function toggle(f, img) {
img_str = img;
img = document.all(img).src;
str_pos = img.lastIndexOf("/");
str_pos = parseInt(str_pos);
img_name = img.substring(str_pos + 1);
            if (img_name == "bt_closed.gif") {
                        document.all(img_str).src = "images/bt_open.gif";
                        document.all(f).className="shown";
            }
            else {
                        document.all(img_str).src = "images/bt_closed.gif";
                        document.all(f).className="hidden";
            }
}