lumen 里路由的 Route::group 怎么用？

http://lumen.laravel.com/docs/routing#route-groups

在 Namespaces 下有一段话：

$app->group(['namespace' => 'Admin'], function()
{
    // Controllers Within The "App\Http\Controllers\Admin" Namespace

    Route::group(['namespace' => 'User'], function()
    {
        // Controllers Within The "App\Http\Controllers\Admin\User" Namespace
    });
});

Remember, by default, the RouteServiceProvider includes your routes.php file within a namespace group, allowing you to register controller routes without specifying the full App\Http\Controllers namespace prefix. So, we only need to specify the portion of the namespace that comes after the base App\Http\Controllers namespace root.

其中 RouteServiceProvider 这个文件的内容怎么写？？？