docs: def yield_event(self, act):
          """
          Hande completion for a request and return an (op, coro) to be
          passed to the scheduler on the last completion loop of a proactor.
          """
          if act in self.tokens:
              coro = act.coro
              op = self.try_run_act(act, self.tokens[act])
              if op:
                  del self.tokens[act]
                  return op, coro