def __add_callback(self, type_, func, serialised_if_crud=True):
        """sync_if_crud indicates whether to serialise this callback (applies only to CRUD)"""
        Validation.callable_check(func)
        with self.__callbacks:
            self.__callbacks[type_].append((func, serialised_if_crud))
sync_if_crud indicates whether to serialise this callback (applies only to CRUD)