<xs:complexType name="SubjectType">
  <xs:annotation>
    <xs:appinfo>
      <mdr:definition>The type used to model the Subject element.</mdr:definition>
    </xs:appinfo>
  </xs:annotation>
  <xs:choice maxOccurs="unbounded">
    <xs:element ref="category"/>
    <xs:element ref="keyword"/>
  </xs:choice>
</xs:complexType>