarchy of minimal propositions that leads to a novel representation of complex assertions that puts a semantic layer on top of the simplified sentences. As a proof of concept, we develop a reference TS implementation.
In a comparative analysis, we demonstrate that our approach outperforms the state of the art in structural TS both in an automatic and a manual analysis.

Sentences that present a complex linguistic structure act as a major stumbling block for semantic applications whose predictive quality deteriorates with sentence length and complexity. The task of Text Simplification (TS) aims to modify sentences in order to make them easier to process, using a set of rewriting operations, such as reordering, deletion or splitting. These transformations are executed with the objective of converting the input into a simplified output, while preserving its main idea and keeping it grammatically sound.

State-of-the-art syntactic TS approaches suffer from two major drawbacks: first, they follow a very conservative approach in that they tend to retain the input rather than transforming it, and second, they ignore the cohesive nature of texts, where context spread across clauses or sentences is needed to infer the true meaning of a statement. We address the first problem by generating a fine-grained output with a simple and regular structure.
For this purpose, we decompose a source sentence into a set of self-contained propositions, with each of them presenting a minimal semantic unit. Moreover, in order to maximize the expressiveness of the simplified sentences, we suggest not only to split the input into isolated sentences, but to also incorporate the semantic context in the form of semantic relationships between the split propositions.

To address this challenge, we present a discourse-aware TS framework that is able to split and rephrase complex English sentences within the semantic context in which they occur.
Our framework differs from previous systems by using a linguistically grounded transformation stage that first transforms syntactically complex sentences into smaller units with a simpler structure using clausal and phrasal disembedding mechanisms. By using a recursive top-down approach, our framework is able to generate a hierarchical representation between those units, capturing both their semantic context and relations to other units in the form of rhetorical relations. In that way, we generate a semantic hierarchy of minimal propositions that benefits downstream Open Information Extraction (IE) tasks.

In a comparative analysis, we demonstrate that our baseline implementation DisSim outperforms the state of the art in structural TS both in an automatic evaluation and a manual analysis, obtaining the highest scores on three simplification datasets from two different domains with regard to SAMSA (0.67, 0.57, 0.54), a recently proposed metric targeted at automatically measuring the syntactic complexity of sentences which highly correlates with human judgments on structural simplicity and grammaticality. Furthermore, a comparative analysis with the annotations contained in the RST Discourse Treebank reveals that we are able to capture the contextual hierarchy between the split sentences with a precision of almost 90% and reach an average precision of approximately 70% for the classification of the rhetorical relations that hold between them. Finally, an extrinsic evaluation shows that when applying our framework as a preprocessing step the performance of state-of-the-art Open IE systems can be improved by up to 346% in precision and 52% in recall.

§ INTRODUCTION

Sentences that present a complex linguistic structure can be hard to comprehend by human readers, as well as difficult to analyze by natural language processing (NLP) applications. Identifying grammatical complexities in a sentence and transforming them into simpler structures - using a set of text-to-text rewriting operations - is the goal of syntactic Text Simplification (TS). The major types of operations that are used to perform this rewriting step are deletion and sentence splitting. The goal of the deletion task is to remove parts of a sentence that contain only peripheral information in order to produce an output proposition that is more succinct and contains only the key information of the input. The splitting operation, on the contrary, aims at preserving the complete information of the input. Therefore, it divides a sentence into several shorter components, with each of them presenting a simpler and more regular structure that is easier to process by both humans and machines and may support a faster generalization in machine learning (ML) tasks.

Syntactic TS with a focus on the task of sentence splitting has been attracting growing interest in the NLP community within the past few years. It has been shown that sentence splitting may benefit both NLP and societal applications. One line of work encompasses syntactic TS approaches that serve as assistive technology for specific target reader populations [30, 80, 72]. The main goal of such approaches is to improve the readability of a text, i.e. to enhance the ease with which it can be understood.
Thus, it aims to make information easier to comprehend for people
with reading difficulties that may arise from, for example, aphasia [17], autism [24] or deafness [40], as well as for those with reduced reading levels, including children [22] and non-native speakers of the respective language [5]. In that way, information is made available to a broader audience.

Apart from substituting a word or phrase that is hard to understand with a more comprehensible synonym (lexical simplification), the most effective operations to improve the reading comprehension for language-impaired humans are splitting long sentences, making discourse relations explicit, avoiding pre-posed adverbial clauses and presenting information in cause-effect order [79].

The second line of work aims at generating an intermediate representation that is easier to analyze by downstream semantic tasks whose predictive quality deteriorates with sentence length and structural complexity, posing problems due to their potential high level of ambiguity. Prior work has established that shortening sentences by dropping constituents or splitting components,
and thus operating on smaller units of text, facilitates and improves the performance
of a variety of applications, including Machine Translation  (MT) [84, 85], Open Information Extraction (IE) [18, 73], Text Summarization [81, 13], Relation Extraction [58], Semantic Role Labeling [92], Question Generation [37, 11], Sentence Fusion [31] and Parsing [19, 41].

Many different methods for addressing the task of TS have been proposed so far. As noted in [83] [83], data-driven approaches outperform rule-based systems in the area of lexical simplification, which aims at substituting a difficult word or phrase with a more comprehensible synonym [35, 68, 67, 95]. In contrast, the state-of-the-art structural simplification approaches are rule-based [80, 30, 72], providing more grammatical output and covering a wider range of syntactic transformation operations, however, at the cost of being very conservative, often to the extent of not making any changes at all.

[doublelined]Subtask 1: Split into Minimal Propositions

Input:     A complex sentence $C$.

Problem:     Produce a sequence of simple sentences $T_1, \dots, T_n$, $n >= 2$, such that

* each simple sentence $T_i$ is grammtically sound. (syntactic correctness)

* each simple sentence $T_i$ presents a minimal semantic unit,
i.e. it cannot be further decomposed into meaningful propositions. (minimality)

* the output sentences $T_1, \dots, T_n$ convey all and only the information in $C$. (semantic correctness)

[doublelined]Subtask 2: Establish a Semantic Hierarchy
Input:     A pair of structurally simplified sentences $T_i$ and $T_j$.

Problem:     Establish a contextual hierarchy between the split sentences and restore their semantic relationship.

Problem description.

In order to overcome the conservatism exhibited by state-of-the-art syntactic TS approaches, i.e. their tendency to retain the input sentence rather than transforming it, we propose a novel sentence splitting approach that breaks down a complex English sentence into a set of minimal propositions, i.e. a sequence of sound, self-contained utterances, with each of them presenting a minimal semantic unit that cannot be further decomposed into meaningful propositions [9] (for an example, see Figure <ref>). Thus, we augment the Split-and-Rephrase task that was proposed in [61] [61] by the notion of minimality.
In the development of our approach, we followed a principled and systematic procedure. First, we performed an in-depth study of the literature on syntactic sentence simplification, followed by a thorough linguistic analysis of the syntactic phenomena that need to be tackled in the sentence splitting task. Next, we materialized our findings into a small set of 35 hand-crafted transformation rules that decompose each source sentence into minimal semantic units and turn them into self-contained propositions that present a regular and grammatically sound structure. In that way, nested structures are removed and related pieces of information are brought closer together, resulting in a fine-grained representation of the input that is easier to process and thus leverages downstream Open IE applications, improving their performance in terms of accuracy and coverage.
Prior work has also shown that sentence splitting may be used as a preprocessing step to facilitate and boost the performance of further artificial intelligence (AI) tasks, including, for instance, Text Summarization [13] and MT [85].

Complex source     A fluoroscopic study which is known as an upper gastrointestinal series is typically the next step in management, although if volvulus is suspected, caution with non water soluble contrast is mandatory as the usage of barium can impede surgical revision and lead to increased post operative complications.
Minimal propositions      A fluoroscopic study is typically the next step in management.

This fluoroscopic study is known as an upper gastrointestinal series.

Volvulus is suspected.

Caution with non water soluble contrast is mandatory.

The usage of barium can impede surgical revision.

The usage of barium can lead to increased post operative complications.

Result of the sentence splitting subtask on an example sentence. A complex source sentence is split into a loose sequence of minimal semantic units that lacks coherence, impeding the interpretability of the input for downstream Open IE applications.

myblock = [rectangle, draw, minimum height=2.5cm, rounded corners]
[scale=0.95, transform shape]
(1)[myblock, text width=4cm,align=center, label=below:Proposition 1]A fluoroscopic study is typically the next step in management.;
(2)[myblock,right =of 1,xshift=0cm, text width=4cm,align=center, label=below:Proposition 2]This fluoroscopic study is known as an upper gastrointestinal series.;
(3)[myblock,right =of 2,xshift=0cm, text width=4cm,align=center, label=below:Proposition 3]Volvulus is suspected.;
(4)[myblock,below =of 1,xshift=0cm, text width=4cm, align=center, label=below:Proposition 4]Caution with non water soluble contrast is mandatory.;
(5)[myblock,below =of 2,xshift=0cm, text width=4cm, align=center, label=below:Proposition 5]The usage of barium can impede surgical revision.;
(6)[myblock,below =of 3,xshift=0cm, text width=4cm, align=center, label=below:Proposition 6]The usage of barium can lead to increased post operative complications.;

Minimal propositions resulting from the sentence splitting subtask on our running example from Figure <ref>. The input is decomposed into a loose sequence of minimal semantic units that lacks coherence.

node distance = 1.5cm and 0cm
myblock = [rectangle, minimum height=0.6cm]

(8)[myblock,fill=Maroon!15, text width=12cm,align=center]A fluoroscopic study which is known as an upper gastrointestinal series is typically the next step in management, although if volvulus is suspected, caution with non water soluble contrast is mandatory as the usage of barium can impede surgical revision and lead to increased post operative complications.;
(0)[myblock, fill=OliveGreen!15, xshift=-3.8cm, text width=4.5cm,align=center, below =of 8]A fluoroscopic study is typically the next step in management.;
(1)[myblock, fill=OliveGreen!15, xshift=2.7cm, text width=4.5cm,align=center, right =of 0]Caution with non water soluble contrast is mandatory.;
(2)[myblock, fill=OliveGreen!15, text width=4.5cm,align=center, below =of 0]This fluoroscopic study is known as an upper gastrointestinal series.;
(3)[myblock, fill=OliveGreen!15, text width=4.5cm,align=center, yshift=-2cm, below =of 2]The usage of barium can impede surgical revision.;
(4)[myblock, fill=OliveGreen!15, xshift=1.2cm, text width=4.5cm,align=center, right =of 3]The usage of barium can lead to increased post operative complications.;
(5)[myblock, fill=OliveGreen!15, text width=2.5cm,align=center, yshift=-1.2cm, xshift=2.5cm, above =of 4]Volvulus is suspected.;

(6)[draw, ultra thick, color=OliveGreen, dashed,fit=(0) (1)] ;
(7)[draw, ultra thick, color=OliveGreen, dashed,fit=(2) (3) (4) (5)] ;
(12)[draw, ultra thick, color=Maroon, dashed,fit=(8)] ;

(9)[text width = 2cm, align=center, right =of 6, xshift=0.2cm]Core;
(11)[text width = 2cm, align=center, right =of 7, xshift=0.2cm]Context;
(13)[text width = 2cm, align=center, right =of 12, xshift=0.2cm]Complex

[-,OliveGreen,very thick] (0) – node[above, align=center] Contrast (1);
[->,OliveGreen,very thick] (2) – node[right, align=center] Enablement (0);
[->,OliveGreen,very thick] (3) – node[below, align=center] Background (1);
[->,OliveGreen,very thick] (4) – node[below, align=center] Background (1);
[-,OliveGreen,very thick] (4) – node[above, align=center] List (3);
[->,OliveGreen,very thick] (5) – node[below, align=center] Condition (1);

Example of the output that is generated by our proposed discourse-aware TS approach. A complex input sentence is transformed into a semantic hierarchy of simplified sentences in the form of minimal, self-contained propositions that are linked via rhetorical relations. The output presents a regular, fine-grained structure that is easy to process, while still preserving the coherence and, hence, the interpretability of the output.

However, any sound and coherent text is not simply a loose arrangement of self-contained units, but rather a logical structure of utterances that are semantically connected [79]. Consequently, when carrying out syntactic simplification operations without considering discourse implications, the rewriting may easily result in a disconnected sequence of simplified sentences, making the text harder to interpret (see the output of the example given in Figure <ref>).
Still, the vast majority of existing structural TS approaches do not take into account discourse level effects that arise from applying syntactic transformations. I.e., they split the input into simplified sentences without considering and preserving their semantic context. Accordingly, the resulting simplified text is prone to lack coherence, consisting of a set of semantically unrelated utterances that miss important contextual information. Therefore, the interpretability of the output for downstream Open IE tasks may be hampered.

