层的滑动展开/折叠

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var act;
function over(s,nMax){
var obj=document.getElementById(s);
var h = parseInt(obj.offsetHeight);
if (h < nMax){
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clearTimeout(act);
act = setTimeout("over('"+s+"',"+nMax+")", 10);
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function out(s,nMin){
var obj=document.getElementById(s);
var h = parseInt(obj.offsetHeight);
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obj.style.height = (h - 2)+"px";
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act = setTimeout("out('"+s+"',"+nMin+")", 10);
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<div id="mytd" onmouseover="over('mytd',200);" onmouseout="out('mytd',30);" style="background:#eee;">层的滑动展开/折叠</div>