protected function getResponseFormat(array $config)
    {
        if (isset($config['responseFormat'])) {
            if (!is_string($config['responseFormat'])) {
                throw new \DomainException(
                    'responseFormat must reference a string',
                    self::ERR_INVALID_RESPONSEFORMAT
                );
            }
            return $config['responseFormat'];
        }
        return '[ %link% ] "%title%" by %author%'
            . '; Length %duration%'
            . '; Published %published%'
            . '; Views %views%'
            . '; Likes %likes%';
    }