I also wrote a slightly more thorough patch that simplifies some of the method implementations (largely just replacing a bunch of for/if/return True/False else return False/True with any/all calls against a generator expression).

The one behavior difference is that I dramatically simplified Sequence's __iter__; the original just kept indexing until it got IndexError, the replacement just runs len(self) times, on the theory that a self-mutating Sequence should write its own __iter__, and we shouldn't jump through hoops to accommodate unsupported behavior like mutating a Sequence during iteration.

All tests pass under both patches.