var allHTMLTags = new Array();

function changeElementByClass(oldClass, newClass) {
    var allHTMLTags = document.getElementsByTagName("*");
    for (i = 0; i < allHTMLTags.length; i++) {
        if (allHTMLTags[i].className == oldClass) {
            allHTMLTags[i].className = newClass;
        }
    }
}

function toggleSubnav(id) {
    var x = document.getElementById(id + "_navitems");
    changeElementByClass("show_subnav", "hide_subnav")
    x.className = "show_subnav";
}