public static function &get_instance()
 {
     // Create class instance if required
     if (!isset(self::$instance)) {
         self::$instance = new SC_Comment_Guestbook();
     }
     // Return class instance
     return self::$instance;
 }
 public function shortcode_comment_guestbook($atts)
 {
     if (NULL == $this->shortcode) {
         require_once 'includes/sc_comment-guestbook.php';
         $this->shortcode = SC_Comment_Guestbook::get_instance();
     }
     return $this->shortcode->show_html($atts);
 }