how do you use 'execOn'

[Suu Quan]

I'm trying to understand the syntax, and it barf on me. Would you point out
my error please.

  <target name='execOn'>
     <execOn executable="sh.exe" >
        <arg value="execEx" />
        <fileset dir="."/>
    </execOn>
  </target>

(Note that the combination sh.exe and execEx works with <exec>)

got this nasty display

myAntfile:41: Could not create task of type: execOn. Common
 solutions are to use taskdef to declare your task, or, if this is an
optional
task, to put the optional.jar in the lib directory of your ant installation
(ANT
_HOME).

In reading the manual, ExecOn is a built-in task