title: When an item in a population doesn't match the "group-by" in an "for-each-group", can it be assigned to a group with an empty current-group-key()?
body: <p>So I have input like:</p>
  
  <pre><code>&lt;Root&gt;
   &lt;Item id="1" group="foo" /&gt;
   &lt;Item id="2" group="bar" /&gt;
   &lt;Item id="3" /&gt;
  &lt;/Root&gt;
  </code></pre>
  
  <p>And a template instruction like:</p>
  
  <pre><code>&lt;xsl:for-each-group select="Root" group-by="@group" /&gt;
  </code></pre>
  
  <p>It would seem that I only get groups for <code>foo</code> and <code>bar</code>. (Why?  I expected I might get a 3rd group with <code>current-group-key() = ''</code>).</p>
  
  <p>I really want that 3rd group.  What options do I have?</p>
  
  <p>thanks,
  David.</p>