public static function getInstance(
        $root_dir = null, $assets_dir = null, $document_root = null, $conflict_flag = self::PRESETS_CONFLICT
    ) {
        if (empty(self::$__instance)) {
            $cls = get_called_class();
            self::$__isStaticInstance = true;
            self::$__instance = new $cls($root_dir, $assets_dir, $document_root, $conflict_flag);
        }
        return self::$__instance;
    }