public function Metadata()
    {
        // variables
        $config = SiteConfig::current_site_config();
        // begin SEO
        $metadata = PHP_EOL . $this->owner->MarkupComment('SEO');
        // register extension update hook
        $this->owner->extend('updateMetadata', $config, $this->owner, $metadata);
        // end
        $metadata .= $this->owner->MarkupComment('END SEO');
        // return
        return $metadata;
    }