code: func (t *Transactor) isSpent(outputID string) (bool, error) {
  	key, err := createInputKey(outputID)
  	if err != nil {
  		return false, err
  	}
  	result, err := t.Ledger.GetState(tokenNameSpace, key)
  	if err != nil {
  		return false, err
  	}
  	if result == nil {
  		return false, nil
  	}
  	return true, nil
  }
docs: // isSpent checks whether an output token with identifier outputID has been spent.