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/**
This function assumes a "fake" menu item as the parent of each sub-menu
*/
function mainMenuSlide() {
    $('li.menu-item-has-children .sub-menu').hide(); // hide all submenus
    $('li.menu-item-has-children .sub-menu li').each(function() { // 
       if($(this).hasClass('current_page_item')) { // if current page is a sub-menu page, don't hide the sub-menu
           $(this).parent('.sub-menu').show();
       }
    });
    $('li.menu-item-has-children > a').click(function(e) {
       e.preventDefault();
       $(this).siblings('.sub-menu').slideToggle();
    });
}