function removeClasses() 
{
    var li_array = document.getElementById("download_tab").getElementsByTagName("li");
    for (var i in li_array) 
    {
        li_array[i].className = "";
    }
}

function showDesc(elem, id) 
{
    removeClasses();
    elem.parentNode.className = "selected";

    var description = document.getElementById("description");

    fx = new Fx.Morph(description, {
        duration: 500
    });

    fx.start({
        'opacity': 0
    }).chain(function () {
        // executes immediately after completion of above effect
        description.innerHTML = document.getElementById("description" + id).innerHTML;
        this.start({
            'opacity': 1
        });
    });
}