<choice xmlns:html="http://www.w3.org/TR/REC-html40" xmlns="http://relaxng.org/ns/structure/1.0" >
     <element name="foo">
          <empty/>
     </element>
     <group>
          <notAllowed/>
          <grammar>
               <define name="foo">
                    <element name="foo">
                         <empty/>
                    </element>
               </define>
          </grammar>
     </group>
</choice>