newbie - problem with sum

I need to sum all the Amounts with a value > 0
I set up a variable

<xsl:variable name="suspensecr">
	<xsl:for-each select="/Report/Suspense">
		<xsl:if test="Amount &gt; 0"><xsl:value-of
select="Amount"/></xsl:if>
	</xsl:for-each>
</xsl:variable>

which seems to contain all the required values but I can't get a sum
produced from this.
I've tried all sorts of <xsl:value-of select="sum(.... but to no avail.
Can someone help please?
Thanks
	Jan




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