code: func (sf *AuxFormatter) Emit(id string, aux interface{}) error {
  	auxJSONBytes, err := json.Marshal(aux)
  	if err != nil {
  		return err
  	}
  	auxJSON := new(json.RawMessage)
  	*auxJSON = auxJSONBytes
  	msgJSON, err := json.Marshal(&jsonmessage.JSONMessage{ID: id, Aux: auxJSON})
  	if err != nil {
  		return err
  	}
  	msgJSON = appendNewline(msgJSON)
  	n, err := sf.Writer.Write(msgJSON)
  	if n != len(msgJSON) {
  		return io.ErrShortWrite
  	}
  	return err
  }
docs: // Emit emits the given interface as an aux progress message