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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 auth(self):
 """ Access the auth :returns: twilio.rest.api.v2010.account.sip.domain.auth_types.AuthTypesList :rtype: twilio.rest.api.v2010.account.sip.domain.auth_types.AuthTypesList """

        if self._auth is None:
            self._auth = AuthTypesList(
                self._version,
                account_sid=self._solution['account_sid'],
                domain_sid=self._solution['sid'],
            )
        return self._auth