Currently, there's no clear statement as to what exactly the fallback is in case `__eq__` returns `NotImplemented`.  It would be good to clarify the behavior of `NotImplemented`; at least for `__eq__`, but perhaps also other rich comparison methods. For example: "When `NotImplemented` is returned from a rich comparison method, the interpreter behaves as if the rich comparison method was not defined in the first place." See http://stackoverflow.com/questions/40780004/returning-notimplemented-from-eq for more discussion.