msxsl:node-set
<div xmlns='http://www.w3.org/1999/xhtml'>
<xsl:variable name="stringArray">
<age>1</age>
<age>1</age>
<age>1</age>
</xsl:variable>
<br />
<xsl:value-of select="sum(msxsl:node-set($stringArray)/age)"/>
<br />
for detail see <a href="http://msdn2.microsoft.com/en-us/library/hz88kef0(vs.71).aspx">this</a>
</div>
<p>Here is an <a href="http://www.xml.com/pub/a/2003/07/16/nodeset.html">example</a> on how to use node-set</p>