title: XML Schema - getting around unique tag constraint
body: <p>I have to make a schema for an XML format that's already being used.  The existing XML is being generated already by a different program, and it sounds like it would be difficult to track down all the places that would need to be changed in order to use a more regular XML format.</p>
  
  <p>Here's an example similar to our XML structure:</p>
  
  <pre><code>&lt;data&gt;
      &lt;summary&gt;some info&lt;/summary&gt;
      &lt;error&gt;error message (only if there was an error)&lt;/error&gt;
      &lt;details&gt;more info
          &lt;x&gt;more&lt;/x&gt;
          &lt;y&gt;even more&lt;/y&gt;
      &lt;/details&gt;
      &lt;error&gt;another error message&lt;/error&gt;
      &lt;z&gt;some extra info&lt;/z&gt;
  &lt;/data&gt;
  </code></pre>
  
  <p>Note that the error tag is reused at the same level and comes after certain items but not others, so I can't just set maxOccurs="unbounded".  I've tried wrapping the associated pairs of error/other tags in an xsd:sequence, but that doesn't do the trick because I'm still effectively breaking the Unique Particle Attribution rule.</p>
  
  <p>Can this even be done, or do I need to let the other developers know this schema isn't going to validate?</p>