code: func (twg *TimeoutWaitGroup) Done() {
  	twg.mutex.Lock()
  	defer twg.mutex.Unlock()
  	twg.workers--
  	if twg.workers < 0 {
  		panic("TimeoutWaitGroup: negative counter")
  	}
  	if twg.workers == 0 && twg.last != nil {
  		close(twg.last)
  	}
  }
docs: // Done decrements the wait group count by one.