<grammar xmlns="http://relaxng.org/ns/structure/1.0" >
  <start>
    <element name="foo">
      <attribute>
        <choice>
          <name>alpha</name>
          <name>bravo</name>
        </choice>
        <choice>
          <value>none</value>
          <value>all</value>
        </choice>
      </attribute>
    </element>
  </start>
</grammar>