(function() {
	var addListener = (function() {
		if (window.addEventListener) { return function(o, t, f) { o.addEventListener(t, f, false); }; }
		else if (window.attachEvent) { return function(o, t, f) { o.attachEvent('on' + t, f); }; }
		else { return function() { }; }
	} ());

	function setSrc(o, src) { return function() { o.src = src; }; }

	function setupImageRollover(img) {
		var imgSrc = img.src, overSrc;
		if (imgSrc) {
			overSrc = imgSrc.replace('up.', 'dn.');
			(new Image()).src = overSrc;
			addListener(img, 'mouseout', setSrc(img, imgSrc));
			addListener(img, 'mouseover', setSrc(img, overSrc));
		}
	}

	function load() {
		var imgs, tn = document.getElementById('topNav');
		if (tn && (imgs = tn.getElementsByTagName('img')) && imgs.length === 6) {
			setupImageRollover(imgs[1]);
			setupImageRollover(imgs[2]);
			setupImageRollover(imgs[3]);
			setupImageRollover(imgs[4]);
			setupImageRollover(imgs[5]);
		}
	}

	if (document.getElementById) {
		addListener(window, 'load', load);
	}
} ());