模式对话框里怎样取得父窗体？

a.htm
-----------------------
<html>
<body>
<input type="button" onclick="showModalDialog('b.htm',window)" value="打开子窗体"></body>
</html>***************************b.htm
-----------------
aaaaaaaaaaaaaaaa
<input type="button" onclick="f()" value="操作父窗口">
<script>
  function f(){
     var parwin=window.dialogArguments;
 alert(parwin.document.body.innerHTML);
 parwin.document.body.innerHTML="ppppppppppppppp";
  }
</script>