Thus, in order to preserve the coherence structure and, hence, the interpretability of the input, we propose a discourse-aware TS approach based on Rhetorical Structure Theory (RST) [50] that establishes a semantic hierarchy on the simplified sentences. For this purpose, we first set up a contextual hierarchy between the split components, distinguishing core sentences that contain the key information of the input from contextual sentences that disclose less relevant, supplementary information. Then, the semantic relationship that holds between the decomposed spans is identified and classified. An example of the resulting context-preserving output is displayed in Figure <ref>.

The contributions of our work can be summarised as:

(i) The proposal of a discourse-aware syntactic TS approach which transforms English sentences that present a complex linguistic structure into a semantic hierarchy of simplified sentences in the form of minimal, self-contained propositions.

(ii) The extension of the task of sentence splitting by the notion of minimality.

(iii) The introduction of a two-layered hierarchical representation of complex text data in the form of core sentences and accompanying contexts.

(iv) The use of rhetorical relations to preserve the coherence structure of the input, resulting in a novel contextual representation that puts a semantic layer on top of the simplified sentences.

(v) A systematic analysis of the syntactic phenomena involved in complex sentence constructions whose findings were materialized in a small set of 35 hand-crafted simplification rules.

(vi) A comprehensive evaluation against state-of-the-art syntactic TS baselines.

(vii) An extrinsic evaluation demonstrating the merits of our approach for downstream Open IE applications.

(viii) A supporting system (DisSim).

(ix) The publication of all the experimental code to support transparency and reproducibility.[<https://github.com/Lambda-3/DiscourseSimplification>]

This manuscript is organised as follows: in Section <ref>, we provide an overview of the state of the art in structural TS. In addition, we discuss approaches addressing the problem of preserving coherence structures in texts.

In Section <ref>, we describe our discourse-aware TS approach that transforms complex input sentences into a semantic hierarchy of minimal propositions, resulting in a novel contextual representation of complex assertions in the form of hierarchically ordered and semantically interconnected sentences.
In Section <ref>, we present the results of the experiments that we conducted to compare the performance of our proposed approach with state-of-the-art syntactic TS baselines. Moreover, we report the results of the evaluation of the semantic hierarchy, as well as the extrinsic evaluation assessing the effect on the performance of state-of-the-art Open IE systems when incorporating our approach as a preprocessing step.

Finally, in Section <ref>, we conclude our findings and present directions for future work.

This article substantially extends the work described in [64] [64]. The main novel contributions of the work reported in this manuscript, as well as the conclusions that can be drawn from it are as follows:

(i) We emphasise and extend the work in the direction of the identification of the discourse structures (see Section <ref>).
While our previous paper is restricted to briefly outlining the core idea of this step, we here present the details that are necessary to fully capture and reproduce
this part of our approach, describing for example the process of selecting the rhetorical relations included in our approach, the final set of rhetorical relations that we apply, and the logic behind differentiating core from context sentences, including important exceptions.

(ii) We conduct a comprehensive evaluation from which we can conclude that our proposed discourse-aware TS approach not only succeeds in transforming syntactically complex sentences into a set of simplified propositions, but also in establishing a semantic hierarchy between them (see Section <ref>). Thus, it generates a fine-grained representation of complex assertions in the form of

(a) hierarchically ordered and

(b) semantically interconnected sentences.

(iii) We present additional detail about the sentence splitting subtask by performing a systematic analysis of the syntactic phenomena involved in complex sentence constructions. The findings are materialized in a small set of transformation patterns whose details are presented in this article for the first time.
It may both serve as a reference and support the re-implementation of the proposed approach.

(iv) We extend the evaluation of the sentence splitting subtask by providing a detailed qualitative analysis of the transformation patterns, including an in-depth analysis of the errors made by the proposed approach (see Section <ref>). These examinations provide further insights into the quality of the specified simplification rules and may serve as a starting point for future improvements.

(v) While [64] [64] is limited to a brief outline of the general approach,
thereby focusing on the sentence splitting subtask, this work provides a both complete and detailed end-to-end narrative of our proposed approach.

To summarise, this article articulates new empirical analyses and communicates the contribution with a level of detail and rigour which are not comparable to previous work. From our perspective this work has the chance of becoming a canonical reference in this specific subfield and in our minds, it constitutes a distinct contribution.

§ RELATED WORK

The following section summarizes previous work in the area of syntactic TS. We discuss the strengths and weaknesses of existing approaches, thereby demonstrating the effectiveness of our proposed context-preserving TS framework. In addition, we give a comprehensive overview of approaches that operate on the level of discourse by taking into account the coherence structure of texts.

§.§ Sentence Splitting

To date, three main classes of techniques for syntactic TS with a focus on the task of sentence splitting have been proposed. The first uses a set of syntax-based hand-crafted transformation rules to perform structural simplification operations, while the second exploits ML techniques where the model learns simplification rewrites automatically from examples of aligned complex source and simplified target sentences. In addition, approaches based on the idea of decomposing a sentence into its main semantic constituents using a semantic parser were described.

§.§.§ Syntax-driven Rule-based Approaches

The line of work on structural TS starts with [19] [19], who manually define a set of rules to detect points where sentences may be split, such as relative pronouns or conjunctions, based on chunking and dependency parse representations. [77] [77] presents a pipelined architecture for a simplification framework that extracts a variety of clausal and phrasal components from a source sentence and transforms them into stand-alone sentences using a set of hand-written grammar rules based on shallow syntactic features.

More recently, [80] [80] propose RegenT, a hybrid TS approach that combines an extensive set of 136 hand-written grammar rules defined over dependency tree structures for tackling 7 types of linguistic constructs with a much larger set of automatically acquired rules for lexical simplification. Taking a similar approach, [30] [30] describe a linguistically-motivated rule-based TS approach called YATS, which relies on part-of-speech tags and syntactic dependency information to simplify a similar set of linguistic constructs, using a set of only 76 hand-crafted transformation patterns in total. A vector space model is applied to achieve lexical simplification.
These two
rule-based structural TS approaches primarily target reader populations with reading difficulties, such as people suffering from dyslexia, aphasia or deafness. According to [79] [79], those groups most notably benefit from substituting difficult words, making discourse relations explicit, reordering sentence components and splitting long sentences that contain clausal constructs.
Therefore, TS approaches that are aimed at human readers commonly represent hybrid systems that operate both on the syntactic and lexical level, with the decomposition of clausal elements representing the main focus of the structural simplification component.

gray!25Linguistic construct     3|c|gray!25#rules
gray!25YATS      gray!25RegenT     gray!25DisSim

Correlatives     4     0     0
Coordinate clauses     2*10     4*85 (lexicalised on conjunctions)      1

Coordinate verb phrases              1

Adverbial clauses     12          6
Reported speech     0     14     4

Relative clauses     17     2*26     9

Appositions     1          2
Coordinate noun phrases     0     0     2
Participial phrases     0     0     4
Prepositional phrases     0     0     3
Adjectival and adverbial phrases     0     0     2
Lead noun phrases     0     0     1

Passive constructions     14     11     0
gray!25Total     gray!2576     gray!25136     gray!2535

Comparison of the linguistic constructs that are addressed by the syntactic TS approaches YATS, RegenT and our proposed approach DisSim based on the number of hand-crafted grammar rules for simplifying these structures.

In contrast to above-mentioned TS frameworks, our proposed TS approach (DisSim) does not address a human audience, but rather aims to produce an intermediate representation that presents a simple and more regular structure that is easier to process for downstream Open IE applications.
For this purpose, we cover a wider range of syntactic constructs. In particular, our approach is not limited to breaking up clausal components, but also splits and rephrases a variety of phrasal elements, resulting in a much more fine-grained output where each proposition represents a minimal semantic unit that is typically composed of a simple subject-predicate-object structure. Though tackling a larger set of linguistic constructs, our framework operates on a much smaller set of only 35 manually defined rules.
A detailed comparison of the linguistic constructs that are handled by the two state-of-the-art rule-based syntactic TS systems YATS and RegenT and our proposed approach DisSim is shown in Table <ref>.

System     Output
gray!25Input     gray!25The house was once part of a plantation and it was the home of Josiah Henson, a slave who escaped to Canada in 1830 and wrote the story of his life.
RegenT     The house was once part of a plantation. And it was the home of Josiah Henson, a slave. This slave escaped to Canada in 1830 and wrote the story of his life.
YATS     The house was once part of a plantation. And it was the home of Josiah Henson. Josiah Henson was a slave who escaped to Canada in 1830 and wrote the story of his life.

#1 0 The house was once part of a plantation.

LIST #2

#2 0 It was the home of Josiah Henson.

ELABORATION #3

LIST #1

#3 1 Josiah Henson was a slave.

ELABORATION #4

ELABORATION #6

#4 2 This slave escaped to Canada.

TEMPORAL #5

LIST #6

#5 3 This was in 1830.

#6 2 This slave wrote the story of his life.

LIST #4

Simplification example (from Newsela).

Table <ref> contrasts the output generated by RegenT and YATS on a sample sentence. As can be seen, RegenT and YATS break down the input into a sequence of sentences that present its message in a way that is easy to digest for human readers. However, the sentences are still rather long and present an irregular structure that mixes multiple semantically unrelated propositions, potentially causing problems for downstream tasks, such as Open IE. On the contrary, our fairly aggressive simplification strategy that splits a source sentence into a large set of very short sentences is not well suited for a human audience and may in fact even hinder reading comprehension. However, we were able to demonstrate that the transformation process we propose improves the accuracy and coverage of the tuples extracted by state-of-the-art Open IE systems, serving as an example of a downstream NLP application that benefits from using our approach as a pre-processing step (see Section <ref>).

In addition, [83] [83] present LexEv and EvLex. They are semantically-motivated, aiming to reduce the amount of descriptions in a text and keeping only event-related information. For this purpose, they make use of a state-of-the-art event extraction system [35] to first extract event mentions from a given text. Then, a set of 11 syntax-based extraction rules is applied to extract arguments of four types: agent, target, location and time, including rules for splitting noun compounds, extracting prepositional objects, nominal subjects and direct objects. For lexical simplification, an unsupervised approach based on word embeddings is used [35].
In comparison to LexEv and EvLex, where content reduction plays a major role, we are interested in preserving the full informational content of an input sentence.

System     Output
gray!25Input     gray!25"The amabassador's arrival has not been announced and he flew in complete secrecy," the official said.
LexEv, EvLex     He arrived in complete secrecy.

#1 0 The ambassador's arrival has not been announced.

LIST #2

ATTRIBUTION #3

#2 0 He flew in complete secrecy.

LIST #1

ATTRIBUTION #3

#3 1 This was what the official said.

Simplification example [83].

Lately, based on the work described in [26] [26], [25] [25] propose a sentence splitting approach that combines data-driven and rule-based methods. In a first step, they use a CRF tagger [46] to identify and classify signs of syntactic complexity, such as coordinators, wh-words or punctuation marks. Then, a small set of 28 manually defined rules for splitting coordinate clauses and a larger set of 125 rules for decomposing nominally bound relative clauses is applied to rewrite long sentences into sequences of shorter single-clause sentences.

§.§.§ Approaches based on Semantic Parsing

While the TS approaches described above are based on syntactic information, there are a variety of methods that use semantic structures for sentence splitting, including the work of [59] [59] and [60] [60], who propose a framework that
takes semantically-shared elements as the basis for splitting and rephrasing a sentence. It first generates a semantic representation of the input (in the form of Discourse Semantic Representations [43])
to identify splitting points in the sentence. In a second step, the split components are then rephrased by completing them with missing elements in order to reconstruct grammatically sound sentences. Lately, with DSS, [89] [89] describe another semantic-based structural simplification framework that follows a similar approach, making use of the semantic parser presented in [39] [39] which supports the direct decomposition of a sentence into individual semantic elements. Furthermore, they propose SENTS, a hybrid TS system that adds a lexical simplification component to DSS by running the split sentences through the neural MT-based system proposed in [67] [67].

Though these approaches pursue a very similar goal as compared to our framework, namely the decomposition of complex source sentences into minimal semantic units, they generally suffer from a poor grammaticality and fluency of the simplified output.

System     Output
gray!25Input     gray!25The tarantula, the trickster character, spun a black cord and, attaching it to the ball, crawled away fast to the east, pulling on the cord with all his strength.
DSS     the tarantula the trickster character spun a black cord. attaching it to the ball. character crawled away fast to the east. character pulling on the cord with all his strength.
SENTS (h4)     the character has a character on the rope, and the player can see the ball.
Hybrid [59]     the tarantula, the trickster character, a black spun cord, and it attaching, crawled, pulling all.

#1 0 The tarantula spun a black cord.

LIST #2

LIST #3

ELABORATION #4

#2 0 The tarantula crawled away fast to the east.

LIST #1

LIST #3

ELABORATION #4

#3 0 The tarantula was pulling on the cord with all his strength.

