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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 sip(self):
 """ Access the sip :returns: twilio.rest.api.v2010.account.sip.SipList :rtype: twilio.rest.api.v2010.account.sip.SipList """

        if self._sip is None:
            self._sip = SipList(self._version, account_sid=self._solution['sid'], )
        return self._sip