##
slider.visible()

    sl.visible(true | false | () )

Like all functions of this kind, the "visible()" method can be called in three ways: without a parameter, in which case it returns the actual visibility of the slider (true or false), or with one parameter: true or false, in which case it makes the slider visible or invisible. Toggling visibility can be done by

    sl.visible ( ! sl.visible() )

An invisible slider can be used to steer the application programmatically or by menu buttons.