code: func (twg *TimeoutWaitGroup) Add(delta int) {
  	twg.mutex.Lock()
  	defer twg.mutex.Unlock()
  	if twg.workers == 0 {
  		// waitgroups are reusable once workers is 0
  		twg.last = nil
  	} else if twg.workers > 0 && twg.last != nil {
  		// This mimics normal WG behavior. Cannot add after Wait() has been
  		// called, unless workers is 0.  Prevents races on Wait()
  		panic("TimeoutWaitGroup misuse: Add called concurrently with Wait")
  	}
  	twg.workers += delta
  }
docs: // Add increments the wait group's count of active workers by delta.