<!DOCTYPE x:a [

<!ELEMENT x:a ANY>
<!ELEMENT x:b ANY>
<!ELEMENT y:a ANY>
<!ELEMENT y:b ANY>
<!ELEMENT z:a ANY>
<!ELEMENT z:b ANY>

<!ATTLIST x:a x:r CDATA #IMPLIED
              y:r CDATA #IMPLIED
              z:r CDATA #IMPLIED>
]>

<x:a xmlns:x="space-x" xmlns:y="space-y" xmlns:z="space-z">
  <b xmlns="space-y" xmlns:z="space-x">
    <z:a x:r="a" y:r="b"/>  <!-- x:r would cause an error! -->
  </b>
  <b xmlns="space-z">
    <z:a/>
  </b>
</x:a>