showing relationships in xforms

Hi,

I'm currently experimenting with xforms and having difficulty displaying
information about relationships. Given a data instance like this:

<persons>
 <person>
  <id>marx</id>
  <name>Karl Marx</name>
 </person>
 <person>
  <id>engels</id>
  <name>Friedrich Engels</name>
  <friend>marx</friend>
 </person>
</persons>

I would expect to be able to display "Karl Marx" from the context
/persons/person[id='engels'] by doing something like:

<xforms:output ref="instance('persons')/person[id=current()/friend]/name"/>

....but, needless to say, I can't.

I realize that the current() function I'm used to from XSLT is not
available in XForms (I use it above for illustration), but I presume that
there must be something functionally equivalent... Can anybody enlighten me?

I'm using Orbeon forms, if that is significant.

Hope somebody can help

Kevin Flynn