Change CSS class with set

How can one change the "class" attribute using set?


I have a CSS embedded stylesheet with the following rule:
------
 .FillGreen {
    fill: green;
    }
 .FillOrange {
    fill: orange;
    }
-----

I have a button:
-------
<g id="button1" >
<use id="button1a" xlink:href="#tbutton1"  x="0" y="0"
	width="54" height="18" class="FillOrange" >

     <set id="SetMouseover" attributeName="class"  to="FillGreen"
        begin="button1a.mouseover" end="button1a.mouseout" />
</use>
</g>

-------

This doesn't work but when actually use the set element to modify the
"fill" attribute the desired affect occurs.

------
<set id="SetMouseover2" xlink:href="#button2a"
attributeName="fill"  to="green"
        begin="button2a.mouseover" end="button2a.mouseout" />
------




Thanks,
--Rick Anderson
"With that amount of fixing it really ought to be better."