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

        if self._domains is None:
            self._domains = DomainList(self._version, account_sid=self._solution['account_sid'], )
        return self._domains