LIST #1

LIST #2

ELABORATION #4

#4 1 The tarantula was the trickster character.

gray!25Input     gray!25Their granddaughter Hélène Langevin-Joliot is a professor of nuclear physics at the University of Paris, and their grandson Pierre Joliot, who was named after Pierre Curie, is a noted biochemist.
DSS     their granddaughter Hélène Langevin Joliot is a professor of nuclear physics at the university of Paris. Joliot was named after Pierre curie.
SENTS (h4)     their granddaughter Hélène Langevin Joliot is a professor of nuclear physics.
Hybrid [59]     their granddaughter hélène langevin-joliot is a professor of physics, and their grandson pierre joliot, who was named, is a biochemist.

#1 0 Hélène Langevin-Joliot is a professor of nuclear physics at the University of Paris.

ELABORATION #2

LIST #3

#2 1 Hélène Langevin-Joliot is their granddaughter.

#3 0 Pierre Joliot is a noted biochemist.

LIST #1

ELABORATION #3

ELABORATION #5

#4 1 Pierre Joliot is their grandson.

#5 1 Pierre Joliot was named after Pierre Curie.

ELABORATION #6

#6 2 Pierre Joliot is their grandson.

Simplification examples (from WikiLarge).

§.§.§ Data-driven Approaches

Recently, data-driven approaches for the task of sentence splitting emerged.
[61] [61] observed that existing TS corpora, such as PWKP [96], EW-SEW [21], Newsela [93] or WikiLarge [95], are ill suited for learning to decompose sentences into shorter, syntactically simplified components, as they contain only a small number of split examples.[For instance, according to [61] [61], the average number of simple sentences per complex source is 1.06 in PWPK. In fact, only 6.1% of the complex input sentences are split into two or more simplified counterparts [59].] Thus, they compiled the WebSplit corpus,[<https://github.com/shashiongithub/Split-and-Rephrase>] the first TS dataset that explicitly addresses the task of sentence splitting, while abstracting away from lexical and deletion-based operations. It was derived in a semi-automatic way from the WebNLG corpus [33], which contains 13K pairs of items, with each of them consisting of a set of RDF triples and one or more texts verbalising them. The resulting WebSplit dataset is composed of over one million tuples that map a single complex sentence to a sequence of structurally simplified sentences expressing the same meaning. [61] [61] describe a set of sequence-to-sequence models [6] trained on this corpus for breaking up complex input sentences into shorter components that present a simpler structure.
[2] [2] further explore this idea, augmenting the presented neural models with a copy mechanism [36, 75]. Though outperforming the models used in [61] [61], they still perform poorly compared to previous state-of-the-art rule-based syntactic simplification approaches.

(1) A Loyal Character Dancer was published by Soho Press, in the United States, where some Native Americans live.

(2) Dead Man's Plack is in England and one of the ethnic groups found in England is the British Arabs.

Characteristic example source sentences from the WebSplit corpus.

Complex source     Starring Meryl Streep, Bruce Willis, Goldie Hawn and Isabella Rossellini, the film focuses on a childish pair of rivals who drink a magic potion that promises eternal youth.
Simplified output     Starring Meryl Streep, Bruce Willis, Goldie Hawn and Isabella Rossellini. The film focuses on a childish pair of rivals who drink a magic potion that promises eternal youth.
Complex source     The Assistant Attorney in Orlando investigated the modeling company, and decided that they were not doing anything wrong, and after Pearlman's bankruptcy, the company emerged unscathed and was sold to a Canadian company.
Simplified output     The Assistant Attorney in Orlando investigated the modeling company, and decided that they were not doing anything wrong. After Pearlman's bankruptcy, the modeling company emerged unscathed and was sold to a Canadian company.

Pairs of aligned complex source and simplified target sentences from WikiSplit.

[12] [12] noticed that the sentences from the WebSplit corpus contain fairly unnatural linguistic expressions using only a small vocabulary and a rather uniform sentence structure.
To overcome this limitation, they present a scalable, language-agnostic method for mining large-scale training data from Wikipedia edit histories. Based on this approach, they compiled WikiSplit[<https://github.com/google-research-datasets/wiki-split>], a new sentence splitting dataset that provides a rich and varied vocabulary over naturally expressed sentences and their extracted splits.
When training the best-performing model of [2] [2] on this corpus, they achieve a strong improvement over their prior best results. However,
[12]s' [12] approach exhibits a strong conservatism since each input sentence is split in exactly two output sentences only, due to the uniform use of a single split per source sentence in the training set.

Consequently, the resulting simplified sentences are still comparatively long and complex, mixing multiple, potentially semantically unrelated propositions that are difficult to handle for downstream Open IE tasks. Though, as opposed to the semantic-based approaches presented above, this approach commonly succeeds in producing split sentences that are grammatically sound.

System     Output
gray!25Input     gray!25Diller suddenly sold the stations to Univision, which converted them into Spanish language outlets, and while some affiliated with Univision, most joined Telefutura, its new sister network that launched in January 2002.
Copy512     Diller suddenly sold the stations to Univision, which converted them into Spanish language outlets. While some affiliated with Univision, most joined Telefutura, its new sister network that launched in January 2002.

#1 0 Diller suddenly sold the stations to Univision.

ELABORATION #2

LIST #4

#2 1 Univision converted them into Spanish language outlets.

#3 0 Some affiliated with Univision.

ELABORATION #2

CONTRAST #4

#4 0 Most joined Telefutura.

LIST #1

CONTRAST #3

ELABORATION #5

#5 1 Telefutura was its new sister network.

ELABORATION #6

#6 2 This new sister network launched in 2002.

Simplification example (from WikiSplit).

§.§ Text Coherence

When simplifying the structure of sentences without considering discourse implications, the rewriting may easily result in a loss of cohesion, hindering the process of making sense of the text. Thus, preserving the coherence of the input is crucial to maintain its interpretability in downstream applications.

§.§.§ Discourse-level Syntactic Simplification

The vast majority of existing structural TS approaches do not take into account discourse level effects that arise from splitting long and syntactically complex sentences into a sequence of shorter utterances.
However, not considering the semantic context of the output propositions, the simplified text is prone to lack coherence, resulting in a set of incoherent propositions that miss important contextual information that is needed to infer the true meaning of the simplified output.
Though, two notable exceptions have to be mentioned.

[78] [78] was the first to use discourse-aware cues in the simplification process, with the goal of generating a coherent output that is accessible to a wider audience. For this purpose, he adds a regeneration stage after the transformation process where, amongst others, simplified sentences are reordered, appropriate determiners are chosen
and cue words in the form of rhetorical relations are introduced to connect them, thus making information easier to comprehend for people with reduced literacy. Though, as opposed to our approach, where a semantic relationship is established for each output sentence, only a comparatively low number of sentences is linked by such cue words in [78]'s [78] framework.

Complex source     The man who had brought it in for an estimate returned to collect it.
Cohesive simplified output      A man had brought it in for an estimate. This man returned to collect it.

Adequate determiner choice to maintain cohesion [78].

Another approach that simplifies texts on the discourse level was proposed by [83] [83]. It presents a semantically motivated method for eliminating irrelevant information from the input by maintaining only those parts that belong to factual event mentions. Our approach, on the contrary, aims to preserve all the information contained in the source.

However, by distinguishing core from contextual information, we are still able to extract only the key information given in the input.

§.§.§ Discourse Parsing

The challenge of uncovering coherence structures in texts is pursued in the area of Discourse Parsing, which aims to identify discourse relations that hold between textual units in a document [51].

The annotated corpora of the Penn Discourse TreeBank (PDTB) [56] and the RST Discourse Treebank (RST-DT) [16] have established two major directions in this area. They are grounded on different taxonomies: whereas PDTB provides a more shallow specification of discourse structures, RST-DT analyzes the hierarchical structure of discourse in texts based on Rhetorical Structure Theory (RST) [50].

In this work, we will concentrate on the latter taxonomy.
In RST, coherence in texts is explained by the existence of rhetorical relations that hold between non-overlapping text spans in a hierarchical structure. Each rhetorical relation is defined on the basis of the function that the connected text spans take up in order to transport a certain message to the reader. These spans are specified as either a nucleus (N) or a satellite (S). The nucleus span embodies the central piece of information, whereas the role of the satellite is to further specify the nucleus. Rhetorical relations that connect a nucleus span and a satellite span are called mono-nuclear, whereas rhetorical relations between two nucleus spans are denoted multi-nuclear. For instance, the rhetorical relation Evidence is defined as follows [50]:

* Constraints on N: The reader might not believe N to a degree satisfactory to the writer.

* Constraints on S: The reader believes S or will find it credible.

* Constraints on the N + S combination: The reader's comprehension of S increases the reader's belief of N.

* The effect: The reader's belief of N is increased.

Relation name     Evidence
Constraints on N     The reader might not believe N to a degree satisfactory to the writer.
Constraints on S     The reader believes S or will find it credible.
Constraints on the N + S combination     The reader's comprehension of S increases the reader's belief of N.
The effect     The reader's belief of N is increased.

Definition of the Evidence relation [50].

An application of the Evidence relation is illustrated in the following excerpt:

[The program as published for calendar year 1980 really works.]N [In only a few minutes, I entered all the figures from my 1980 tax return and got a result which agreed with my hand calculations to the penny.]S [50]

Here, a positive practical experience of an income tax program is presented in the satellite span as an evidence to the reader in order to increase her belief that the program really works (nucleus span).
Besides the set and characteristics of rhetorical relations, [50] [50] also introduced schema types that define the structural constituency arrangements of texts. In this way, RST is able to identify semantic relations both locally between neighbored elementary textual units and for the full document by using a hierarchical representation of nested schema type applications that cover long-range dependencies up to the whole document.

Over time, the original set of 25 relations presented in [50] [50] has been extended. RST-DT currently contains 53 mono-nuclear and 25 multi-nuclear relations.
Approaches to identify and detect rhetorical structure arrangements in texts range from early rule-based approaches [52] to supervised data-driven models that were trained on annotated corpora such as the RST-DT [29, 48]. Most commonly, these approaches separate the task of extracting discourse structures into two major subtasks: (1) elementary discourse unit (EDU) segmentation, where the input text is partitioned into basic textual units, and (2) a tree-building step, where the identified EDUs are interconnected by rhetorical relations to form a discourse tree.
Due to the simplicity of the first task and the high accuracy achieved (90% by [32] [32]), recent research focuses on the more challenging task of the discourse tree construction, which includes the task of determining rhetorical relations between two consecutive text spans [29, 42].
Recent approaches often solve the tree-building task by training two individual classifiers: a binary structure classifier for finding pairs of consecutive text spans that are to be joined together, and a multi-class relation classifier for identifying the rhetorical relation that holds between them [38, 48, 29, 42].
The most prominent features that are used by state-of-the-art systems for the rhetorical relation identification step are a mix of lexical and syntactic features, including cue phrases, part-of-speech tags, and information derived from syntactic parse trees [29, 48].

[scale=0.9, level distance=3cm, sibling distance=0.25cm, every tree node/.style=align=center, transform shape]

.[style=draw,rectangle, fill=rgb:orange,1;yellow,2;pink,5] 1-4: Condition;
node[midway, left] N; [.[style=draw,rectangle, fill=rgb:orange,1;yellow,2;pink,5] 1-2: Elaboration;
node[midway, left] N; [.(a)A fluoroscopic
node[midway, right] S; [.(b)known as an
upper gastrointestinal
series is typically
the next step
in management,;]
node[midway, right] S; [.[style=draw,rectangle, fill=rgb:orange,1;yellow,2;pink,5] 3-4: List;
node[midway, left] N; [.(a)although if volvulus
is suspected,
caution with non
water soluble contrast
is mandatory as
the usage of
barium can impede
surgical revision;]
node[midway, right] N; [.(b)and lead to
post operative

Rhetorical structure tree of the example sentence from Figure <ref> generated using the RST parser proposed by <cit.> (N: nucleus, S: satellite).

Although the segmentation step is related to the task of sentence splitting, it is not possible to simply use an RST parser for the sentence splitting subtask in our TS approach.
Such a parser does not return grammatically sound sentences, but rather just copies the components that belong to the corresponding EDUs from the source. In order to reconstruct proper sentences, rephrasing is required. For this purpose, amongst others, referring expressions have to be identified, and phrases have to be rearranged and inflected. Moreover, the textual units resulting from the segmentation process are too coarse-grained for our purpose, since RST parsers mostly operate on clausal level. The goal of our approach, though, is to split the input into minimal semantic units, which requires to go down to the phrasal level in order to produce a much more fine-grained output in the form of minimal propositions.

§ DISCOURSE-AWARE SENTENCE SPLITTING

We present DisSim, a discourse-aware sentence splitting approach that creates a semantic hierarchy of simplified sentences.[To support transparency and reproducibility, the source code of our framework [63] is publicly available: <https://github.com/Lambda-3/DiscourseSimplification>.]
It takes a sentence as input and performs a recursive transformation stage that is based upon a small set of 35 hand-crafted grammar rules, encoding syntactic and lexical features that are derived from a sentence's phrase structure. Each rule specifies (1) how to split up and rephrase the input into structurally simplified sentences  (see Section <ref>) and (2) how to set up a contextual hierarchy between the split components (see Section <ref>) and identify the semantic relationship that holds between those elements (see Section <ref>). In that way, a semantic hierarchy of simplified sentences is established, providing an intermediate representation of the input that can be used to facilitate and improve the performance of downstream Open IE tasks.

