code: def service_define(self, service, ty):
          
  
          assert service not in self._data
          assert service not in self._algebs + self._states
  
          self._service.append(service)
          self._service_ty.append(ty)
docs: Add a service variable of type ``ty`` to this model
  
          :param str service: variable name
          :param type ty: variable type
          :return: None