function Navitem_onclick(parent_id) {
    var li = document.getElementById('navitem_'+parent_id);
    if (Element.Methods.hasClassName(li, 'open')) {
        Element.Methods.removeClassName(li, 'open');
        Element.Methods.addClassName(li, 'closed');
    } else {
        Element.Methods.removeClassName(li, 'closed');
        Element.Methods.addClassName(li, 'open');
    }
}