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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 pexpire(self, ttl=None):
 """ Expire the given key in the given number of milliseconds. If ``ttl`` is ``None``, then any expiry will be cleared and key will be persisted. """

        if ttl is not None:
            self.database.pexpire(self.key, ttl)
        else:
            self.database.persist(self.key)