type

<attribute name="type" use="required">
  <simpleType>
    <restriction base="string">
      <enumeration value="identity"/>
      <enumeration value="table"/>
      <enumeration value="discrete"/>
      <enumeration value="linear"/>
      <enumeration value="gamma"/>
    </restriction>
  </simpleType>
  <!-- these all need to be stripped out and checked -->
</attribute>