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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 dir_tails_top(self, rr_id) -> str: """ Return top of tails tree for input rev reg id. :param rr_id: revocation registry identifier :return: top of tails tree """


        return join(self.dir_tails_hopper, rr_id) if self.external else self.dir_tails