code: function(on /*...rest */ ) {
  		var rest = gui.Array.from(arguments).slice(1);
  		if (on) {
  			return this.add.apply(this, rest);
  		} else {
  			return this.remove.apply(this, rest);
  		}
  	}
docs: Invokes `add` or `remove` according to first argument given.
  The remaining arguments are applied to the method we invoke.
  @param {boolean} on
  @returns {gui.Tracker}