code: def next(self):
          
          self._running_future = TracebackFuture()
  
          if self._finished:
              self._return_result(self._finished.popleft())
  
          return self._running_future
docs: Returns a `.Future` that will yield the next available result.
  
          Note that this `.Future` will not be the same object as any of
          the inputs.