code: func (ch *Cheque) Cash(session *contract.ChequebookSession) (string, error) {
  	v, r, s := sig2vrs(ch.Sig)
  	tx, err := session.Cash(ch.Beneficiary, ch.Amount, v, r, s)
  	if err != nil {
  		return "", err
  	}
  	return tx.Hash().Hex(), nil
  }
docs: // Cash cashes the cheque by sending an Ethereum transaction.