$router[] = new Route('<list [0-9,]+>/', array(
    ...,
    'list' => array(
        Route::FILTER_IN => function ($list) {
            return is_array($list) ? $list ? explode(',', $list);
        },
        Route::FILTER_OUT => function ($list) {
            return is_array($list) ? implode(',', $list) ? $list;
        },
    )
));