finalize4.patch repairs the comment typos, adds a new comment, and removes the unused `old` argument.  I think the code is ready to ship with this.  

I still don't have a reasonably simple fails-before-works-after test case.    But that doesn't bother me much, since "the problem" is obvious once it's been seen, and the patch obviously fixes it, and any way of trying to provoke this from pure Python will rely on implementation accidents to get exactly the right pieces of cyclic trash in exactly the right order in gc's internal lists.  Still, hope springs eternal ;-)