public function getPreviousFeed($feed, $className = null)
    {
        $previousLink = $feed->getPreviousLink();
        if (!$previousLink) {
            return null;
        }
        $previousLinkHref = $previousLink->getHref();

        if ($className === null) {
            $className = get_class($feed);
        }

        return $this->getFeed($previousLinkHref, $className);
    }