关于菜单的问题

Question

<script language="JavaScript">
var objEle
function Menu_Clicked(obj)
{
if(objEle!=null)
objEle.nextSibling.style.display="none"
if(obj.nextSibling.style.display=='none')
{
obj.nextSibling.style.display='block';
}
else
{
obj.nextSibling.style.display='none';
}
objEle=obj;
}
</script>
<table>
  <tr onclick=Menu_Clicked(this)>
    <td>菜单1</td>
  </tr>
  <tr style="display:none">
    <td>
      <table>
        <tr>
          <td>子菜单一</td>
        </tr>
        <tr>
          <td>子菜单二</td>
        </tr>
</table>
    </td>
  </tr>
    <tr onclick=Menu_Clicked(this)>
    <td>菜单2</td>
  </tr>
  <tr style="display:none">
    <td>
      <table>
        <tr>
          <td>子菜单一</td>
        </tr>
        <tr>
          <td>子菜单二</td>
        </tr>
</table>
    </td>
  </tr>
    <tr onclick=Menu_Clicked(this)>
    <td>菜单3</td>
  </tr>
  <tr style="display:none">
    <td>
      <table>
        <tr>
          <td>子菜单一</td>
        </tr>
        <tr>
          <td>子菜单二</td>
        </tr>
</table>
    </td>
  </tr>
    <tr onclick=Menu_Clicked(this)>
    <td>菜单4</td>
  </tr>
  <tr style="display:none">
    <td>
      <table>
        <tr>
          <td>子菜单一</td>
        </tr>
        <tr>
          <td>子菜单二</td>
        </tr>
</table>
    </td>
  </tr>
</table>