protected function addExtraNamespaces( $nodeset )
    {
        $this->addNamespace( 'hasImages', $nodeset, 'xmlns:image', "http://www.google.com/schemas/sitemap-image/1.1" );
        $this->addNamespace( 'hasVideos', $nodeset, 'xmlns:video', "http://www.google.com/schemas/sitemap-video/1.1" );
        $this->addNamespace( 'hasMobile', $nodeset, 'xmlns:mobile',
            "http://www.google.com/schemas/sitemap-mobile/1.0" );
        $this->addNamespace( 'hasNews', $nodeset, 'xmlns:news', "http://www.google.com/schemas/sitemap-news/0.9" );
    }