It is silly, but here's my XSLT that sums cost elements and replaces it with a total:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:template match="add">
<total><xsl:value-of select="sum(cost)"/></total>
</xsl:template>

<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>

</xsl:stylesheet>

Any non-add or cost element will be returned unchanged.

The element can exist in any context.