title: How to do mutually exclusive attributes in XML schema?
body: <p>I'm trying to make two XML attributes to be mutually exclusive. How can one create an XSD schema to capture this kind of scenario?</p>
  
  <p>I would like to have one of these</p>
  
  <pre><code>&lt;elem value="1" /&gt;
  &lt;elem ref="something else" /&gt;
  </code></pre>
  
  <p>but not</p>
  
  <pre><code>&lt;elem value="1" ref="something else" /&gt;
  </code></pre>