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