ANT_17_BRANCH/src/etc/testcases/types/mappers/define.mapperresult.xml

<project>
  <typedef name="mapperresult"
           classpath="../../../../../build/testcases"
           classname="org.apache.tools.ant.types.mappers.MapperResult"/>

  <!-- this is what you get with no result -->           
  <property name="no-results" value="&lt;NULL&gt;" />           
</project>