code: def _on_trigger(self, my_task):
          
          self.queued += 1
          # All tasks that have already completed need to be put back to
          # READY.
          for thetask in my_task.workflow.task_tree:
              if thetask.thread_id != my_task.thread_id:
                  continue
              if (thetask.task_spec == self and
                      thetask._has_state(Task.COMPLETED)):
                  thetask._set_state(Task.FUTURE, True)
                  thetask._ready()
docs: Enqueue a trigger, such that this tasks triggers multiple times later
          when _on_complete() is called.