function switch_main3_layer(id)
{
    var maincell = document.getElementById("main");
    var layerlist = maincell.getElementsByTagName("div");
    var i = 0;
    for (i = 0; i < layerlist.length; i++)
    {
        layerlist[i].className = "clayer_hidden";
    }
    document.getElementById(id).className = "clayer_shown";
}