public function message($message, single_button $button = null) {
        $output  = $this->output->box_start('generalbox boxaligncenter');
        $output .= $message;
        if ($button !== null) {
            $output .= $this->output->box($this->output->render($button), 'lessonbutton standardbutton');
        }
        $output .= $this->output->box_end();
        return $output;
    }