The transformation patterns were heuristically determined in a rule engineering process that was carried out on the basis of an in-depth study of the literature on syntactic sentence simplification [78, 79, 77, 80, 26, 25, 37, 92, 76, 57, 49, 90, 30, 19], followed by a thorough linguistic analysis of the syntactic phenomena that need to be tackled in the sentence splitting task. Our main goal was to provide a best-effort set of rules, targeting the challenge of being applied in a recursive fashion and to overcome biased or incorrectly structured parse trees. We empirically determined a fixed execution order of the specified rules by examining which sequence achieved the best simplification results in a qualitative manual analysis conducted on a development test set of 100 randomly sampled Wikipedia sentences. The grammar rules are applied recursively in a top-down fashion on the source sentence, until no more simplification pattern matches (see Algorithm <ref>). For reproducibility purposes, the complete set of transformation patterns is detailed in the appendix.

ROOT $<<:$ (S $<$ (NP $\$..$ (VP $<+$(VP) (SBAR $<,$ (IN $\$+$ (S $<$ (NP $\$..$ VP)))))))

Example of a transformation pattern (for decomposing adverbial clauses with post-posed subordinative clauses).
They are specified in terms of Tregex patterns [47].
A boxed pattern represents the part of a sentence that is extracted from the input and transformed into a new stand-alone sentence. A pattern in bold is deleted from the source. The underlined part is labelled as a context sentence, while the remaining part represents core information. The italic pattern is used as a cue phrase for identifying the rhetorical relation that holds between the decomposed spans.

Rule     Tregex pattern     Extracted sentence

Rule #2     gray!25ROOT $<<:$ (S $<$ (NP $\$..$ (VP $<+$(VP) (SBAR $<$ (S $<$ (NP $\$..$ VP))))))     gray!25S $<$ (NP $\$..$ VP).
3p12.9cm Example: It is also used by those who commute into the city using Metrorail, as the two systems are connected at the Government Center and Brickell stations.

An example of a transformation rule pattern (for decomposing adverbial clauses with post-posed subordinate clauses). They are specified in terms of Tregex patterns (see [47] for details on the rule syntax). A pattern in bold represents the part of a sentence that is extracted from the input and transformed into a new stand-alone sentence. An underlined pattern (solid line) will be deleted from the source sentence. An italic pattern will be labelled as a context sentence, while the remaining part will represent core information. The pattern that is underlined with a dotted line will be used as a cue phrase for identifying the rhetorical relation that holds between the two decomposed sentence parts. The semantic hierarchy that results from applying above-mentioned transformation pattern on the given example sentence is depicted in Figure <ref>.

[scale=0.9, level distance=2.4cm, sibling distance=1cm, every tree node/.style=align=center, transform shape]

.[style=draw,rectangle, fill=rgb:orange,1;yellow,2;pink,5] SUBORD
node[midway, left] core; [.(a)It is also used by those
who commute into the city
using Metrorail.;]
node[midway, right] context; [.(b)The two systems are connected
at the Government Center
and Brickell stations.;]

Semantic hierarchy that results from applying the transformation rule from Table <ref> on the input sentence.

An example of a transformation rule is provided in Table <ref>. In total, we manually defined a set of 35 patterns that are applied in a pre-determined order on a given input sentence. By disembedding clausal and phrasal constituents that contain only supplementary information, source sentences that present a complex linguistic structure are recursively split into shorter, syntactically simplified components. These elements are then converted into self-contained sentences. Hereby, the input is reduced to its key information (“core sentence”) and augmented with a number of associated contextual sentences that disclose additional information about it, resulting in a novel hierarchical representation in the form of core sentences and accompanying contexts. Moreover, we determine the rhetorical relations by which the split sentences are linked in order to preserve their semantic relationship. The resulting representation consisting of a set of hierarchically ordered and semantically interconnected sentences (see Figure <ref>) that present a simplified syntax will be denoted a linked proposition tree in the following.

Transform into Semantic Hierarchy

[1]
complex source sentence $str$
linked proposition tree $tree$
$new\_leaves$ $\gets$ source sentence $str$
$new\_node \gets$ create a new parent node for $new\_leaves$
$new\_node.labels$ $\gets$ None
$new\_node.rel$ $\gets$ ROOT
linked proposition tree $tree$ $\gets$ initialize with $new\_node$
Process leaves (i.e. propositions) from left to right
$leaf$ in $tree.leaves$

Check transformation rules in fixed order
$rule$ in $TRANSFORM\_RULES$

(a) Sentence splitting

$simplified\_propositions \gets$ decompose $leaf$ into a
set of simplified propositions
$new\_leaves \gets$ convert $simplified\_propositions$
into leaf nodes

(b) Constituency Type Classification

$new\_node \gets$ create a new parent node for $new\_leaves$

$new\_node.labels \gets$ link each leaf in $new\_leaves$ to
$new\_node$ and label each edge with the leaf's constituency
type $c \in CL$

(c) Rhetorical Relation Identification
$cue\_phrase \gets$ extract cue phrase from $leaf.parse\_tree$

$new\_node.rel \in REL \gets$ match $cue\_phrase$ against a
predefined set of rhetorical cue words

Update Tree
$tree.replace(leaf, new\_node)$

§.§ Subtask 1: Splitting into Minimal Propositions

In a first step, source sentences that present a complex linguistic form are transformed into regular, compact structures by disembedding clausal and phrasal components that contain only supplementary information. For this purpose, the transformation rules encode both the splitting points and rephrasing procedure for reconstructing proper sentences.
Each rule takes a sentence's phrasal parse tree[generated by Stanford's pre-trained lexicalized parser [82]] as input and encodes a pattern that, in case of a match, will extract textual parts from the tree. The decomposed text spans, as well as the remaining constituents are then transformed into proper stand-alone sentences. In order to ensure that the resulting simplified output is grammatically sound, the extracted spans are combined with their corresponding referents from the main sentence or appended to a simple canonical phrase (e.g. “This is”), if necessary.
Table <ref> provides an overview of the linguistic constructs that are tackled by our approach, including the number of patterns that were specified for the respective syntactic phenomenon.

Clausal/Phrasal type     Hierarchy     # rules

4c Clausal disembedding

1     Coordinate clauses     coordinate     1

2     Adverbial clauses     subordinate     6

3a     Relative clauses (non-restrictive)     subordinate     5

3b     Relative clauses (restrictive)     subordinate     4

4     Reported speech     subordinate     4

4c Phrasal disembedding

5     Coordinate verb phrases     coordinate     1

6      Coordinate noun phrases     coordinate     2

6      Participial phrases     subordinate     4

8a      Appositions (non-restrictive)     subordinate     1

8b     Appositions (restrictive)     subordinate     1

9     Prepositional phrases     subordinate     3

10     Adjectival and adverbial phrases     subordinate     2

11     Lead NPs     subordinate     1

Total         35

Linguistic constructs addressed by DisSim, including both the hierarchical level and the number of simplification rules that were specified for each syntactic construct.

For a better understanding of the splitting procedure, Figure <ref> visualizes the application of the first grammar rule that matches the input sentence from our running example.
The upper part of the box represents the complex source sentence, which is matched against the specified simplification pattern (see Table <ref>). The lower part then depicts the result of the corresponding transformation operation.

Example: SubordinationPostExtractor
Input: “A fluoroscopic study which is known as an upper gastrointestinal series is typically the next step in management, although if volvulus is suspected, caution with non water soluble contrast is mandatory as the usage of barium can impede surgical revision and lead to increased post operative complications.”
Matched Pattern:

[scale=0.69, every tree node/.style=align=center]
[.NP [roof]; A fluoroscopic
... series ]
[.VBZ is ]
[.ADVP typically ]
[.NP [roof]; the next step
in management ]
[., , ]
[.IN although ]
[.SBAR [roof]; if volvulus
is suspected ]
[., , ]
[.NP [roof]; caution
... contrast ]
[.VP [roof]; is mandatory
... complications ]
[.. . ]

Semantic Hierarchy (after the first transformation pass):

[scale=1, level distance=3cm, sibling distance=1.5cm, every tree node/.style=align=center]
[.[style=draw,rectangle] (3) “although” $\rightarrow$ Contrast;
node[midway, left] (2) core; (1)
A fluoroscopic study
which is known as an upper
gastrointestinal series
is typically the next
step in management.
node[midway, right] (2) core; (1)
If volvulus is suspected,
caution with non water soluble contrast
is mandatory as the  usage of barium
can impede surgical revision  and lead
to increased post operative complications.

(Subtask 1) The source sentence is split up and rephrased into a set of syntactically simplified sentences. (Subtask 2) Then, the split sentences are connected with information about their constituency type to establish a contextual hierarchy between them. (Subtask 3) Finally, by identifying and classifying the rhetorical relation that holds between the simplified sentences, their semantic relationship is restored which can be used to inform downstream Open IE applications.

§.§ Subtask 2: Establishing a Semantic Hierarchy

Each split will create two or more sentences with a simplified syntax. In order to establish a semantic hierarchy between them, two subtasks are carried out: constituency type classification and rhetorical relation identification.

§.§.§ Constituency Type Classification

First, we set up a contextual hierarchy between the split sentences. For this purpose, we connect them with information about their constituency type. According to [28] [28], clauses can be related to one another in two ways: first, there are parallel clauses that are linked by coordinating conjunctions, and second, clauses may be embedded inside another, introduced by subordinating conjunctions. The same applies to phrasal elements. Since subordinations commonly express less relevant information, we denote them “context sentences”. In contrast, coordinations are of equal status and typically depict the key information contained in the input. Therefore, they are called “core sentences” in our approach. To differentiate between those two types of constituents, the transformation patterns encode a simple syntax-based approach where subordinate clauses and subordinate phrasal elements are classified as context sentences, while their superordinate counterparts as well as coordinate clauses and coordinate phrases are labelled as core (see Table <ref>).

However, two exceptions have to be noted. In case of an attribution, the subordinate clause expressing the actual statement (e.g. “Economic activity continued to increase.”) is assigned a core tag, while the superordinate clause containing the fact that it was uttered by some entity (e.g. “This was what the Federal Reserve noted.”) is labelled as contextual information. The reason for this is that the latter is considered less relevant as compared to the former, which holds the key information of the source sentence. Moreover, when disembedding adverbial clauses of contrast (as determined in the rhetorical relation identification step, see Section <ref>), both the superordinate and the subordinate clause are labelled as a core sentence (see the example in Figure <ref>), in accordance with the theory of RST, where text spans that present a “Contrast” relationship are considered both as nuclei.

This approach allows for the distinction of core information capturing the key message of the input from contextual information that provides only supplementary material, resulting in a two-layered hierarchical representation in the form of core facts and accompanying contextual information.
This directly relates to the concept of nuclearity in RST, which specifies each text span as either a nucleus or a satellite. The nucleus span embodies the central piece of information and is comparable to what we denote a core sentence, whereas the role of the satellite is to further specify the nucleus, corresponding to a context sentence in our case.

§.§.§ Rhetorical Relation Identification

Second, we aim to preserve the semantic relationship between the decomposed spans. For this purpose, we identify and classify the rhetorical relations that hold between the simplified sentences, making use of both syntactic and lexical features that are encoded in the transformation patterns. While syntactic features are manifested in the phrasal composition of a sentence's parse tree, lexical features are extracted from the parse tree in the form of cue phrases. The determination of potential cue words and their positions in specific syntactic environments is based on the work of [45] [45]. The extracted cue phrases are then used to infer the type of rhetorical relation. For this task, we utilize a predefined list of rhetorical cue words adapted from the work of [91] [91], which assigns them to the relation that they most likely trigger.[The full list of cue phrases that serve as lexical features for the identification of rhetorical relations in our approach, as well as the corresponding relations to which they are mapped,
is provided in the appendix.] For example, the transformation rule in Table <ref>, which is the first pattern that matches our running example, specifies that the phrase “although” is the cue word here, which is mapped to a “Contrast” relationship. The semantic hierarchy that results from the first transformation pass is depicted in the lower part of Figure <ref>.

