Rowspan and colspan

Hi All,
I've an xml document like this in foot and I've to transform it in xhtml 
tables.
Now I've done the tr and th correct rendering and nesting, but what put 
me into troubles is the calculation of their attributes colspan and rowspan.
For the rowspan I've thought to calculate the maximum number from the 
child of the preceding:sibling and following:heading using the also the 
max() function from EXSLT...
<xsl:variable name="rowspan">   
   <xsl:for-each select="following-sibling::heading"> <!-- sorry 
everyone! -->
       <xsl:sort select="count(ancestor-or-self::heading)" 
order="descending"/>
           <xsl:if test="position()=1">
               <xsl:value-of select="count(ancestor-or-self::heading)"/>
           </xsl:if>
   </xsl:for-each>
</xsl:variable>

Anyone could give me some advice?
Thanks for the help!

Danilo

-------------------------------------------

<?xml version="1.0" encoding="UTF-8"?>
<set>
   <tabForm>
       <title>Table caption</title>
       <rowHeader>
           <heading>th1
               <heading>th1_a</heading>
               <heading>th1_b
                   <heading>th1_b_a</heading>
                   <heading>th1_b_b</heading>
               </heading>
           </heading>
           <heading>th2</heading>
       </rowHeader>
   </tabForm>
</set>

smime.p7s
Description: S/MIME Cryptographic Signature