Using increment correctly with maskprocessor.

I'm trying to use the increment option with maskprocessor, but when I use this command:

I get the following result, which contains invalid rules:

Quote:$
$_
$_$
$_$a
$_$a$
$_$a$b

What is the correct syntax that I need to use, to get the correct output of:

Quote:$_
$_$a
$_$a$b