Hello Michael,
I was trying to use xsl:use-when but it is not finding a variable.
 
This works:
<xsl:if test="$is-featured">
   <span>content</span>
</xsl:if>
 
This doesn't:
<span xsl:use-when="$is-featured">content</span>
 
Saxon.Api.StaticError: Error in use-when expression. XPath syntax error at char 12 on line 224 in {$is-featured}:
   Variable $is-featured has not been declared
 
I'm using 9.0.0.2 on .net
 
--
Max