public function render_index( $sitemaps ) {
    header( 'Content-Type: application/xml; charset=UTF-8' );

    $this->check_for_simple_xml_availability();

    $index_xml = $this->get_sitemap_index_xml( $sitemaps );

    if ( ! empty( $index_xml ) ) {
      // All output is escaped within get_sitemap_index_xml().
      echo $index_xml;
    }
  }