code: func onMessage(cm gcm.CcsMessage) error {
  	toylog.Infoln("Message, from:", cm.From, "with:", cm.Data)
  	// Echo the message with a tag.
  	cm.Data["echoed"] = true
  	m := gcm.HttpMessage{To: cm.From, Data: cm.Data}
  	r, err := gcm.SendHttp(*serverKey, m)
  	if err != nil {
  		toylog.Errorln("Error sending message.", err)
  		return err
  	}
  	toylog.Infof("Sent message. %+v -> %+v", m, r)
  	return nil
  }
docs: // onMessage receives messages, logs them, and echoes a response.