add one response object to the list

def add(self, response, label=None):
        """add one response object to the list
        """
        if not isinstance(response, sip_response.sip_response):
            raise Exception(
                'can only add sip_reponse.sip_response objects'
            )
        self.objects.append(response)

        if label is None:
            self.labels.append('na')
        else:
            self.labels.append(label)