public function boot()
{
   // …

   $this ->routes(function () {
      Route::middleware('api')
         ->prefix('api')
         ->group(base_path('routes/api.php'));

      Route::middleware('web')
         ->group(base_path('routes/web.php'));

      Route::middleware('is_admin')
         ->namespace($this ->namespace)
         ->group(base_path('routes/admin.php'));
    });
}
