How can I get values from named controls?  I don't seem to have access
to anything on the page.  When I use this stylesheet, I always get the
following error:
System.ArgumentException: Last Trace Point=[]. Object required

<?xml version="1.0" encoding="utf-8" ?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/
Transform" xmlns:msxsl="urn:schemas-microsoft-com:xslt"
xmlns:user="urn:schemas-microsoft-com:xslt">
	<msxsl:script language="JavaScript" implements-prefix="user">
	<![CDATA[
						function GetCheckboxes(f)
						{
							var prodIds = ""
							for(var i = 0;i<f.chk.length;i++)
							{
								if(f.chk[i].checked)
								{
									prodIds = prodIds + "," + f.chk[i].value;
								}
							}

							return prodIds
						}
	            ]]>
</msxsl:script>
<xsl:template match="/">
	<div id="centerFull">
				<br/>
				<br/>

				<center>
					<input type="checkbox" name="chk" id="c1" value="val1" />
					<input type="checkbox" name="chk" id="c2" value="val2" />
					<input type="button" value="Compare"
onclick='window.open("compare.aspx?
productIDs={user:GetCheckboxes(this)}")'></input>
				</center>
	</div>
</xsl:template>
</xsl:stylesheet>