<?xml version="1.0" encoding="UTF-8"?>
<doctype>
  <modelGroup name="e1">
    <sequence/>
  </modelGroup>
  <modelGroup name="e2">
    <sequence>
      <modelGroupRef name="e1"/>
      <elementRef>
        <name>foo</name>
      </elementRef>
      <elementRef>
        <name>bar</name>
      </elementRef>
    </sequence>
  </modelGroup>
  <element>
    <name>doc</name>
    <modelGroupRef name="e2"/>
  </element>
  <element>
    <name>foo</name>
    <sequence/>
  </element>
  <element>
    <name>bar</name>
    <sequence/>
  </element>
</doctype>
