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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 to_dict(self):
 """ Convert the tree node to its dictionary representation. :return: an expansion dictionary that represents the type and expansions of this tree node. :rtype dict[list[union[str, unicode]]] """

        expansion_strings = []

        for expansion in self.expansions:
            expansion_strings.extend(expansion.to_strings())

        return {
            self.type: expansion_strings,
        }