code: func (i *Inbox) Cash() (string, error) {
  	if i.cheque == nil {
  		return "", nil
  	}
  	txhash, err := i.cheque.Cash(i.session)
  	i.log.Trace("Cashing in chequebook cheque", "amount", i.cheque.Amount, "beneficiary", i.beneficiary)
  	i.cashed = i.cheque.Amount
  
  	return txhash, err
  }
docs: // Cash attempts to cash the current cheque.