$(window).addEvent("domready", function() {

 $$(".submenu").setStyles({
   display: "block",
   opacity: 0
 });
 
 $$("#leftmenu>li").addEvent("mouseenter",function() {
   this.getElement(".submenu").tween("opacity",1);
 });
 $$("#leftmenu>li").addEvent("mouseleave",function() {
   this.getElement(".submenu").tween("opacity",0);
 });
});