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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 get(self, uid: int) -> FrozenSet[Flag]: """Return the session flags for the mailbox session. Args: uid: The message UID value. """

        recent = _recent_set if uid in self._recent else frozenset()
        flags = self._flags.get(uid)
        return recent if flags is None else (flags | recent)