Shell command completers

[Martin Lichtin]

I'm struggling a bit with shell command argument completers.
My command has two arguments, I need a completer only for the second 
argument.
Below configuration does not seem to work (myCompleter is not called):
            <completers>
                <null />
                <ref component-id="myCompleter" />
                <null />
            </completers>

Is this configuration ok and supported?
Also, is the "terminating" <null /> indeed necessary?

Martin