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