Any changes here including the latest pull request might conflict with

http://bugs.python.org/issue29553

Argparser does not display closing parentheses in nested mutex groups

If someone uses nested mutually exclusive groups (which I don't think they should, but anyways ...), the usage formatter gets confused by the nested brackets, and weeds out the wrong one(s).

That issue hasn't been settle yet; my feeling is that fixing this assertion error properly is more important.