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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 registrations(self):
 """ Access the registrations :returns: twilio.rest.api.v2010.account.sip.domain.auth_types.auth_registrations_mapping.AuthTypeRegistrationsList :rtype: twilio.rest.api.v2010.account.sip.domain.auth_types.auth_registrations_mapping.AuthTypeRegistrationsList """

        if self._registrations is None:
            self._registrations = AuthTypeRegistrationsList(
                self._version,
                account_sid=self._solution['account_sid'],
                domain_sid=self._solution['domain_sid'],
            )
        return self._registrations