Some nested args in function definitions can crash bpython

See http://bpaste.net/show/8135/

---
- Bitbucket: https://bitbucket.org/bobf/bpython/issue/133
- Originally Reported By: [Andreas Stührk](http://bitbucket.org/Trundle)
- Originally Created At: 2010-07-22T02:03:04.253
