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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 _cleanSessions(self):
 """ Clean expired sesisons. """

        tooOld = extime.Time() - timedelta(seconds=PERSISTENT_SESSION_LIFETIME)
        self.store.query(
            PersistentSession,
            PersistentSession.lastUsed < tooOld).deleteFromStore()
        self._lastClean = self._clock.seconds()