code: func (w *window) add(win uint32) bool {
  	// a zero sized window adjust is a noop.
  	if win == 0 {
  		return true
  	}
  	w.L.Lock()
  	if w.win+win < win {
  		w.L.Unlock()
  		return false
  	}
  	w.win += win
  	// It is unusual that multiple goroutines would be attempting to reserve
  	// window space, but not guaranteed. Use broadcast to notify all waiters
  	// that additional window is available.
  	w.Broadcast()
  	w.L.Unlock()
  	return true
  }
docs: // add adds win to the amount of window available
  // for consumers.