Adding numbers in xsl loop

Perhaps somebody can help me with the following: 

I want in an xsl:for-each loop to add a found value to a certain variable. Something like this:

<xsl:for-each select="projects/project[medewerker[login=$loginMatch]]">
	<xsl:variable name="currentPro" select="name"/>
	<xsl:variable name="totaalX" select="sum(../../hours/entry[login=$loginMatch][date/year=$jaarX][date/month=$maandX][projectname=$currentPro]/duration)"/>
	(now i want something like, so that absTotal get's in the loop everytime the value of totaalX plus the value of itself:)	
	<xsl:variable name="absTotal" select="$absTotal + $totaalX"/>
</xsl:for-each>

Apparently, this does not work in xsl. I can't figure out, how to use a basic programming issue like i = i + 1 in a loop???????????

Can anybody help me, or has an idea?? 

regards ard

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