operator

<attribute name="operator" default="over">
  <simpleType>
    <restriction base="string">
      <enumeration value="over"/>
      <enumeration value="in"/>
      <enumeration value="out"/>
      <enumeration value="atop"/>
      <enumeration value="xor"/>
      <enumeration value="arithmetic"/>
    </restriction>
  </simpleType>
</attribute>