reccursive sum ?

For each "ME" node in LIST.xml, i calculate the number of "devoirs". To do
so I must open each MATIERE.XML and making a count of all "DEVOIR" nodes.

LIST.xml looks like this :

<LISTE-ME>
	<ME id="0010"/>
	<ME id="0045"/>
	<ME id="0152"/>
</LISTE-ME>

Each ME has a description in his own file, for instance the file 0010.xml
looks like this :

<ME ref="0010">
	<FOURN label="anything">
		<DEVOIR name="anything">
		<DEVOIR name="anything">
	</FOURN>
	<FOURN label="anything">
		<DEVOIR name="anything">
		<DEVOIR name="anything">
		<DEVOIR name="anything">
	</FOURN>
</ME>

The XSL apply on LIST.xml looks like this :

<xsl:template match="LISTE-ME">
	<xsl:for-each select="ME">
		<xsl:variable name="ThisMEid">
			<xsl:value-of select="@id"/>
		</xsl:variable>
		ME <xsl:value-of select="@id"/> :
		<xsl:call-template name="NbreDevoirsME">
			<xsl:with-param name="MEid" select="$ThisMEid"/>
		</xsl:call-template>
		 devoirs <br/>
	<xsl:for-each/>
</xsl:template>

<xsl:template name="NbreDevoirsME">
	<xsl:param name="MEid"/>
	<xsl:variable name="MEfile">
		<xsl:value-of select="$MEid">.xml
	</xsl:variable>
	<xsl:value-of select="count(document($MEfile)/ME/FOURN/DEVOIR"/>
</xsl:template>

The Output would be here something like this :

ME 0010 : 5 devoirs
ME 0045 : 3 devoirs
ME 0152 : 7 devoirs

What i'd like is the total number of devoirs for all ME. ==> TOTAL = 15
devoirs

At this point I do not manage anymore :

* I m pretty sure the solution must be a reccursive template, but i did not
find it ...
* I thought about a way to calculate this sum :
	if I write <xsl:value-of select="5+3+7">, it gives 15.
	So I put in a variable "string" the expression to be calculated :

					<xsl:variable name="String">
						<xsl:for-each select="ME">
							<xsl:variable name="ThisMEid">
								<xsl:value-of select="@id"/>
							</xsl:variable>
							<xsl:call-template name="NbreDevoirsME">
								<xsl:with-param name="MEid" select="$ThisMEid"/>
							</xsl:call-template>
							<xsl:if test="position()!=last()">
							+
							</xsl:if>
						</xsl:for-each>
					</xsl:variable>

	Off course, when I write <xsl:value-of select="$String"/> the processor
don't calculate it, it just dysplay the string.
	Is there a way to tell him, $string must be interpreted as a Xpath
expression which you must calculate ?





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