code: def set(self):
          
          self._flag = True
          old_future = self._waiting_future
          # Replace _waiting_future with a fresh one incase someone woken up by set_result()
          # sets this AsyncEvent to False before waiting on it to be set again.
          self._waiting_future = tornado_Future()
          old_future.set_result(True)
docs: Set event flag to true and resolve future(s) returned by until_set()
  
          Notes
          -----
          A call to set() may result in control being transferred to
          done_callbacks attached to the future returned by until_set().