function open_popup ( address, name ) {
        my_window = window.open( address, name, "width=450,height=550,left=250,top=200");
        my_window.focus();
    }

    function toggle_activation (element) {
        if (element.type == 'checkbox') {
          var groupname = element.value;
          if (document.getElementsByName (groupname)) {
            var group = document.getElementsByName (groupname);
            for (var i = 0; i < group.length; i++) {
                if ( element.checked ) {
                    group[i].disabled = true;
                    group[i].readOnly = true;
                    group[i].value = "--kein Student--";
                } else {
                    group[i].disabled = false;
                    group[i].readOnly = false;
                    group[i].value = "";
                }
            }
          }
        }
    }