Create xsl key by "joining" elements - xslt

Question

<t>
<rendition xml:id="b">color: blue</rendition>
<rendition xml:id="r">color: red</rendition>
<tagUsage gi="p" render="b" />
<tagUsage gi="emph" render="r" />
</t>
How would I create an XSL 1.0 key into rendition elements based on #gi in the tagUsage element, joining rendition/#xml:id to tagUsage/#render? Something like
<xsl:key name="rendition-by-tagName"
match="rendition"
use="//tagUsage[#xml:id of rendition = #render of tagUsage]/#gi" />
so that given "p", the key would return the blue rendition; given "emph", the key would return the red rendition.

Answer 1

Use:
<xsl:key name="kRendByUsageGi" match="rendition"
use="../tagUsage[#render=current()/#xml:id]/#gi"/>
Here is a complete verification:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:key name="kRendByUsageGi" match="rendition"
use="../tagUsage[#render=current()/#xml:id]/#gi"/>
<xsl:template match="/">
<xsl:copy-of select="key('kRendByUsageGi', 'p')/text()"/>
========
<xsl:copy-of select="key('kRendByUsageGi', 'emph')/text()"/>
</xsl:template>
</xsl:stylesheet>
When this transformation is applied on the provided XML document:
<t>
<rendition xml:id="b">color: blue</rendition>
<rendition xml:id="r">color: red</rendition>
<tagUsage gi="p" render="b" />
<tagUsage gi="emph" render="r" />
</t>
the wanted, correct result is produced:
color: blue
========
color: red

Answer 2

I found that the following, which uses a second key(), works with xsltproc, so, if that's your target processor, this should help. It doesn't work with Saxon, though.
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:key name='kTagUsage' match='tagUsage' use='#render'/>
<xsl:key name="kRendByUsageGi" match="rendition"
use="key('kTagUsage', #xml:id)/#gi"/>
<xsl:template match="/">
<xsl:copy-of select="key('kRendByUsageGi', 'p')/text()"/>
========
<xsl:copy-of select="key('kRendByUsageGi', 'emph')/text()"/>
</xsl:template>
</xsl:stylesheet>