title: XSD choice of elements and simple types
body: <p>How to write XSD, where should be the choice between simple and complex types.</p>
  
  <p>For example, all of this XMLs:</p>
  
  <pre><code>&lt;a&gt;
    &lt;b/&gt;
  &lt;/a&gt;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>&lt;a&gt;
    &lt;c/&gt;
  &lt;/a&gt;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>&lt;a&gt;
    100
  &lt;/a&gt;
  </code></pre>
  
  <p>(there are only integers allowed in last XML) should be valid.</p>
  
  <p>PS: only one element allowed within &lt;a> tag.</p>
  
  <pre><code>&lt;a&gt;
    &lt;b/&gt;
    &lt;c/&gt;
  &lt;/a&gt;
  </code></pre>
  
  <p>is invalid.</p>