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