public function move_item_up_down($direction, $id) {
        $peers = $this->get_items_peers($id);
        $itemkey = array_search($id, $peers);
        switch ($direction) {
            case 'down' :
                if (isset($peers[$itemkey+1])) {
                    $olditem = $peers[$itemkey+1];
                    $peers[$itemkey+1] = $id;
                    $peers[$itemkey] = $olditem;
                } else {
                    print_error('listcantmoveup');
                }
                break;

            case 'up' :
                if (isset($peers[$itemkey-1])) {
                    $olditem = $peers[$itemkey-1];
                    $peers[$itemkey-1] = $id;
                    $peers[$itemkey] = $olditem;
                } else {
                    print_error('listcantmovedown');
                }
                break;
        }
        $this->reorder_peers($peers);
    }