In triangle \( \triangle ABC \), \( AB = AC \) and the measure of \( \angle BAC = 40^\circ \). Points \( D \) and \( E \) are on sides \( AB \) and \( AC \) respectively, such that \( \angle BDC = \angle BEC = 70^\circ \). Find the measure of \( \angle BDE \). Let's think step by step and output the final answer within \boxed{}.
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