<oTopBar>:getPrev() --> nPosition

        Returns a numeric value that indicates the position within the top
        bar menu of the previous selectable item.  getPrev() returns 0 in the
        event that the current item is the first selectable item or the top
        bar menu does not contain a selectable item.

        getPrev() is a method of the TopBarMenu class that is used for
        determining the position of the previous selectable item in a top bar
        menu.  getPrev() searches for the previous selectable item starting
        at the item immediately before the current item.  The term selectable
        is defined as a menu item that is enabled and whose caption is not a
        menu separator.

        Note:  getPrev() does not change the currently selected menu item.
        In order to change the currently selected top bar menu item, you must
        call the TopBarMenu:select() method.