javascript 多个层拖动效果

<style>
<!--
div {
background: #cccccc;
position: absolute;
width: 50;
height: 50;
}
-->
</style>
<div onmousedown="oMove(this)"></div>
<div onmousedown="oMove(this)"></div>
<div onmousedown="oMove(this)"></div>
<script>
function oMove(odiv)
{
var otop,oleft;
otop=event.y-odiv.offsetTop;
oleft=event.x-odiv.offsetLeft;
odiv.setCapture();
odiv.onmousemove=function()
{
  odiv.style.left=event.x-oleft;
  odiv.style.top=event.y-otop;
}
odiv.onmouseup=function()
{
  odiv.onmousemove=null;
  odiv.releaseCapture();
}
}
</script>