title: Perform xpath function on an xform output node
body: <p>I've got an xform with an output element that references a "time" node in my model that contains a string something like "00:12:34,567".  I'd like the text of the output element to be the substring "00:12:34".  I didn't quite expect this to work but maybe you'll get what I'm trying to do.</p>
  
  <pre><code>&lt;xf:output ref="substring-before(//time[1],',')"&gt;
      &lt;xf:label&gt;Time&lt;/xf:label&gt;
  &lt;/xf:output&gt;
  </code></pre>