<?xml version="1.0" encoding="UTF-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <start>
    <element>
      <choice>
        <name>c</name>
        <choice>
          <name>a</name>
          <name>b</name>
        </choice>
      </choice>
      <empty/>
    </element>
  </start>
</grammar>
