There's a problem with introducing ,_ as a new operator. $ python3.6 Python 3.6.2 (default, Jul 29 2017, 00:00:00) [GCC 4.8.4] on linux Type "help", "copyright", "credits" or "license" for more information. >>> 2 + 2 4 >>> 4 ,_ + 6 (4, 10) Hint: >>> _ (4, 10) >>> 3 ,_ (3, (4, 10)) -- Jonathan