code: def append(self, future):
          
          if future._ended() and future.index is None:
              self.inprogress.add(future)
          elif future._ended() and future.index is not None:
              self.ready.append(future)
          elif future.greenlet is not None:
              self.inprogress.add(future)
          else:
              self.movable.append(future)
  
              # Send the oldest future in the movable deque until under the hwm
              over_hwm = self.timelen(self.movable) > self.highwatermark
              while over_hwm and len(self.movable) > 1:
                  sending_future = self.movable.popleft()
                  if sending_future.id[0] != scoop.worker:
                      sending_future._delete()
                  self.socket.sendFuture(sending_future)
                  over_hwm = self.timelen(self.movable) > self.highwatermark
docs: Append a future to the queue.