private static function registerSymfonyYaml()
    {
        if (
            ! isset(self::$classMap['yaml'])
            and ! isset(self::$classMap['yml'])
            and class_exists(self::SYMFONY_YAML)
        ) {
            self::registerExtension('yaml', self::YAML_ADAPTER);
            self::registerExtension('yml', self::YAML_ADAPTER);
        }
    }