$ ./factor foo.factor

$ ./factor foo.factor --help
Usage:
    factor foo.factor [--help] [command]

Arguments:
    command    {add,subtract}

Options:
    --help    show this help and exit

$ ./factor foo.factor add --help
Usage:
    factor foo.factor add [--help] [a]

Arguments:
    a

Options:
    --help    show this help and exit

$ ./factor foo.factor multiply
ERROR: Invalid value 'multiply' for option 'command'