public static function topLevels(): SetInterface
    {
        if (self::$topLevels === null) {
            self::$topLevels = (new Set('string'))
                ->add('application')
                ->add('audio')
                ->add('font')
                ->add('example')
                ->add('image')
                ->add('message')
                ->add('model')
                ->add('multipart')
                ->add('text')
                ->add('video');
        }

        return self::$topLevels;
    }