<t:root xmlns:t="test"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="test test.xsd">

  <!-- valid extension but not restriction: y attribute -->
  <restriction-b>
    <a>a</a>
    <b>b</b>
  </restriction-b>

</t:root>
