Xpath expression with condition sought...

I have XML like:
<outer>
  .....
          <a>someArbitaryText</a>
.....
          <b>alsoSomeArbitaryText</b>
.....
          <c>likewise</c>
......
          <d>likewise</d>
 
</outer>
I'm looking for a single Xpath ___expression which
evaulates to the text at <c> if the text at <a>
matches the text at <b>, otherwise to the text at <d>.
I'm using Xalan. And yes, I know I could do it in
stages in XSLT but I need a SINGLE XPath
___expression. (Why?: because my transform is driven
by an XML configuration file defining the XPaths of
the items of interest in the source document. My XSLT
reads the XPath __expression from the configuration
file, and evaluates it as an XPath using
xalan:evaluate(). In this particular case the location
of the item to be used depends on whether 2 other
elements have the same text values.)
 
I can do it this way:
//c|//d[position()=2-number(//a=//b)]
but this, I believe (correct me if I'm wrong) assumes
c & d appear in this order. Is there a way which is
independent of the order of c and d  in the source
document (whilst being a single Xpath _expression)?
 
 
Suggestions greatly appreciated.
Pete


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