Recursion Help with Summing Durations

Hello,

I am trying to use the EXSLT template add-duration.  I have a node-set
of durations that I'd like to sum together.  My named template is not
working, and I was hoping someone could give me some hints or tips on
where to go to make this recursion work properly.

My environment:

Java 1.3.1
Xalan2
Cocoon 2.0.2

I've tested that I'm getting the correct node-set by using count(). 
I've also hard-coded some values into add-duration to make sure it
works.

I've been scanning documentation all day, and coming up empty.  I might
be getting tripped up with the difference between node-sets and RTFs,
but that's where my XSLT gets fuzzy.  Any tips or suggestions would be
greatly appreciated!

Thanks so much,
Seth

My template:

    <xsl:template name="total-durations">
        <xsl:param name="durations"/>

        <xsl:choose>
            <xsl:when test="$durations">
                <xsl:variable name="first" select="$durations[1]"/>
                <xsl:variable name="rest">
                    <xsl:call-template name="total-durations">
                        <xsl:with-param name="durations"
                                select="$durations[position()!=1]"/>
                    </xsl:call-template>
                </xsl:variable>

                <xsl:call-template name="date:add-duration">
                    <xsl:with-param name="duration1" select="$first"/>
                    <xsl:with-param name="duration2" select="$rest"/>
                </xsl:call-template>

            </xsl:when>
            <xsl:otherwise>P0M</xsl:otherwise>
        </xsl:choose>

    </xsl:template>

I call it with this (while I am in the timecard element):

<xsl:call-template name="total-durations">
    <xsl:with-param name="durations" select="shift/duration"/>
</xsl:call-template>

And my original document looks like:

<timecard>
    <shift>
       <duration>P13M</duration>
    </shift>
    <shift>
       <duration>P43M</duration>
    </shift>
</timecard>



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