John Nolan and Don Ryan - I haven't looked at it yet, but if you think there's a fixable flaw in the proof of Theorem 1.88, it would be good to report it here:

* [Seven Sketches book: Typos, comments, questions, and suggestions.](

If there's a problem, Brendan and David will fix it.

By the way, it's also good to download the [latest copy of the book]( (as of 7 April 2018), which has posets restored to their rightful place, and various other mistakes caught by the students in this course fixed.