function opensubmenu() { $('ul.sub_menu', this).css('visibility', 'visible'); }
function closesubmenu() { $('ul.sub_menu', this).css('visibility', 'hidden' ); }
function addevent() {
$('.drop_down_menu li.toggle_sub_menu').hover( opensubmenu, closesubmenu); }
$(document).ready(addevent);