光标定位，帮忙看看

<TEXTAREA NAME="txt" ROWS="10" COLS="100%"></TEXTAREA>
<SCRIPT LANGUAGE="JavaScript">
function test(obj,n)
{
document.getElementById("txt").value += "XXXXX()";
obj.focus(); 
var rng=document.selection.createRange(); 
rng.moveStart("character", n); 
rng.select(); 
document.getElementById("txt").focus();
}
</SCRIPT>
<input type="button" id="b" value="Set" onclick="test(txt,6)">