function fetch_object(idname)
{
    var my_obj = document.getElementById(idname);
    return my_obj;
}

function toggle_nav(obj)
{
    for (i = 1; i<= 5; i++  )
    {
        var nav_img = fetch_object("nav_" + i);
        var sub_nav = fetch_object("sub_nav_" + i);
        var sub_nav_index =  fetch_object("sub_nav_0");
        sub_nav_index.style.display = "none";
        if (obj == i)   {
            sub_nav.style.display = "block";
        }
        else   {
            sub_nav.style.display = "none";
        }
    }
}