<grammar xmlns=“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>