1<?xml version="1.0"?> 2<grammar ns="http://www.example.com/choice" 3 xmlns="http://relaxng.org/ns/structure/1.0"> 4 <start> 5 <element name="doc" ns=""> 6 <ref name="elem"/> 7 <optional> 8 <ref name="elem"/> 9 <optional> 10 <ref name="elem"/> 11 </optional> 12 </optional> 13 </element> 14 </start> 15 16 <define name="elem"> 17 <element name="elem" ns=""> 18 <text/> 19 </element> 20 </define> 21</grammar> 22