title: XSD validation: Element Value should be sum of 2 other Elements
body: <pre><code>&lt;xml&gt;
     &lt;root&gt;
       &lt;courses&gt;
           &lt;course&gt;
               &lt;ceu&gt;2000&lt;/ceu&gt;
               &lt;first_year&gt;1000&lt;/first_year&gt;
               &lt;second_year&gt;1000&lt;/first_year&gt;
           &lt;/course&gt;
       &lt;/courses&gt;
     &lt;/root&gt;
  </code></pre>
  
  <p>How do I add XSD validation so that the element CEU's value is always equal to the sum of first_year and second_year values? </p>