As for a work-around, the following works with xsltproc, but
interestingly, not with Saxon.  Saxon complains "key() function cannot
be used here".  (This is a variant of the XSLT that Dimitre posted on
SO):

       <xsl:key name='kTagUsage' match='tagUsage' use='@render'/>
       <xsl:key name="kRendByUsageGi" match="rendition"
         use="key('kTagUsage', @xml:id)/@gi"/>

	Martin Honnen --- MVP Data Platform Development
	http://msmvps.com/blogs/martin_honnen/