code: public static void keepWithinStage (Actor actor) {
  		Stage stage = actor.getStage();
  		if (stage == null) {
  			throw new IllegalStateException("keepWithinStage cannot be used on Actor that doesn't belong to any stage. ");
  		}
  		keepWithinStage(actor.getStage(), actor);
  	}
docs: Makes sures that actor will be fully visible in stage. If it's necessary actor position will be changed to fit it
  on screen.
  @throws IllegalStateException if actor does not belong to any stage.