public function collection(string $collection, ?string $format = null) : string {
        list($collection, $extension) = preg_split('/\.(css|js)/', $collection, 2, PREG_SPLIT_DELIM_CAPTURE);

        $group = $extension == 'css' ? 'stylesheets' : 'javascripts';

        return $this->serve($collection, $group, $format);
    }