protected function getNewFeedItems(array $new, array $old)
    {
        $map = array();
        $getKey = function($item) {
            return $item->getPermalink();
        };
        $logger = $this->getLogger();
        foreach ($new as $item) {
            $key = $getKey($item);
            $logger->debug('New: ' . $key);
            $map[$key] = $item;
        }
        foreach ($old as $item) {
            $key = $getKey($item);
            $logger->debug('Old: ' . $key);
            unset($map[$key]);
        }
        $logger->debug('Diff: ' . implode(' ', array_keys($map)));
        return array_values($map);
    }