code: func (cb *Chequebook) MarshalJSON() ([]byte, error) {
  	var file = &chequebookFile{
  		Balance:  cb.balance.String(),
  		Contract: cb.contractAddr.Hex(),
  		Owner:    cb.owner.Hex(),
  		Sent:     make(map[string]string),
  	}
  	for addr, sent := range cb.sent {
  		file.Sent[addr.Hex()] = sent.String()
  	}
  	return json.Marshal(file)
  }
docs: // MarshalJSON serialises a chequebook.