function execute_demo()
 {
     global $config;
     $config_old = $config;
     foreach ($this->captcha_vars as $captcha_var => $template_var) {
         $config[$captcha_var] = request_var($captcha_var, (int) $config[$captcha_var]);
     }
     parent::execute_demo();
     $config = $config_old;
 }