title: how to extend attributes using XML Schema?
body: <p>I have on the XML:</p>
  
  <pre><code>&lt;Devices&gt;
      &lt;Device name="Phone" number="123456789"/&gt;
      &lt;Device name="Computer" ip="192.168.0.1"/&gt;
  &lt;/Devices&gt;
  </code></pre>
  
  <p>and I want to set an Schema for this, where I have 'devices' and I can declare a device, but if the device has the name="phone" a number has to be declared as required, but if the device is name="computer", well, the ip is required just for 'computer'</p>
  
  <p>is there a way to do this, is it possible?</p>