code: def sequential_id(self, client):
          
          if client.client_id not in self._sequential_ids:
              id_ = sequential_id("e:{0}:users".format(self.name), client.client_id)
              self._sequential_ids[client.client_id] = id_
          return self._sequential_ids[client.client_id]
docs: Return the sequential id for this test for the passed in client