public function export_to_xml($question, qformat_xml $format, $extra=null) {
        $expout = '';
        $expout .= $format->write_combined_feedback($question->options,
                                                    $question->id,
                                                    $question->contextid);
        $extraquestionfields = $this->extra_question_fields();
        array_shift($extraquestionfields);
        foreach ($extraquestionfields as $extra) {
            $expout .= "    <{$extra}>" . $question->options->$extra . "</{$extra}>\n";
        }
        return $expout;
    }