Notification.java

public class Notification {
   String aspect;
   Object arg;
   Notification (String aspect) {
      this(aspect,null);
   }
	Notification (String aspect, Object arg) {
		this.aspect = aspect;
		this.arg = arg;
	}
	public String aspect() {return aspect;}
	public Object arg() {return arg;}
	public String toString() {
		if (arg == null) return aspect;
		return aspect+" "+arg;
	}
}