表格里的展开和收回做法

<script language="Javascript">
function td_click(div_id,tdbz)
{
   var ggbz=document.all('ggbz')
    if (div_id.style.display=='block')　//如果展开的
      {
          div_id.style.display='none' ;   //设为隐藏
          tdbz.style.backgroundImage='url(../../images/tdjia.gif)';   //图片改为加号图片
         }
    else          //   否则就展开
      {
          div_id.style.display='block';
          tdbz.style.backgroundImage='url(../../images/tajian.gif)';
       }
}
</script>


 <td id="yonghuzong" class="listcell" colspan="2"
onclick="td_click(document.all('gonggong'),document.all('ggbz'))"  style="cursor: hand; height: 20px" valign="middle">