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,
        }
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]]]