<?xml version=“1.0”?> <grammar ns=“www.example.com/choice

       xmlns="http://relaxng.org/ns/structure/1.0">
<start> 
  <element name="doc" ns="">
    <ref name="elem"/>
    <optional>
      <ref name="elem"/>
      <optional>
        <ref name="elem"/>
      </optional>
    </optional>
  </element>
</start>

<define name="elem">
  <element name="elem" ns="">
    <text/>
  </element>
</define>

</grammar>