<!DOCTYPE root [
  <!ATTLIST x id ID #IMPLIED>
]>
<root xmlns:xi="http://www.w3.org/2001/XInclude" xml:base="http://www.example.org/">
<x id="foo">
  <x id="bar"/>
</x>
<x id="foo">
  <x id="bar"/>
</x>
<x id="bar"/>
</root>