[Jelly] How to select an XML node where attribute = a jelly var

[Dave Garthwaite]

I have parsed an XML doc using <parse>.
I want to select a node where an attribute equals the value of a jelly variable.
I tried <jx:set var="sourceNode" select="$doc/[EMAIL PROTECTED]'${sourceName}']"/> but 
this does not work.
If I use a literal instead of ${sourceName}, it does work.
What am I doing wrong ?