XPath function after grouping

I would like to use XPath function like sum() or count() for groups I've
created with the Muenchian Method but I can't get it right to run a e.g.
sum() for all members of a sub-group.
Could someone give me a hint how to get it working?!

Thanks,
-Michael


<?xml version="1.0" encoding="UTF-8"?>
<Result>
	<Observation>
		<SiteNumber>1</SiteNumber>
		<Cond>
			<value>10.4</value>
		</Cond>
	</Observation>
	<Observation>
		<SiteNumber>130001</SiteNumber>
		<Cond>
			<value>10.2</value>
		</Cond>
	</Observation>
	<Observation>
		<SiteNumber>1</SiteNumber>
		<Cond>
			<value>9.0</value>
		</Cond>
	</Observation>
	<Observation>
		<SiteNumber>2</SiteNumber>
		<Cond>
			<value>0</value>
		</Cond>
	</Observation>
</Result>



<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:fo="http://www.w3.org/1999/XSL/Format">
	<xsl:key name="site_" match="Members/Observation" use="SiteNumber"/>
	<xsl:template match="Result">
		<xsl:apply-templates select="Members/Observation[generate-id(.) =
generate-id(key('site_', SiteNumber))]"/>
	</xsl:template>
	<xsl:template match="Observation">
		<xsl:value-of select="SiteNumber"/><br/>
		<xsl:value-of select="sum(????)"/>
	</xsl:template>
</xsl:stylesheet>


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