[xsl] Selecting all repeated elements

[Jason Foster <jason.foster@xxxxxxxxxxx>]

I've been hunting for an elegant way to obtain a sequence containing  
those elements that share a common attribute (in this case <course>  
elements with the same "code" attribute.  I think that I've got a  
kludged solution using <xsl:for-each> as follows:

<xsl:for-each select="child::course">
  <xsl:variable name="targetCode" select="attribute::code"/>
  <xsl:variable name="duplicates">
    <if test="count(/descendant::course[attribute::code =  
$targetCode]) > 1">
      <xsl:value-of select="concat(attribute::code,ancestor::term/ 
attribute::name, ' ')"/>
    </xsl:if>
  </xsl:variable>
</xsl:for-each>

This is ugly, and the result isn't a nodeset, so in general I'm not  
impressed.  The thing I haven't been able to figure out is whether  
XPath has something equivalent to a RegExp backreference, which would  
allow something like:

<xsl:variable
  name="duplicates"
  select="child::course[count(/descendant::course 
[attribute::code=???]) &gt; 1]"
/>

The trick is what should replace ???

Any assistance would be greatly appreciated.  For what's it's worth,  
I'm using XSLT to generate a linear program that represents the  
courses our students can take in order to find out what's the  
smallest number of hours that they'll experience over their program.   
Sometimes they can choose courses in multiple locations during their  
program.

Thanks!

Jason Foster