code: def _start(self, start_message):
          
          if start_message.contents.get(
              ACTION_STATUS_FIELD, None) != STARTED_STATUS:
              raise InvalidStartMessage.wrong_status(start_message)
          if start_message.task_level.level[-1] != 1:
              raise InvalidStartMessage.wrong_task_level(start_message)
          return self.set(start_message=start_message)
docs: Start this action given its start message.
  
          @param WrittenMessage start_message: A start message that has the
              same level as this action.
  
          @raise InvalidStartMessage: If C{start_message} does not have a
              C{ACTION_STATUS_FIELD} of C{STARTED_STATUS}, or if it has a
              C{task_level} indicating that it is not the first message of an
              action.