There are two forms of the mod command. The first form adds
a module to the list of available modules, and the second selects one of the
available modules as the one to be used.
The command to add a card is mod +$N where the
$N is the module number on the current card as selected by the
card command.
When the command "mod $N", where $N is a
number, selects the Nth module. For exmaple, if no modules are open, then
mod +2 mod +4 mod 0
opens the modules in positions two and four on the current card, and selects the first of these (the module in position two) for use.
This is a command understood by the pathtest test program.