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