code: func (p *Selection) MarshalJSON() ([]byte, error) {
  	conds, err := json.Marshal(p.Conditions)
  	if err != nil {
  		return nil, errors.Trace(err)
  	}
  	buffer := bytes.NewBufferString("{")
  	buffer.WriteString(fmt.Sprintf(""+
  		" \"condition\": %s,\n"+
  		" \"child\": \"%s\"\n}", conds, p.children[0].GetID()))
  	return buffer.Bytes(), nil
  }
docs: // MarshalJSON implements json.Marshaler interface.