code: func (v *Verifier) isSpent(spentKey string, simulator ledger.LedgerReader) (bool, error) {
  	verifierLogger.Debugf("checking if input with ID '%s' has been spent", spentKey)
  	result, err := simulator.GetState(tokenNameSpace, spentKey)
  	return result != nil, err
  }
docs: // isSpent checks whether an output token with identifier outputID has been spent.