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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 authenticatedUserForKey(self, key):
 """ Find a persistent session for a user. @type key: L{bytes} @param key: The persistent session identifier. @rtype: L{bytes} or C{None} @return: The avatar ID the session belongs to, or C{None} if no such session exists. """

        session = self.store.findFirst(
            PersistentSession, PersistentSession.sessionKey == key)
        if session is None:
            return None
        else:
            session.renew()
            return session.authenticatedAs