code: def _add_child(self, message):
          
          self._validate_message(message)
          level = message.task_level
          return self.transform(('_children', level), message)
docs: Return a new action with C{message} added as a child.
  
          Assumes C{message} is not an end message.
  
          @param message: Either a C{WrittenAction} or a C{WrittenMessage}.
  
          @raise WrongTask: If C{message} has a C{task_uuid} that differs from the
              action's C{task_uuid}.
          @raise WrongTaskLevel: If C{message} has a C{task_level} that means
              it's not a direct child.
  
          @return: A new C{WrittenAction}.