function nav_menu(e)
{
    var targetId, srcElement, targetElement; 

    srcElement = e.srcElement ? e.srcElement : e.target; 

    if (srcElement.className == "submenu") 
    { 
        targetId = srcElement.id + "ul"; 
        targetElement = document.getElementById(targetId); 

        if (targetElement.style.display == "none") 
            targetElement.style.display = ""; 
        else 
            targetElement.style.display = "none"; 
    }
}