public function restart_preview_button() {
        global $OUTPUT;
        if ($this->is_preview() && $this->is_preview_user()) {
            return $OUTPUT->single_button(new moodle_url(
                    $this->start_attempt_url(), array('forcenew' => true)),
                    get_string('startnewpreview', 'quiz'));
        } else {
            return '';
        }
    }