<element name="foo" xmlns:html="http://www.w3.org/TR/REC-html40" xmlns="http://relaxng.org/ns/structure/1.0" >
     <grammar>
          <start name="x" combine="choice">
               <element name="bar1">
                    <empty/>
               </element>
          </start>
          <start name="x">
               <element name="bar2">
                    <empty/>
               </element>
          </start>
          <start name="x" combine="interleave">
               <element name="bar3">
                    <empty/>
               </element>
          </start>
     </grammar>
</element>