operator

<xs:attribute name="operator" use="required">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:enumeration value="equal"/>
      <xs:enumeration value="notEqual"/>
      <xs:enumeration value="lessThan"/>
      <xs:enumeration value="lessOrEqual"/>
      <xs:enumeration value="greaterThan"/>
      <xs:enumeration value="greaterOrEqual"/>
      <xs:enumeration value="isMissing"/>
      <xs:enumeration value="isNotMissing"/>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>