<xsd:simpleType name="SimpleOperatorType">
  <xsd:annotation>
    <xsd:documentation>Exists, Not Exists, ...</xsd:documentation>
  </xsd:annotation>
  <xsd:restriction base="NonEmptyTokenType">
    <xsd:enumeration value="exists">
      <xsd:annotation>
        <xsd:documentation>Field must exist</xsd:documentation>
      </xsd:annotation>
    </xsd:enumeration>
    <xsd:enumeration value="missing">
      <xsd:annotation>
        <xsd:documentation>Field must not exist</xsd:documentation>
      </xsd:annotation>
    </xsd:enumeration>
    <xsd:enumeration value="isNull">
      <xsd:annotation>
        <xsd:documentation>Field exists but empty or null</xsd:documentation>
      </xsd:annotation>
    </xsd:enumeration>
  </xsd:restriction>
</xsd:simpleType>