code: func SyncC128ProofOfWork(trytes Trytes, mwm int, parallelism ...int) (Trytes, error) {
  	syncC128ProofOfwork.Lock()
  	defer syncC128ProofOfwork.Unlock()
  	nonce, err := c128ProofOfWork(trytes, mwm, nil, parallelism...)
  	if err != nil {
  		return "", err
  	}
  	return nonce, nil
  }
docs: // SyncC128ProofOfWork is like C128ProofOfWork() but only runs one ongoing Proof-of-Work task at a time.