Null Type

<simpleType name="NullEnumeration?">

 <restriction base="string">
  <enumeration value="inapplicable"/>
  <enumeration value="missing"/>
  <enumeration value="template"/>
  <enumeration value="unknown"/>
  <enumeration value="withheld"/>
 </restriction>

</simpleType> <simpleType name="NullType">

 <union memberTypes="gml:NullEnumeration? anyURI"/>

</simpleType>