code: func (tx *Tx) SignBytes() ([]byte, error) {
  	bs, err := json.Marshal(tx)
  	if err != nil {
  		return nil, fmt.Errorf("could not generate canonical SignBytes for Payload %v: %v", tx.Payload, err)
  	}
  	return bs, nil
  }
docs: // Produces the canonical SignBytes (the Tx message that will be signed) for a Tx