如何使用JS折叠菜单？

/* Prototype JavaScript framework
* (c) 2005 Sam Stephenson <sam@conio.net>
* Prototype is freely distributable under the terms of an MIT-style license.
* For details, see the Prototype web site: http://prototype.conio.net/
/*--------------------------------------------------------------------------*/

//note: modified & stripped down version of prototype, to be used with moo.fx by mad4milk (http://moofx.mad4milk.net).

var Class = {
create: function() {
return function() {
this.initialize.apply(this, arguments);
}
}
}

Object.extend = function(destination, source) {
for (property in source) destination[property] = source[property];
return destination;
}

Function.prototype.bind = function(object) {
var __method = this;
return function() {
return __method.apply(object, arguments);
}
}

Function.prototype.bindAsEventListener = function(object) {
var __method = this;
return function(event) {
__method.call(object, event || window.event);
}
}

function $() {
if (arguments.length == 1) return get$(arguments[0]);
var elements = [];
$c(arguments).each(function(el){
elements.push(get$(el));
});
return elements;

function get$(el){
if (typeof el == 'string') el = document.getElementById(el);
return el;
}
}

if (!window.Element) var Element = new Object();

Object.extend(Element, {
remove: function(element) {
element = $(element);
element.parentNode.removeChild(element);
},

hasClassName: function(element, className) {
element = $(element);
if (!element) return;
var hasClass = false;
element.className.split(' ').each(function(cn){
if (cn == className) hasClass = true;
});
return hasClass;
},

addClassName: function(element, className) {
element = $(element);
Element.removeClassName(element, className);
element.className += ' ' + className;
},

removeClassName: function(element, className) {
element = $(element);
if (!element) return;
var newClassName = '';
element.className.split(' ').each(function(cn, i){
if (cn != className){
if (i > 0) newClassName += ' ';
newClassName += cn;
}
});
element.className = newClassName;
},

cleanWhitespace: function(element) {
element = $(element);
$c(element.childNodes).each(function(node){
if (node.nodeType == 3 && !//S/.test(node.nodeValue)) Element.remove(node);
});
},

find: function(element, what) {
element = $(element)[what];
while (element.nodeType != 1) element = element[what];
return element;
}
});

var Position = {
cumulativeOffset: function(element) {
var valueT = 0, valueL = 0;
do {
valueT += element.offsetTop || 0;
valueL += element.offsetLeft || 0;
element = element.offsetParent;
} while (element);
return [valueL, valueT];
}
};

document.getElementsByClassName = function(className) {
var children = document.getElementsByTagName('*') || document.all;
var elements = [];
$c(children).each(function(child){
if (Element.hasClassName(child, className)) elements.push(child);
});
return elements;
}

//useful array functions
Array.prototype.iterate = function(func){
for(var i=0;i<this.length;i++) func(this[i], i);
}
if (!Array.prototype.each) Array.prototype.each = Array.prototype.iterate;

function $c(array){
var nArray = [];
for (var i=0;i<array.length;i++) nArray.push(array[i]);
return nArray;
}