code: func (r Response) MarshalJSON() ([]byte, error) {
  	if (r.Result == nil || len(*r.Result) == 0) && r.Error == nil {
  		return nil, errors.New("can't marshal *jsonrpc2.Response (must have result or error)")
  	}
  	type tmpType Response // avoid infinite MarshalJSON recursion
  	b, err := json.Marshal(tmpType(r))
  	if err != nil {
  		return nil, err
  	}
  	b = append(b[:len(b)-1], []byte(`,"jsonrpc":"2.0"}`)...)
  	return b, nil
  }
docs: // MarshalJSON implements json.Marshaler and adds the "jsonrpc":"2.0"
  // property.