title: Schema Choice Element Condition
body: <p>How can the choice keyword be manipulated to meet the following condition:</p>
  
  <pre><code>(Subject &amp;&amp; MessageBody) || (Subject || MessageBody)
  </code></pre>
  
  <p>I have:</p>
  
  <pre><code>&lt;xs:choice&gt;
      &lt;xs:element ref="Subject"/&gt;
      &lt;xs:element ref="MessageBody"/&gt;
  &lt;/xs:choice&gt;
  </code></pre>
  
  <p>However Subject and MessageBody cannot co-exist in the complexType since the set enforces a XOR condition.</p>