protected function _register($id, $className)
    {
        $id = (string) $id;
        if (class_exists($className)) {
            self::$_loaded[$id] = $className;
            $this[$id] = function () use ($className) {
                return new $className();
            };
        } else {
            throw new Exception("Helper \"{{$className}}\" not found!");
        }
    }