code: func (cb *Chequebook) Save() error {
  	data, err := json.MarshalIndent(cb, "", " ")
  	if err != nil {
  		return err
  	}
  	cb.log.Trace("Saving chequebook to disk", cb.path)
  
  	return ioutil.WriteFile(cb.path, data, os.ModePerm)
  }
docs: // Save persists the chequebook on disk, remembering balance, contract address and
  // cumulative amount of funds sent for each beneficiary.