[scale=0.68, level distance=2.4cm, sibling distance=0cm, every tree node/.style=align=center, transform shape]

.[style=draw,rectangle] COORD
node[midway, left] core; [
.[style=draw,rectangle] SUBORD
node[midway, left] core; [.(a)A fluoroscopic study
is typically the next
step in management.;]
node[midway, right] context; [.(b)A fluoroscopic study
is known as an upper
gastrointestinal series.;]
node[midway, right] core; [
.[style=draw,rectangle] SUBORD
node[midway, left] context; [.(c)Volvulus is
node[midway, right] core;[.[style=draw,rectangle] SUBORD
node[midway, left] core; [.(d)Caution with non
water soluble contrast
is mandatory.;]
node[midway, right] context; [.[style=draw,rectangle] COORD
node[midway, left] core; [.(e)The usage of
barium can impede
surgical revision.;]
node[midway, right] core; [.(f)The usage of
barium can lead
to increased post
operative complications.;]

Final linked proposition tree of the example sentence.

The process of selecting the rhetorical relations to include in our approach was guided by the following two questions:

(i) Which rhetorical relations are the most relevant for downstream Open IE applications?

(ii) Which of them are likely to be recognized by syntactic and lexical features according to the work of [91] [91]?

Based on the above considerations, we employ a subset of the classical set of RST relations defined in [50] [50]. This comprises the Contrast, List, Disjunction, Cause, Result, Background, Condition, Elaboration and Purpose relationships. Additionally, we adopt a number of relations from the extended set of rhetorical relations defined in the RST-DT, including Temporal-After, Temporal-Before and Attribution. In order to deal with the problem of linking split sentences whose connecting rhetorical relation could not be identified, the custom relations Unknown-Coordination and Unknown-Subordination are introduced. Moreover, representing classes of context that were frequently encountered in our example sentences, we add the relations Spatial and Temporal for the identification of semantic relationships that provide local or temporal information about a presented situation.

Accordingly, we end up with the following set of rhetorical relations:

{Contrast, List, Disjunction, Cause, Result,
Background, Condition, Elaboration, Attribution, Purpose, Temporal-Before, Temporal-After, Temporal, Spatial, Unknown-Coordination, Unknown-Subordination}[Following the approach described in [91] [91], we later map the List and Disjunction relationships to a common Joint class and integrate
the Result relation into the semantic category of Cause relationships. Moreover, both Temporal-After and Temporal-Before become part of the Temporal class of rhetorical relations. Finally, Purpose is embedded in the class of Enablement relationships.]

The lower part of Figure <ref> depicts the result of the first transformation pass on the example sentence.
Afterwards, the resulting leaf nodes are recursively simplified in a top-down fashion. When no more rule matches, the algorithm stops and returns the generated linked proposition tree,[A linked proposition tree is a labeled binary tree whose leaf nodes represent the minimal propositions, whereas its inner nodes represent rhetorical relations that hold between a pair of associated split sentences. Every node of the tree is connected to its parent by an edge that presents either a core or a context label. For more details on the data model, see [66] [66].] representing the semantic hierarchy of simplified sentences for the given input sentence (see Figure <ref>).[For a detailed step-by-step example demonstrating the complete transformation process, the interested reader may refer to the appendix.]

Output generated by our discourse-aware TS approach. The structurally simplified sentences are semantically linked to one another via rhetorical relations. The number following the sentences' ID indicates the hierarchical level of the split components. It refers to the sentences' context layer cl. Sentences with cl = 0 carry the core information of the source, whereas sentences with a cl$\geq$1 provide contextual information about a sentence with a context layer of cl-1.

§ EVALUATION

To evaluate our proposed discourse-aware TS approach, we first compare its performance with state-of-the-art syntactic simplification systems with respect to the sentence splitting task (subtask 1). Next, we assess the accuracy of the generated semantic hierarchy (subtask 2) on the basis of the RST-DT corpus. In addition, we extrinsically evaluate the usefulness of our approach as a preprocessing step for downstream semantic applications in the context of the task of Open IE.

§.§ Experimental Setup

In the following, we present the experimental setup for evaluating the performance of our reference implementation DisSim with regard to the two subtasks of (1) splitting and rephrasing syntactically complex input sentences into a set of minimal propositions, and (2) setting up a semantic hierarchy between the split components, based on constituency type classification and rhetorical relation identification. In addition, we describe the setting for assessing the effectiveness of our approach for downstream Open IE tasks.

§.§.§ Subtask 1: Splitting into Minimal Propositions

To evaluate the performance of our discourse-aware TS framework with regard to the sentence splitting subtask, we conducted experiments on three commonly applied TS corpora from two different domains. The first dataset we used was WikiLarge [94]. It consists of 359 sentences from the PWKP corpus [96], which is made up of aligned complex English Wikipedia sentences and Simple English Wikipedia counterparts. In WikiLarge, each complex sentence from PWKP is paired with eight simplification variants that were collected by crowdsourcing. Moreover, to demonstrate domain independence, we compared the output generated by our TS approach with that of the various baseline systems on the Newsela corpus [93], which is composed of 1077 sentences from newswire articles. In addition, we assessed the performance of our simplification system using the 5000 test sentences from the WikiSplit benchmark [12], which was mined from Wikipedia edit histories.

We compared our reference implementation DisSim against several state-of-the-art TS baseline systems that have a strong focus on syntactic transformations through explicitly modeling splitting operations. For WikiLarge, these include (i) the recent semantic-based DSS model that decomposes a sentence into its main semantic constituents using a semantic parser; (ii) SENTS, which is an extension of DSS that runs the split sentences through the NTS system proposed in [67] [67]; (iii) Hybrid [59], a further TS approach that takes semantically-shared elements as the basis for splitting and rephrasing a sentence; (iv) the hybrid TS system YATS; and (v) RegenT, another hybrid state-of-the-art simplification approach for combined lexical and syntactic simplification. In addition, we report evaluation scores for both the complex input sentences, which allows for a better judgment of system conservatism, and the corresponding simple reference sentences. With respect to the Newsela dataset, we considered the same baseline systems, with the exceptions of DSS and SENTS, whose outputs were not available. Finally, regarding the WikiSplit corpus, we restricted the comparison of our TS approach to the best-performing system in [12] [12], Copy512, which is a sequence-to-sequence neural model augmented with a copy mechanism and trained over the WikiSplit dataset.

Automatic metrics The automatic metrics that were calculated in the evaluation procedure comprise a number of basic statistics, including (i) the average sentence length of the simplified sentences in terms of the average number of tokens per output sentence (#T/S); (ii) the average number of simplified output sentences per complex input (#S/C); (iii) the percentage of sentences that are copied from the source without performing any simplification operation (%SAME), serving as an indicator for system conservatism, i.e. the tendency of a system to retain the input sentence rather than transforming it; and (iv) the averaged Levenshtein distance from the input (LDSC), which provides further evidence for a system's conservatism. Furthermore, in accordance with prior work on TS, we report average BLEU [69] and SARI [94] scores for the rephrasings of each system. Finally, we computed the SAMSA and SAMSAabl scores of each system, which are the first metrics that explicitly target syntactic aspects of TS [88]. The SAMSA metric is based on the idea that an optimal split of the input is one where each predicate-argument structure is assigned its own sentence in the simplified output and measures to what extent this assertion holds for the input-output pair under consideration. Accordingly, the SAMSA score is maximized when each split sentence represents exactly one semantic unit in the
input. SAMSAabl, in contrast, does not penalize cases where the number of sentences in the simplified output is lower than the number of events contained in the input, indicating separate semantic units that should be split into individual target sentences for obtaining minimal propositions.

Human evaluation Human evaluation was carried out on a subset of 50 randomly sampled sentences per corpus by 2 in-house non-native, but fluent English speakers who rated each input-output pair for the different systems according to three parameters: grammaticality, meaning preservation and structural simplicity. The detailed guidelines presented to the annotators are given in
the supplementary material. Regarding the grammaticality and meaning preservation dimensions, we adopted the guidelines from [83] [83], with some minor deviations to better reflect our goal of simplifying the structure of the input sentences, while retaining their full informational content. Besides, since the focus of our work is on structural rather than lexical simplification, we followed the approach taken in [89] [89] in terms of the third parameter, simplicity, and neglected the lexical simplicity of the output sentences. Instead, we restricted our analysis to the syntactic complexity of the resulting sentences, which was measured on a scale that ranges from -2 to 2 in accordance with [67] [67].

1. Grammaticality (G): Is the output fluent and grammatical?

5 The output is meaningful and there are no grammatical mistakes.

4 One or two minor errors, but the meaning can be easily understood.

3 Several errors, but it is still possible to understand the meaning.

2 It is hard to understand the meaning due to many grammatical errors.

1 The output is so ungrammatical that it is impossible to infer any meaning.

2. Meaning Preservation (M): Does the output preserve the meaning of the input?

5 The simplification has the same meaning as the original sentence. No piece of information from the input is lost.

* The core meaning is the same as in the original sentence but with some subtle differences.

* If there are several output sentences, one is missing some important piece of information from the original sentence but the others fully preserve the original meaning.

* The simplified sentence(s) contain(s) a part of the relevant information from the original sentence, but another important part of relevant information is missing (there is no spurious information).

* Incorrect pronoun/attachment resolution.

2 The simplification has the opposite or very different meaning compared to the original sentence.

1 Simplified sentence(s) have no coherent meaning (i.e. it is not possible to compare it/them to the original sentence).

3. Structural Simplicity (S): Is the output simpler than the input, ignoring the complexity of the words?

2 The simplified output is much simpler than the original sentence.

1 The simplified output is somewhat simpler than the original sentence.

0 The simplified output is equally difficult as the original sentence.

-1 The simplified output is somewhat more difficult than the original sentence.

-2 The simplified output is much more difficult than the original sentence.

Annotation guidelines for the human evaluation task.

Manual analyses In order to get further insights into the quality of our implemented simplification patterns, we performed an extensive qualitative analysis of the 35 hand-crafted transformation rules, including an examination of the frequency distribution of the syntactic constructs that are addressed by our TS approach, a manual recall-based analysis of the simplification patterns that we defined, and a detailed error analysis. For this purpose, we compiled a dataset consisting of 100 Wikipedia sentences per syntactic phenomenon tackled by our TS approach.[available under <https://github.com/Lambda-3/DiscourseSimplification/blob/master/supplemental_material/dataset_pattern_analysis.zip>] In the construction of this corpus we ensured that the collected sentences exhibit a great syntactic variability to allow for a reliable predication about the coverage and accuracy of the specified transformation rules.

§.§.§ Subtask 2: Establishing a Semantic Hierarchy

We evaluated the constituency type classification and rhetorical relation identification steps by mapping the minimal propositions that were generated in the previous subtask to the EDUs of the RST-DT corpus, a collection of 385 Wall Street Journal articles annotated for rhetorical relations.

Constituency type classification To determine whether the hierarchical relationship that was assigned by our framework between a pair of simplified sentences is correct, we check if the hierarchy of the contextual layers corresponds to the nuclearity of the aligned text fragments from RST-DT.

Rhetorical relation identification For each matching sentence pair, we examine whether the rhetorical relation assigned by our approach equates the relation that connects the corresponding sentences in the RST-DT dataset. For this purpose, we use the more coarse-grained classification scheme from [91] [91], who group the full set of 78 rhetorical relations that are contained in the RST-DT corpus into 19 classes of relations that share rhetorical meaning.

§.§.§ Extrinsic Evaluation

The fine-grained representation of complex sentences in the form of hierarchically ordered and semantically interconnected propositions may serve as an intermediate representation for downstream tasks.
An application area that may benefit greatly from our approach
as a preprocessing step is the task of Open IE [8].

The goal of this task is

to turn unstructured information expressed in natural language text into a structured representation in the form of relational tuples that consist of a set of arguments and a phrase denoting a relation between them, e.g. $\langle$arg1: Barack Obama; rel: served as; arg2: the 44th President of the US$\rangle$. In contrast to traditional IE systems, Open IE approaches allow for a domain-independent discovery of relations in large amounts of text, as they do not depend on any relation-specific human input [62].

In order to demonstrate the merits of our approach for the task of Open IE, we integrated it as a preprocessing step into a variety of state-of-the-art Open IE systems, including OpenIE-4 [54], ClausIE [23], Stanford Open IE [4], ReVerb [27] and Ollie [55]. We then investigated whether these systems profit from our proposed discourse-aware TS approach along two different dimensions.
In a first step, we measured if their performance was improved in terms of recall and precision of the generated relational tuples when splitting complex source sentences into a set of syntactically simplified minimal propositions with the help of our TS approach before performing the extraction step. In a second step, we examined whether the semantic hierarchy generated by our approach was beneficial in preserving the interpretability of the output.

§.§ Results and Discussion

Below, we present and discuss the results of our evaluation procedure.

§.§.§ Subtask 1: Splitting into Minimal Propositions

In the following, we describe the evaluation results with respect to the first subtask of our approach, whose goal is to decompose sentences that present a complex linguistic structure into a set of syntactically simplified propositions, with each of them representing a minimal semantic unit.

