code: func TailTransactionHash(bndl Bundle) Hash {
  	if bndl == nil || len(bndl) == 0 {
  		return ""
  	}
  	for i := range bndl {
  		tx := &bndl[i]
  		if tx.CurrentIndex != 0 {
  			continue
  		}
  		if len(tx.Hash) > 0 {
  			return tx.Hash
  		}
  		return transaction.TransactionHash(tx)
  	}
  	return ""
  }
docs: // TailTransactionHash returns the tail transaction's hash.