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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 credentials_lists(self):
 """ Access the credentials_lists :returns: twilio.rest.trunking.v1.trunk.credential_list.CredentialListList :rtype: twilio.rest.trunking.v1.trunk.credential_list.CredentialListList """

        if self._credentials_lists is None:
            self._credentials_lists = CredentialListList(self._version, trunk_sid=self._solution['sid'], )
        return self._credentials_lists