Unlike the <jar> and <zip> tasks of ant, the <tar> task does not provide an
'update' parameter to specify, wheather the archive should be overwritten or not.
How can I then add something to an existing tar archive? For example, when I
have

 <target ...>
    ....
    <tar destfile="exp.tar" >
       <tarfileset mode="0660" dir="." includes="foo,bar"/>
    </tar>
    ....
    <tar destfile="exp.tar" >
       <tarfileset mode="0660" dir="." includes="baz"/>
    </tar>
 </target>

Ant says:

     ....
     [tar] Building tar: H:\exp\exp.tar
     ....
     [tar] Nothing to do: H:\exp\exp.tar is up to date.

and exp.tar contains only foo and bar, but not baz.

Ronald