Automatic metrics

The upper part of Table <ref> reports the results that were achieved on the 359 sentences from the WikiLarge corpus, using a set of automatic metrics. Transforming each sentence of the dataset, our reference implementation DisSim reaches the highest splitting rate among the TS systems under consideration, together with Hybrid, DSS and SENTS. With 2.82 split sentences per input on average, our framework outputs by a large margin the highest number of structurally simplified sentences per source. Moreover, consisting of 11.01 tokens on average, the DisSim approach returns the shortest sentences of all systems, on an average halving the length of the input.
The relatively high word-based Levenshtein distance of 11.90 confirms previous findings, suggesting that our structural TS approach tends to extensively split the source sentences instead of retaining the input by simply copying the input without performing any simplification operations. SENTS (13.79) is the only system that shows an even higher Levenshtein distance to the source.

#T/S     #S/C     %SAME     LDSC     BLEU     SARI     SAMSA     SAMSAabl
9c359 test sentences from the Wikilarge corpus
gray!25Complex source     gray!2522.06     gray!251.03     gray!25100     gray!250.00      gray!2594.25     gray!2532.53     gray!250.59     gray!250.96

gray!25Simple reference     gray!2520.19     gray!251.14     gray!250.00     gray!257.14      gray!2599.48     gray!2543.09     gray!250.48     gray!250.78

DisSim     11.01     2.82     0.00     11.90               63.03              35.05     0.67     0.84

DSS                          12.91              1.87              0.00     8.14                 74.42              34.32     0.64              0.75

SENTS                   14.17              1.09              0.00     13.79           54.37              29.76     0.40              0.58

Hybrid                      13.44              1.03           0.00      13.04             48.97          26.19         0.47         0.76

YATS                         18.83              1.40              18.66            4.44              73.07              33.03     0.56              0.80

RegenT                       18.20              1.45              41.50            3.77         82.49              32.41     0.61              0.85
9c1077 test sentences from the Newsela corpus
gray!25Complex source     gray!2523.34     gray!251.01     gray!25100     gray!250.00      gray!2520.91     gray!259.84     gray!250.49     gray!250.96

gray!25Simple reference     gray!2512.81     gray!251.01     gray!250.00     gray!2516.25      gray!25100     gray!2591.13     gray!250.25     gray!250.46

DisSim     11.20     2.96     0.00     13.00               14.54              49.00     0.57     0.84

Hybrid         12.49                 1.02              0.00         13.46     14.42             40.34           0.38           0.74

YATS                         18.71              1.42             16.16           5.03          17.51              36.88     0.50              0.83

RegenT                       16.74              1.61              33.33           5.03       18.96             32.83     0.55             0.85
9c5000 test sentences from the WikiSplit corpus
gray!25Complex source     gray!2532.01     gray!251.10     gray!25100     gray!250.00      gray!2574.28     gray!2529.91     gray!250.37     gray!250.95

gray!25Simple reference     gray!2518.14     gray!252.08     gray!250.00     gray!257.48      gray!25100     gray!2594.71     gray!250.49     gray!250.75

DisSim     11.91     4.09     0.76     19.10      51.96              39.33     0.54     0.84

Copy512                         16.55              2.08             13.30            2.39         76.42              61.51     0.51             0.78

Automatic evaluation results. We report evaluation scores for a number of basic statistics. These include the average sentence length of the simplified sentences in terms of the average number of tokens per output sentence (#T/S) and the average number of simplified output sentences per complex input (#S/C). Furthermore, in order to assess system conservatism, we measure the percentage of sentences that are copied from the source without performing any simplification operation (%SAME) and the averaged Levenshtein distance from the input (LDSC). Moreover, we calculate the average BLEU and SARI scores for the rephrasings of each system, as well as their SAMSA and SAMSAabl scores, which are the first metrics targeting syntactic aspects of TS. The highest score by each evaluation criterion is shown in bold.

With regard to SARI, our DisSim framework (35.05) again outperforms the baseline systems.[Note that SARI is a lexical simplicity metric that measures the goodness of words that are added, deleted and kept in the simplified output generated by the TS systems [94]. As detailed in the appendix, regarding SARI, splitting-only approaches have a natural advantage over systems that perform lexical simplification operations, as well as simple syntactic transformations in the form of deletions. This explains DisSim's comparatively high SARI score.]
However, it is among the systems with the lowest BLEU score (63.03).
Though, [87] [87] recently demonstrated that BLEU is ill suited for the evaluation of TS approaches when sentence splitting is involved, since in this case
it negatively correlates with structural simplicity, thus penalizing sentences that present a simplified syntax. Moreover, they showed that the BLEU score presents no correlation with the grammaticality and meaning preservation dimensions. For this reason, we only report these scores for the sake of completeness and to match past work.

According to [88] [88], the recently proposed SAMSA and SAMSAabl metrics are better suited for the evaluation of the sentence splitting task. With a score of 0.67, the DisSim framework shows the best performance for SAMSA, while its score of 0.84 for SAMSAabl is just below the one obtained by the RegenT system (0.85). As reported in [88] [88], SAMSA highly correlates with human judgments for structural simplicity and grammaticality, while SAMSAabl achieves the highest correlation for meaning preservation.

The results on the Newsela dataset, depicted in the middle part of Table <ref>, support our findings on the WikiLarge corpus, indicating that our TS approach can be applied in a domain independent manner.
The lower part of Table <ref> illustrates the numbers achieved on the WikiSplit dataset. Though the Copy512 system beats our approach in terms of BLEU and SARI, the remaining scores are clearly in favour of the DisSim system. With an average length of 11.91 tokens per simplified sentence, it reduces the source sentences to about one third of its original length. The tendency to perform a large amount of splitting operations is underlined by both the high number of simple sentences per complex input (4.09) and the large Levenshtein distance from the source (19.10), as well as the low percentage of unmodified input sentences (0.76%). The SAMSA (0.54) and SAMSAabl scores (0.84) convey further evidence that our approach succeeds in producing structurally simplified sentences that are grammatically sound and preserve the original meaning of the input.

Manual analyses
In the following, we report the results of the human judgments and the manual qualitative analysis of the transformation patterns.

G     M     S     average

5cWikiLarge test set
gray!25Simple reference     gray!254.70     gray!254.56     gray!25-0.2     gray!253.02

DisSim     4.36     4.50     1.30     3.39

DSS     3.44     3.68     0.06     2.39

SENTS     3.48     2.70     -0.18     2.00

Hybrid     3.16     2.60     0.86     2.21

YATS     4.40     4.60     0.22     3.07

RegenT     4.64     4.56     0.28     3.16
5cNewsela test set
gray!25Simple reference     gray!254.92     gray!252.94     gray!250.46     gray!252.77

DisSim     4.44     4.60     1.38     3.47

Hybrid     2.97     2.35     0.93     2.08

YATS     4.26     4.42     0.32     3.00

RegenT     4.54     4.70     0.62     3.29
5cWikiSplit test set
gray!25Simple reference     gray!254.72     gray!254.32     gray!250.44     gray!253.16

DisSim     4.36     4.36     1.66     3.46

Copy512     4.72     4.72     0.92     3.45

Human evaluation ratings on a random sample of 50 sentences from each dataset. Grammaticality (G) and meaning preservation (M) are measured using a 1 to 5 scale. A -2 to 2 scale is used for scoring the structural simplicity (S) of the output relative to the input sentence. The last column (average) presents the average score obtained by each system with regard to all three dimensions. The highest score by each evaluation criterion is shown in bold.

Human judgments

The results of the human evaluation are displayed in Table <ref>.
The inter-annotator agreement was calculated using Cohen's $\kappa$ [20], resulting in rates of 0.72 (G), 0.74 (M) and 0.60 (S).
The assigned scores demonstrate that our DisSim approach outperforms all other TS systems in the simplicity dimension (S). With a score of 1.30 on the WikiLarge sample sentences, it is far ahead of the baseline approaches, with Hybrid (0.86) coming closest. However, this system receives the lowest scores for grammaticality (G) and meaning preservation (M). RegenT obtains the highest score for G (4.64), while YATS is the best-performing approach in terms of M (4.60). With a rate of only 0.22, though, it achieves a low score for S, indicating that the high score in the M dimension is due to the conservative approach taken by YATS, resulting in only a small number of simplification operations. This explanation also holds true for RegenT's high mark for G. Still, our DisSim approach follows closely, with a score of 4.50 for M and 4.36 for G, suggesting that it obtains its goal of returning fine-grained simplified sentences that achieve a high level of grammaticality and preserve the meaning of the input. Considering the average scores of all systems under consideration, our approach is the best-performing system (3.39), followed by RegenT (3.16). The human evaluation ratings on the Newsela and WikiSplit sentences show similar results, again supporting the domain independence of our proposed approach.

Qualitative analysis of the transformation rule patterns

Table <ref> shows the results of the recall-based qualitative analysis of the transformation patterns. With an average rate of more than 80% for the syntactic phenomena under consideration, the overall ratio of the simplification rules being fired whenever the corresponding clausal or phrasal component is present in the given input sentence is very high. However, the patterns for the extraction of coordinate noun phrases, as well as prepositional phrases exhibit a much lower recall ratio. Regarding the former, the main reason is that we follow a very conservative approach when splitting lists of noun phrases, since this type of syntactic construct is prone to parsing errors and hard to distinguish from appositions. When decomposing prepositional phrases, we are confronted with the complex problem of resolving attachment ambiguities that is pervasive when dealing with this kind of phrasal element [34, 7].

frequency     %fired     %correct transformations

4c Clausal disembedding
Coordinate clauses     113     93.8%     99.1%

Adverbial clauses     113     84.1%     96.8%

Relative clauses (non-defining)     108     88.9%     70.8%

Relative clauses (defining)     103     86.4%     75.3%

Reported speech     112     82.1%     75.0%
4c Phrasal disembedding
Coordinate verb phrases     109     85.3%     89.2%

Coordinate noun phrases     115     48.7%     82.1%

Participial phrases     111     76.6%     72.9%

Appositions (non-restrictive)     107     86.0%     83.7%

Appositions (restrictive)     122     87.7%     72.0%

Prepositional phrases     163     68.1%     75.7%
Total     1276     80.7%     81.1%

Recall-based qualitative analysis of the transformation rule patterns. This table presents the results of a manual analysis of the performance of the hand-crafted simplification patterns. The first column lists the syntactic phenomena under consideration, the second column indicates its frequency in the dataset, the third column displays the percentage of the grammar fired, and the fourth column reveals the percentage of sentences where the transformation operation results in a correct split.

With respect to the accuracy of the specified transformation patterns in terms of the percentage of input sentences that were correctly split into syntactically simplified output sentences, the rules for disembedding coordinate and adverbial clauses perform remarkably well, approaching an accuracy rate of almost 100%. On average, correct transformations are carried out in over 80% of the cases. The syntactic construct that provides the lowest accuracy are non-restrictive relative clauses, which are prone to missing some essential part from the source sentence or assigning the wrong attachment phrase to the extracted clausal component, as revealed by the error analysis.

Error 1     Error 2     Error 3     Error 4     Error 5     Error 6

7c Clausal disembedding
Coordinate clauses     1     0     0     0     0     0

Adverbial clauses     1     1     0     1     0     0

Relative clauses (non-defining)      5     8     0     0     14     1

Relative clauses (defining)      8     8     2     0     5     1

Reported speech     5     1     13     1     2     1
7c Phrasal disembedding
Coordinate verb phrases     4     3     2     1     0     0

Coordinate noun phrases     3     3     0     3     1     0

Participial phrases     2     2     4     2     13     0

Appositions (non-restrictive)     0     5     3     0     7     0

Appositions (restrictive)      1     21     3     0     0     0

Prepositional phrases     3     11     4     6     4     0
Total     33      63      31      14     46     3

(17%)     (33%)     (16%)     (7%)     (24%)     (2%)

Results of the error analysis. Six types of errors were identified (Error 1: additional parts; Error 2: missing parts; Error 3: morphological errors; Error 4: wrong split point; Error 5: wrong referent; Error 6: wrong order of the syntactic elements).

Error analysis

Representing about 60% of the erroneous simplifications, missing elements of the input when constructing the simplified sentences and allocating the wrong referent are the most frequently experienced problems that were identified in the qualitative analysis of the performance of the hand-crafted simplification rules. In total, six types of errors were identified, as detailed in Table <ref>. With the help of an example sentence, each error class is illustrated in the appendix.

Type     Error     Input      Output

1     additional parts     The largest populations of Mennonites are in the Democratic Republic of Congo and the United States.      The largest populations of Mennonites are in the Democratic Republic of Congo. The largest populations of Mennonites are in the Democratic Republic of the United States.
2     missing parts     Chun was later pardoned by Kim Young-sam on the advice of then President-elect Kim Dae-jung.     Chun was later pardoned by Kim Young-sam on the advice of Kim Dae-jung. Kim Dae-jung was President-elect.
3     morphological errors     Mars has two Moons, Phobos and Deimos.     Mars has two Moons. Phobos and Deimos is two moons.
4     wrong split point     Peter Ermakov killed her with a gun shot to the left side of her head.      Peter Ermakov killed her. This was to the left side of her head. This was with a gun shot.

