code: func (s *SequentialWaitGroup) Add(sequence int64, delta int) {
  	s.mutex.Lock()
  	defer s.mutex.Unlock()
  
  	_, ok := s.semaphores[sequence]
  	if ok {
  		s.semaphores[sequence] += delta
  	} else {
  		s.semaphores[sequence] = delta
  	}
  	if s.semaphores[sequence] <= 0 {
  		delete(s.semaphores, sequence)
  		s.change.Broadcast()
  	}
  }
docs: // Add adds the given delta to the waitgroup at the given sequence