public function extract_pages(tpost $post, $s)
 {
     $post->deletepages();
     $pages = explode('<!--nextpage-->', $s);
     $firstpage = $this->filter(array_shift($pages));
     foreach ($pages as $page) {
         $page = trim($page);
         if ($page) {
             $post->addpage($this->filter($page));
         }
     }
     return $firstpage;
 }