title: xslt multi-part key grouping
body: <p>Lets say I have some xml like this:</p>
  
  <pre><code>&lt;root&gt;
  &lt;object id='1'/&gt;
  &lt;object id='1' otherid='2'/&gt;
  &lt;object otherid='2'/&gt;
  &lt;object otherid='2' yetanotherid='3'/&gt;
  &lt;object yetanotherid='3'/&gt;
  &lt;object id='2'&gt;I'm not the same&lt;/object&gt;
  &lt;/root&gt;
  </code></pre>
  
  <p>Lovely, I know. Now these are all the same object, but they have an assortment of ids. How can I group them? Any of these id's will uniquely identify, so id[1]=otherid[2]=yetanotherid[3].</p>
  
  <p>A complete example:</p>
  
  <pre><code>&lt;root&gt;
  &lt;object id='1'&gt;&lt;property1&gt;&lt;/property1&gt;&lt;/object&gt;
  &lt;object id='1' otherid='2'&gt;&lt;property1&gt;&lt;/property1&gt;&lt;/object&gt;
  &lt;object otherid='2'&gt;&lt;property2&gt;&lt;/property2&gt;&lt;/object&gt;
  &lt;object otherid='2' yetanotherid='3'&gt;&lt;property3&gt;&lt;/property3&gt;&lt;/object&gt;
  &lt;object yetanotherid='3'&gt;&lt;property2&gt;&lt;/property2&gt;&lt;property4&gt;&lt;/property4&gt;&lt;/object&gt;
  &lt;object id='2'&gt;&lt;property2&gt;&lt;/property2&gt;&lt;/object&gt;
  &lt;/root&gt;
  </code></pre>
  
  <p>Outcome:</p>
  
  <pre><code>&lt;root&gt;
  &lt;object id='1' otherid='2' yetanotherid='3'&gt;
  &lt;property1&gt;&lt;/property1&gt;
  &lt;property2&gt;&lt;/property2&gt;
  &lt;property3&gt;&lt;/property3&gt;
  &lt;property4&gt;&lt;/property4&gt;
  &lt;/object&gt;
  &lt;object id='2'&gt;
  &lt;property2&gt;&lt;/property2&gt;
  &lt;/object&gt;
  &lt;/root&gt;
  </code></pre>