public function __construct() {
    parent::__construct( JP_SITEMAP_MAX_ITEMS, JP_SITEMAP_MAX_BYTES, '1970-01-01 00:00:00' );

    $this->doc->appendChild(
      $this->doc->createComment( "generator='jetpack-" . JETPACK__VERSION . "'" )
    );

    $this->doc->appendChild(
      $this->doc->createProcessingInstruction(
        'xml-stylesheet',
        'type="text/xsl" href="' . $this->finder->construct_sitemap_url( 'sitemap-index.xsl' ) . '"'
      )
    );
  }