5      wrong referent     Pushkin and his wife Natalya Goncharova, whom he married in 1831, later became regulars of the court society.      Pushkin and his wife Natalya Goncharova later became regulars of the court society. He married Pushkin and his wife Natalya Goncharova in 1831.
6      wrong order of the syntactic elements     She is the adoptive mother of actor Dylan McDermott, whom she adopted when he was 18.     She is the adoptive mother of actor Dylan McDermott. Dylan McDermott she adopted when he was 18.

Error classification.

To give a clearer picture of the source of the errors, we complemented the analysis described above with a tracing where the errors start. We found that they can be attributed to two causes: either the parse tree constructed by the constituency parser is erroneous or the simplification rules we specified fail to capture slight nuances or variations of the input presenting a rarely occurring sentence structure, as illustrated in the following examples:

* “Chun was later pardoned by President Kim Young-sam on the advice of then President-elect Kim Dae-jung.”: Here, the adverb modifier “then” is missed when decomposing the appositive phrase “then President-elect”.

* “Mars has two Moons, Phobos and Deimos.”: Here, the appositive phrase “Phobos and Deimos” is extracted and transformed into a stand-alone sentence. However, the plural form is not recognized by our approach, resulting in a simplified sentence with an incorrectly inflected verb (“is” instead of “are”).

* “She is the adoptive mother of actor Dylan McDermott, whom she adopted when he was 18.”: The simplified sentence resulting from the given relative clause presents an unusual arrangement of its syntactic elements. This is due to the fact that the specified transformation rules capture only simple pronoun-subject-verb constructs in the case of relative clauses that are introduced by the pronoun “whom”.

As Table <ref> shows, morphological errors and wrong orderings of the syntactic elements in the simplified output mostly appear due to particular structures that are not covered by the specified transformation patterns. In contrast, additional parts in the simplified sentences, wrong split points and wrong referents can typically be traced back to erroneous parses:

* “The largest populations of Mennonites are in the Democratic Republic of Congo and the United States.”: In this example, the noun phrase “the United States” is mistakenly attributed as a prepositional phrase referring to “the Democratic Republic” due to an erroneous parse tree, leading to a simplified sentence that incorrectly includes an additional noun phrase.

* “Peter Ermakov killed her with a gun shot to the left side of her head.”: This input is split at the wrong point due to an erroneous parse tree that mistakenly attributes the prepositional phrase “to the left side of her head” to the pronoun “her” instead of the noun phrase “a gun shot” to which it actually refers. Therefore, it is transformed into a separate simplified sentence that lacks coherence with the surrounding simplified sentences.

* “Pushkin and his wife Natalya Goncharova, whom he married in 1831, later became regulars of the court society.”: Here, the parser fails to identify the correct referent of the relative clause (“Natalya Goncharova”). Instead, it mistakenly recognizes the full coordinate noun phrase of the superordinate clause as its antecedent.

Errors that can be attributed to erroneous parse trees will diminish as the performance of the underlying parser increases. Thus, more attention should be paid to errors caused by underspecified simplification rules. According to Table <ref>, there is the most potential for improvement with respect to reported speech and restrictive appositive phrases.

Error 1     Error 2     Error 3     Error 4     Error 5     Error 6     Total

8c Clausal disembedding
Coordinate clauses     0 - 1     —     —     —     —     —     0.00

Adverbial clauses     0 - 1     0 - 1     —     1 - 0     —     —     0.33

Relative clauses (non-def.)      4 - 1     1 - 7     —     —     8 - 6     0 - 1     0.46

Relative clauses (def.)      8 - 0     8 - 0     1 - 1     —     5 - 0     0 - 1     0.92

Reported speech     2 - 3     0 - 1     0 - 13     1 - 0     2 - 0     1 - 0     0.26
8c Phrasal disembedding
Coordinate verb phrases     3 - 1     0 - 3     0 - 2     1 - 0     —     —     0.40

Coordinate noun phrases     3 - 0     0 - 3     —     2 - 1     0 - 1     —     0.50

Participial phrases     0 - 2     0 - 2     4 - 0     0 - 2     9 - 4     —     0.57

Appositions (non-restr.)     —     5 - 0     1 - 2     —     5 - 2     —     0.73

Appositions (restr.)      0 - 1     7 - 14     0 - 3     —     —     —     0.28

Prepositional phrases     0 - 3     7 - 4     0 - 4     4 - 2     2 - 2     —     0.46
Total     0.61     0.44     0.19     0.64     0.67     0.33     0.50

Total number of errors that can be attributed to wrong constituency parses (first number) compared to rules not covering the respective sentence's structure (second number) (Error 1: additional parts; Error 2: missing parts; Error 3: morphological errors; Error 4: wrong split point; Error 5: wrong referent; Error 6: wrong order of the syntactic elements). The last row and column (“Total”) indicate the percentage of errors that can be attributed to erroneous constituency parse trees.

Type     Rule     WikiLarge     Newsela     WikiSplit     Total

Coordinate clauses     Rule #1
38     123     1388     1549 (8.77%)
6Adverbial clauses     Rule #2
40     123     832     995 (5.63%)
Rule #3
6     59     189     254 (1.44%)
Rule #4
10     49     221     280 (1.59%)
Rule #5
0     6     8     14 (0.08%)
Rule #6
0     0     3     3 (0.02%)
Rule #7
0     0     2     2 (0.01%)
5Relative clauses (non-restrictive)     Rule #8
1     2     33     36 (0.20%)
Rule #9
6     16     127     149 (0.84%)
Rule #10
1     0     6     7 (0.04%)
Rule #11
0     0     15     15 (0.08%)
Rule #12
10     55     377     442 (2.50%)

4Relative clauses (restrictive)     Rule #13
0     0     0     0 (0.00%)
Rule #14
0     0     17     17 (0.10%)
Rule #15
24     121     612     757 (4.29%)
Rule #16
7     35     254     296 (1.68%)

4Reported speech     Rule #17
2     33     38     73 (0.41%)
Rule #18
0     4     0     4 (0.02%)
Rule #19
0     22     0     22 (0.12%)
Rule #20
3     99     140     242 (1.37%)

Coordinate verb phrases     Rule #21
38     107     1268     1413 (8.00%)
2Coordinate noun phrases     Rule #22
51     81     959     1091 (6.18%)
Rule #23
7     32     168     207 (1.17%)
4Participial phrases     Rule #24
8     11     266     285 (1.61%)
Rule #25
40     76     652     768 (4.35%)
Rule #26
23     38     405     466 (2.64%)
Rule #27
4     7     156     167 (0.95%)
Appositions (non-restrictive)     Rule #28
38     147     844     1029 (5.83%)
Appositions (restrictive)     Rule #29
32     66     863     961 (5.44%)
3Prepositional phrases     Rule #30
117     421     2965     3503 (19.83%)
Rule #31
65     152     1314     1531 (8.67%)
Rule #32
21     66     490     570 (3.23%)
2Adjectival/adverbial phrases     Rule #33
17     46     194     257 (1.46%)
Rule #34
14     22     212     248 (1.40%)
Lead noun phrases     Rule #35
0     0      2     2 (0.01%)

Total         623     2019     15020     17662

Rule application statistics.

Rule application statistics

In addition, we calculated a frequency distribution of the grammar rules that were triggered by our DisSim approach during the transformation of the sentences from WikiLarge, Newsela and WikiSplit. The results are displayed in Table <ref>. The ten most matched rules are shown in bold for each dataset. It becomes apparent that the transformation patterns that are applied the most often are the same for all three corpora, with the notable exception of the SubordinationPostAttributionExtractor rule, which is among the top ten rules for Newsela, but not for the other two datasets. This pattern encodes a form of reported speech. Since this type of speech is far more prevalent in newswire texts than in Wikipedia articles, this represents a result that was to be expected. Furthermore, the table shows that the ten most matched simplification rules (i.e. less than 30% of the 35 manually defined grammar rules) account for about 75% of the rule applications.

§.§.§ Subtask 2: Establishing a Semantic Hierarchy

In the following section, we describe the evaluation results with regard to the second subtask, whose goal is to set up a semantic hierarchy between the split sentences.

Constituency type classification

In 88.88% of the matched sentence pairs, the hierarchical relationship that was allocated between a pair of simplified sentences by our TS approach corresponds to the nuclearity status of the aligned EDUs from RST-DT, i.e. in case of a nucleus-nucleus relationship in RST-DT, both output sentences from DisSim are assigned to the same context layer, while in case of a nucleus-satellite relationship the sentence mapped to the nucleus EDU is allocated to the context layer cl, whereas the sentence mapped to the satellite span is assigned to the subordinate context layer cl+1.
For comparison, PAR-s [42], a state-of-the-art sentence-level discourse parser, reaches a precision of 75.2% on the RST-DT test set in the nuclearity labeling task when automatic discourse segmentation is used, i.e. when it is fed the output of its discourse segmenter. Thus, the performance of our approach with respect to the constituency type classification outperforms current intra-sentential discourse parsers.

Source     2p10cmWhile lawyers arranged individual tie-ups before, the formal network of court reporters should make things easier and cheaper.

Mapped Span 1     Mapped Span 2

DisSim     Lawyers arranged individual tie-ups.      The formal network of court reporters should make things easier and cheaper.

Nuclearity     gray!25context layer 0     gray!25context layer 1

RST-DT     While lawyers arranged individual tie-ups before,     the formal network of court reporters should make things easier and cheaper.

Nuclearity     gray!25nucleus     gray!25satellite

Constituency type classification example.

The majority of the cases where our TS approach assigns a hierarchical relationship that differs from the nuclearity in the RST-DT corpus can be attributed to relative clauses.
In RST-DT, given a sentence that contains a relative clause, the EDUs are typically assigned a nucleus-nucleus relationship. In contrast, we regard the information contained in relative clauses as background information that further describes the entity to which it refers.
Therefore, we classify a simplified sentence that originates from such a type of clause as a contextual sentence that contributes additional information about its referent contained in the superordinate clause.

Source     2p10cmThe consolidated firm, which would rank among the 10 largest in Texas, would operate under the name Jackson & Walker.

Mapped Span 1     Mapped Span 2

DisSim     The consolidated firm would operate under Jackson & Walker.     The consolidated firm would rank among the 10 largest in Texas.

Nuclearity     gray!25context layer 0     gray!25context layer 1

RST-DT     would operate under the name Jackson & Walker.     The consolidated firm, which would rank among the 10 largest in Texas,

Nuclearity     gray!25nucleus     gray!25nucleus

Source     2p10cmCiba Corning, which had been a 50-50 venture between Basel-based Ciba-Geigy and Corning, has annual sales of about $300 million, the announcement said.

Mapped Span 1     Mapped Span 2

DisSim     Ciba Corning has annual sales of about $ 300 million.     Ciba Corning had been a 50-50 venture between Basel-based Ciba-Geigy and Corning.

Nuclearity     gray!25context layer 0     gray!25context layer 1

RST-DT     has annual sales of about $300 million,     Ciba Corning, which had been a 50-50 venture between Basel-based Ciba-Geigy and Corning,

Nuclearity     gray!25nucleus     gray!25nucleus

Examples of constituency type classifications of sentences that contain a relative clause.

Rhetorical relation identification

Table <ref> displays the frequency distribution of the 19 classes of rhetorical relations that were specified in [91] [91] over the RST-DT corpus. The ten most frequently occurring classes make up for 89.45% of the relations that are present in the dataset. We decided to limit ourselves to these classes in the evaluation of the rhetorical relation identification step, with two exceptions.

Source     2p10cmMr. Volk, 55 years old, succeeds Duncan Dwight, who retired in September.
Mapped Span 1     Mapped Span 2
DisSim     Volk succeeds Duncan Dwight.     Duncan Dwight retired in September.
RST-DT     Mr. Volk, 55 years old, succeeds Duncan Dwight,     who retired in September.
Rhet. rel.     2cgray!25Elaboration
Source     2p10cmThree seats currently are vacant and three others are likely to be filled within a few years, so patent lawyers and research-based industries are making a new push for specialists to be added to the court.
Mapped Span 1     Mapped Span 2
DisSim     Three seats are vacant.     Three others are likely to be filled within a few years.
RST-DT     Three seats currently are vacant     and three others are likely to be filled within a few years,
Rhet. rel.     2cgray!25Joint
Source     2p10cmMr. Carpenter notes that these types of investors also are `sophisticated” enough not to complain about Kidder's aggressive use of program trading.
Mapped Span 1     Mapped Span 2
DisSim     These types of investors are “sophisticated” enough not to complain about Kidder's aggressive use of program trading.      This is what Carpenter notes.
RST-DT     that these types of investors also are “sophisticated” enough not to complain about Kidder's aggressive use of program trading.      Mr. Carpenter notes
Rhet. rel.     2cgray!25Attribution

Result of the rhetorical relation identification step on example sentences.

