그누 코드 중에, f.mb_birth 처럼 앞에 f가 붙는 게 있던데..

<form action="<?php echo $action_url ?>" onsubmit="return fwrite_submit(this);">        <tr>            <th scope="row"><label for="wr_subject">제목<strong class="sound_only">필수</strong></label></th>            <td>                <input type="text" name="wr_subject" value="<?php echo $subject ?>" id="wr_subject" required class="frm_input required" size="50" maxlength="255">            </td>        </tr>         </tbody>        </table>    </div>     <div class="btn_confirm">        <input type="submit" value="작성완료" id="btn_submit" accesskey="s" class="btn_submit">    </div>    </form>     <script>    function fwrite_submit(f)    {         if (!f.wr_subject.value) {            alert("제목을 입력해 주세요.");            f.wr_subject.focus();            return false;        }         return true;    }    </script>