title: Assign unique attribute to elements from different types in XSD schema
body: <p>I'm new to XML and XSD and I have a question about XML-Schema validation. Consider XML file:</p>
  
  <pre><code>&lt;RequestList&gt;
    &lt;Request1 id="1"&gt;...&lt;/Request1&gt;
    &lt;Request2 id="2"&gt;...&lt;/Request2&gt;
    .
    .
    .
  &lt;/RequestList&gt;
  </code></pre>
  
  <p>I want to force id attribute to be unique. Google helped me much in case that elements have the same type. But I can't make a sequence of sinlge element 'Request', because each request can contain different information, according to his type. What should I do?
  Thanks in advance for help. </p>