var myrules = {
// changes ul images to their respective mouse-overs - replaces the img.src
// ".gif" extension with ".over.gif" on mouse in, and vice versa
    'ul li a' : function(element) {
        element.onmouseover = function() {
            var imgs = this.getElementsByTagName("img");
            if(imgs.length > 0) imgs[0].src = imgs[0].src.replace(".gif",".over.gif");
        }

        element.onmouseout = function() {
            var imgs = this.getElementsByTagName("img");
            if(imgs.length > 0) imgs[0].src = imgs[0].src.replace(".over.gif",".gif");
        }
    }
};

Behaviour.register(myrules);