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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def trees_to_dict(trees_list):
 """ Convert a list of `TreeNode`s to an expansion dictionary. :param trees_list: A list of `TreeNode` instances :type trees_list: list[TreeNode] :return: An expansion dictionary that represents the expansions detailed in the provided expansions tree nodes :rtype: dict[union[str, unicode]] """

        result = {}

        for tree in trees_list:
            result.update(tree.to_dict())

        return result