<!DOCTYPE doc [
  <!ENTITY ent1 "<elem/>">
  <!ENTITY ent2 "<ns:elem/>">
]>
<doc>
    <a xmlns="urn:a">&ent1;</a>
    <b xmlns="urn:b">&ent1;</b>
    <a xmlns:ns="urn:a">&ent2;</a>
    <b xmlns:ns="urn:b">&ent2;</b>
</doc>