Closure under difference

I am having a hard time seeing why the constraint on <data> causes RELAX NG
not to be closed under difference.  I am not convinced we are closed at the
moment, but the constraint doesn't seem to make things worse.  The hardest
case I have found is:

<difference>
  <mixed>
    <element name="foo"/>
      <empty/>
    </element>
  </mixed>
  <data type="int"/>
</difference>

but this is equivalent to:

<choice>
  <difference>
    <mixed>
      <element name="foo"/>
        <empty/>
      </element>
    </mixed>
    <text/>
  </difference>
  <difference>
    <data type="string"/>
    <data type="int"/>
  </difference>
</choice>

and I can't see that the proposed restriction affects either:

  <difference>
    <mixed>
      <element name="foo"/>
        <empty/>
      </element>
    </mixed>
    <text/>
  </difference>

or

  <difference>
    <data type="string"/>
    <data type="int"/>
  </difference>

James