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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 sessions(self):
 """ Access the sessions :returns: twilio.rest.proxy.v1.service.session.SessionList :rtype: twilio.rest.proxy.v1.service.session.SessionList """

        if self._sessions is None:
            self._sessions = SessionList(self._version, service_sid=self._solution['sid'], )
        return self._sessions