public function actionFlush($category = null)
    {
        if (is_null($category)) {
            $category = $this->prompt("Category (blank for all): ");
        }
        if (empty($category)) {
            $category = 'all';
        } else {
            $category = ['category', $category];
        }
        Cacher::invalidateGroup($category);
        $this->out("Done!");
    }