public static function getStandard()
    {
        if (self::$standard === null) {
            $cn = self::$standardCN;
            if (!class_exists($cn, true)) {
                throw new \LogicException('The standard FS implementation not installed');
            }
            if (!is_subclass_of($cn, self::$interface)) {
                throw new \LogicException('OMG, standard FS is fake');
            }
            self::$standard = new $cn;
        }
        return self::$standard;
    }