protected function getUrls(array $config)
    {
        if (!isset($config['urls'])
            || !is_array($config['urls'])
            || array_filter($config['urls'], 'is_string') != $config['urls']) {
            throw new \DomainException(
                'urls must be a list of strings containing feed URLs',
                self::ERR_URLS_INVALID
            );
        }
        return $config['urls'];
    }