maker and retrieve-marker question

Hi all:
I am having a problem in using the marker and
retrieve-marker to calculate page subtotals.

here is what i am doing:
in a table cell, i create two markers with the
cumulative sum of a nodeset as follows:


<fo:marker
marker-class-name="previous_cumulative_total_marker">
    <xsl:value-of
select="sum((preceding-sibling::Quantity)/NetValue)"/>
</fo:marker>

<fo:marker
marker-class-name="this_cumulative_total_marker">
    <xsl:value-of
select="sum((.|preceding-sibling::Quantity)/NetValue)"/>
</fo:marker>



then, in the static block, i retrieve the
previous_cumulative_total_marker using the
"first-statring-within-page" and
this_cumulative_total_marker using the
"last-ending-within-page", and i put them in two
variables as follows :


<xsl:variable name="previous_cumulative_total">
    <fo:retrieve-marker
retrieve-class-name="previous_cumulative_total_marker"
retrieve-boundary="page"
retrieve-position="first-starting-within-page"/>
</xsl:variable>

<xsl:variable name="this_cumulative_total">
    <fo:retrieve-marker
retrieve-class-name="this_cumulative_total_marker"
retrieve-boundary="page"
retrieve-position="last-ending-within-page"/>
</xsl:variable>


then, in the same static block, i want to show the
result of the 
subtraction of those two variables from each other, so
i do the 
following:
<xsl:value-of select="$this_cumulative_total -
$previous_cumulative_total"/>

but this does not show anything.
does anyone know why this does not work?

Now, if i do 
<xsl:copy-of select"$previous_cumulative_total"/> , it
shows me the 
number. same as for <xsl:copy-of
select"$this_cumulative_total"/>
but i cannot subtract those from each other? can i?

why does it work with copy-of but not with value-of?

and how can i get the result of the subtraction of the
two variables? 
is there another way of doing this?

any help is greatly appreciated.

hilz


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