When creating nested mutually exclusive groups, all closing brackets except one are omitted.

Example:
parser = ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument('-a')
group.add_argument('-b')
group2 = group.add_mutually_exclusive_group()
group2.add_argument('-c')
group2.add_argument('-d')
group3 = group2.add_mutually_exclusive_group()
group3.add_argument('-e')
group3.add_argument('-f')

prints a usage line of:
usage: test.py [-h] [-a A | -b B | [-c C | -d D | [-e E | -f F]

it should print something like:
usage: test.py [-h] [-a A | -b B | [-c C | -d D | [-e E | -f F]]]