title: How to Implement Conditional Schema Validation
body: <p>How Can i implement a schema validation that can serve the following requirement</p>
  
  <pre><code>&lt;test&gt;
    &lt;e1&gt;abc&lt;/e1&gt;
    &lt;e2&gt;
       &lt;e21&gt;xxx&lt;/e21&gt;
       &lt;e22&gt;yyy&lt;/e22&gt;
    &lt;/e2&gt;
  &lt;/test&gt;
  </code></pre>
  
  <p>so my requirement is that e22 cannot be null if e21 is not null, so how can i design my xsd schema</p>