<xs:simpleType name="DocumentFunctionCode">
  <xs:restriction base="xs:token">
    <!-- Lower-case values -->
    <xs:pattern value="delete"/>
    <xs:pattern value="original"/>
    <xs:pattern value="replace"/>
    <xs:pattern value="update"/>
    <!-- Upper-case values -->
    <xs:pattern value="DELETE"/>
    <xs:pattern value="ORIGINAL"/>
    <xs:pattern value="REPLACE"/>
    <xs:pattern value="UPDATE"/>
    <!-- Ucfirst-values -->
    <xs:pattern value="Delete"/>
    <xs:pattern value="Original"/>
    <xs:pattern value="Replace"/>
    <xs:pattern value="Update"/>
  </xs:restriction>
</xs:simpleType>