public function dump_links($links = null)
    {
        if ($links === null) {
            $links = $this->links;
        }
        $dump = array();
        foreach ($links as $key => $link) {
            if (isset($link['url'])) {
                $dump[$key] = $link['url'];
            }
            if ($key === '*') {
                $dump = array_merge($dump, $this->dump_links($link));
            } else if (isset($link['links'])) {
                $dump[$key] = $this->dump_links($link['links']);
            }
        }

        return $dump;
    }