First, we did not take into account two of the classes included in this set, namely “Topic-change” and “Same-unit”. The former encompasses relations that connect large sections of text when there is an abrupt change between topics [74]. Accordingly, this type of relation is relevant when examining larger paragraphs, but of no importance when considering intra-sentential relationships only, as is the case in our approach. The latter is not a true coherence relation. In RST-DT, it is used to link parts of units separated by embedded units
or spans [91].[For example, in the sentence “The petite, 29-year-old Ms. Johnson, dressed in jeans and a sweatshirt, is a claims adjuster with Aetna Life Camp Casualty.”, a “Same-unit” relation holds between the two underlined text spans [44].] Second, similar to [10] [10], we merged the two highly related classes of “Cause” and “Explanation” into a single category, since they both indicate a causal relationship. Consequently, we ended up with a set of seven rhetorical relations, aggregating 75.74% of the rhetorical relations included in RST-DT.

Rhetorical Relation     Count     Percentage     Precision
Elaboration     6,879     25.9%     0.6656
Joint     6,186     23.3%     0.6678
Attribution     2,641     9.9%     0.9452
Same-unit     2,530     9.5%      —
Contrast     1,312     4.9%     0.7342
Topic-change     1,269     4.8%     —
Explanation     855     3.2%     2*0.7407
Cause     660     2.5%
Temporal     837     3.2%     0.5789
Background     786     3.0%     0.4918
Evaluation     518     2.0%
Enablement     500     1.9%     (0.5766)
Comparison     394     1.5%
Textual organization     344     1.3%
Condition     269     1.0%     (0.7429)
Topic-comment     205     0.8%
Manner-means     191     0.7%
Summary     188     0.7%
Span     1     0.00%
$\sum$ 26,564     $\sum$ 100%     avg.: 0.6892

Frequency distribution of the 19 classes of rhetorical relations that are distinguished in [91] over the RST-DT corpus (left), and the precision of DisSim's rhetorical relation identification step (right).

Note that the Spatial relation is not included in the RST-DT gold annotations. Therefore, it is not possible to include an analysis of the performance of our approach with respect to this type of rhetorical relation.

Rhetorical Relation     Count     Percentage     Precision

Elaboration     7,675     25.65%     0.5550

Joint     7,116     23.78%     0.6673

Attribution     2,984     9.97%     0.9601

Same-unit     2,788     9.32%      —

Contrast     1,522     5.09%     0.7421

Topic-change     1,315     4.39%     —

Explanation     966     3.21%

Cause     754     2.52%     -2*0.7037

Temporal     964     3.22%     0.7895

Background     897     2.30%     0.4459

avg.: 0.6948

4p0.96Evaluation (2.0%), Enablement (1.8%), Comparison (1.5%), Textual organization (1.2%), Condition (1.1%), Topic-comment (0.9%), Manner-means (0.7%), Summary (0.7%), Span (0.0%). Note that the Spatial relation is not included in the RST-DT gold annotations. Therefore, it is not possible to include an analysis of the performance of our approach with respect to this type of rhetorical relation.

Frequency distribution of the 19 classes of rhetorical
relations from that are distinguished in [91] [91] over the RST-DT corpus,
and the precision of DisSim's rhetorical relation identification step.

The right column in Table <ref> displays the precision of our TS approach for each class of rhetorical relation when run over the sentences from RST-DT.

With a score of 96.01%, the “Attribution” relation reaches by far the highest precision. The remaining relations, too, show decent scores, with a precision of around 70%. The only exception is “Background”.
The difficulty with this type of relationship is that it signifies a very broad category that is not signalled by discourse markers and therefore hard to detect by our approach [91].

Source     3p11.5cmMerrill Lynch and Prudential-Bache Securities both lowered the stock's investment rating immediately after the results were issued Friday, according to PIR.
Span 1     Span 2      Rhet. Rel.
DisSim     Merrill Lynch lowered the stock's investment rating.     The results were issued Friday.     gray!25Temporal
RST-DT     Merrill Lynch and Prudential-Bache Securities both lowered the stock's investment rating     immediately after the results were issued Friday,     gray!25Background
Source     3p11.5cmMr. Downey's previous show, a one-hour shout fest, syndciated by MCA Inc. and produced by Quantum Media Inc., was canceled in July after advertisers and stations abandoned it.
Span 1     Span 2      Rhet. Rel.
DisSim     Downey's previous show syndciated by MCA Inc. and produced by Quantum Media Inc. was canceled in July.     Advertisers and stations abandoned it.     gray!25Temporal
RST-DT     Mr. Downey's previous show, a one-hour shout fest, syndciated by MCA Inc. and produced by Quantum Media Inc., was canceled in July     after advertisers and stations abandoned it.     gray!25Cause

Some examples of sentences where DisSim assigns a “Temporal” relation which differs from the ground truth rhetorical relation of RST-DT. Still, an inherent temporal relationship is present between the related text spans.

Source     3p11.5cmIn addition, economists are forecasting a slowdown in foreign direct investments as businessmen become increasingly wary of China's deteriorating political and economic environment.
Span 1     Span 2     Rhet. Rel.
DisSim     Economists are forecasting a slowdown in foreign direct investments.     Businessmen become increasingly wary of China's deteriorating political and economic environment.     gray!25Background
RST-DT     In addition, economists are forecasting a slowdown in foreign direct investments      as businessmen become increasingly wary of China's deteriorating political and economic environment.     gray!25Temporal
Source     3p11cmMeanwhile, analysts said Pfizer's recent string of lackluster quarterly performances continued, as earnings in the quarter were expected to decline by about 5%.
Span 1     Span 2     Rhet. Rel.
DisSim     Pfizer's recent string of lackluster quarterly performances continued.     Earnings in the quarter were expected to decline by about 5. %     gray!25Background
RST-DT     Pfizer's recent string of lackluster quarterly performances continued,     as earnings in the quarter were expected to decline by about 5%.     gray!25Elaboration

Examples of sentences where a “Background” relation is assigned by DisSim, differing from RST-DT's gold rhetorical relation. However, the background relationship is valid in those cases as well.

With an average precision of 68.5%,[The average precision refers to the scores of the selected subset of rhetorical relations only, i.e. the ones that are printed in bold in Table <ref>.] our approach surpasses PAR-s in the relation labeling task, which achieves a precision of 66.1% on the RST-DT corpus based on automatic discourse segmentation.[Note that PAR-s' precision score is computed based on the full set of 19 relation classes that are used for the annotation of the RST-DT. Consequently, its average precision score is not directly comparable to the score achieved by our approach, as they do not include exactly the same set of rhetorical relations. However, as the relations that are not considered in the evaluation of our approach make up for less than a quarter of the relations occurring in the RST-DT corpus, we assume that it still allows for a reliable approximation of the performance of the PAR-s baseline as compared to the approach we propose.]

§.§.§ Extrinsic Evaluation

In a comparative analysis, we demonstrated that the semantic hierarchy of minimal propositions benefits state-of-the-art Open IE systems in two dimensions:

(a) The canonical subject-predicate-object structure of the simplified sentences reduces the complexity of the relation extraction step, leading to an improved precision (up to 32%) and recall (up to 30%) of the extracted relational tuples on a large Open IE benchmark corpus [86].

(b) The semantic hierarchy
in terms of intra-sentential rhetorical structures and hierarchical relationships can be leveraged to extract relational tuples within their semantic context, resulting in more informative and coherent predicate-argument structures.

In that way, the shallow semantic representation of state-of-the-art Open IE systems is transformed into a canonical context-preserving representation of relational tuples, resulting in a novel standardized output scheme for complex text data. For details, see [65] [65].

We demonstrate that state-of-the-art Open IE systems may leverage the fine-grained representation of complex sentences in the form of hierarchically ordered and semantically interconnected propositions for

(a) facilitating the extraction of relational tuples (see Section <ref>, part 1) and

(b) enriching their output with semantic information (see Section <ref>, part 2). In this context, we introduce a novel context-preserving representation of relational tuples.

The results achieved in the automatic comparative evaluation conducted over a set of state-of-the-art Open IE systems demonstrate the effectiveness of using our proposed discourse-aware TS approach as a pre-processing step to improve the coverage (by up to 52%) and accuracy (by up to 346%) of the extracted relational tuples.[Note that this analysis was already briefly discussed in [64]. However, the work reported in this article provides additional detail, in particular Table <ref> and <ref>. [We also performed a similar analysis in [18]. However, while applying a customized function for matching the tuples extracted by an Open IE system with gold tuples, we instead use the default function that is provided by the benchmark framework in the work reported in this article.]] We conclude that by applying our framework as a pre-processing step, Open IE systems are capable of correcting erroneous extractions, as indicated by the improved overall precision. Moreover, the higher recall scores suggest that the split sentences substantially facilitate the extraction of the relations contained in the input.
We also point out to what extent the semantic hierarchy of minimal propositions generated by our discourse-aware TS approach can be leveraged to transform the shallow semantic representation of existing Open IE systems into a canonical context-preserving representation of relational tuples. We show that the proposed representation allows for a simplistic unified representation of predicate-argument structures that can easily be enriched with contextual information in terms of intra-sentential rhetorical structures and hierarchical relationships between the extracted tuples. Thus, we introduce an innovative representation for Open IE where the extracted relations are put into a logical structure that preserves the semantic context of the extractions, resulting in an output that is more informative and coherent.
In addition, we performed an in-depth qualitative analysis of the extracted predicate-argument relations to support and strengthen the findings of the preceding analyses (see Section <ref>, part 3). In this context, we analyze by hand the effects of applying our TS approach as a pre-processing step for the task of Open IE. The results show that in many cases the specific deficiencies of the Open IE systems are eliminated when taking advantage of the structurally simplified sentences.
Instead, a novel standardized output scheme is created where each simplified sentence results in a normalized (mostly) binary predicate-argument structure, in which both the predicate and the argument slots are reduced to their essential components.

§ CONCLUSION

In this work, we present a context-preserving sentence splitting approach that transforms syntactically complex sentences into a novel two-layered representation in the form of core sentences and accompanying contexts that are semantically linked by rhetorical relations. In that way, a semantic hierarchy of propositions is created, with each of them presenting a minimal semantic unit that cannot be further decomposed into meaningful utterances.

§.§ Contributions

The goal of our proposed discourse-aware TS approach is to break up sentences with a complex syntax into a set of minimal propositions by decomposing clausal and phrasal elements in a recursive top-down fashion. In that way, we generate a fine-grained intermediate representation that presents a canonical structure which is easier to process for downstream Open IE applications.

However, when splitting complex input sentences into a sequence of self-contained propositions without preserving the semantic relationship between the individual components, we end up with a set of incoherent utterances which lack important contextual information that is needed for a proper interpretation of the output. Hence, such isolated propositions easily mislead to incorrect reasoning in downstream Open IE applications. To address this challenge, we developed a novel contextual representation that puts a semantic layer on top of the simplified sentences.

The proposed framework was built upon a transformation stage where syntactically complex sentences are recursively transformed into a hierarchical representation of simplified core and contextual sentences. For this purpose, we defined a set of transformation rules that decompose clausal and phrasal elements from a given source sentence and turn them into stand-alone sentences that present a simplified structure. Moreover, distinguishing constructs that provide key information from those that contribute only some piece of background information, we set up a contextual hierarchy between the split components. Finally, we identify the rhetorical relations that connect them using a cue phrase mapping. In that way, we establish a semantic hierarchy of simplified sentences. The framework that we presented was evaluated within our reference implementation DisSim. To enable reproducible research, all code is provided online.

§.§ Summary of the Results

Both a thorough manual analysis and an automatic evaluation across three TS datasets from two different domains demonstrate that our proposed framework outperforms the state of the art in structural TS. In a comparative analysis, we demonstrate that our TS approach achieves the highest scores on all three simplification corpora with regard to SAMSA (0.67, 0.57, 0.54), a metric targeted at automatically measuring the syntactic complexity of sentences which highly correlates with human judgments on structural simplicity and grammaticality. In addition, it comes no later than a close second in terms of SAMSAabl (0.84, 0.84, 0.84), which shows a high correlation with human ratings on meaning preservation. These findings are supported by the other scores of the automatic evaluation, as well as the human annotation ratings, suggesting that our proposed approach succeeds in splitting complex input sentences into minimal semantic units, producing a sequence of output sentences that present a simpler and more regular structure that is easy to process.

Moreover, a detailed qualitative analysis of the 35 hand-crafted transformation patterns shows that syntactically complex sentences are split into well-formed simplified sentences in almost 80% of the cases under consideration on average. Together with the error analysis, these investigations allow for determining starting points for future research on how to improve the performance of our approach.
Furthermore, a comparative analysis with the annotations contained in the RST Discourse Treebank reveals that we are able to capture the contextual hierarchy between the split sentences with a precision of 89% and reach an average precision of 69% for the classification of the rhetorical relations that hold between them.