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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 sync(self):
 """ Access the Sync Twilio Domain :returns: Sync Twilio Domain :rtype: twilio.rest.sync.Sync """

        if self._sync is None:
            from twilio.rest.sync import Sync
            self._sync = Sync(self)
        return self._sync