A problem with numbered pages

Hi, 
I have a small problem but it will be great if somebody can resolve it. 

From a XML document and a XSL-FO stylesheet, i generate a PDF document within there are several documents (i use Fop). I would like to have a notation like "1/2" for each document within my PDF document.

I have already had a notation like "1/3" but for all my PDF document.
My stylesheet looks like :
<fo:layout-master>
   <fo:simple-page-master ...>
   .....
   .....
   </fo:simple-page-master>
   <fo:page-sequence-master ...>
   .....
   </fo:page-sequence-master>
</fo:layout-master>

<xsl:for-each select="aDocument">
  <fo:page-sequence ...>
    ........
    ........
    <fo:static-content flow-name="xsl-region-after">
      page <fo:page-number/> / <fo:page-number-citation ref-id="endofdoc"> 
    </fo....>
    ........
    ........
    <fo:block id="endofdoc"> </fo:block>
  </fo:page-sequence>
</xsl:for-each>

With this stylesheet, I have this error : the variable "endofdoc" already exists. 
this error is normal : i must have a unique id for the block but i try a lot of thinks and i didn't succeed to resolv my problem.

Anyone has an idea ? Is it possible to do that with xsl:fo or xslt ? I am interested in any help.

I hope you have understand my problem (sorry for my english!!!).

Thanks in advance for your help.

Marina