So what problem(s) would a dynamic ordered set really be aiming at?  I
don't know.  Consistency & predictability I understand and appreciate,
but little beyond that.  Even FIFO ordering is somewhat a PITA, since
`next(iter(set))` is an overly elaborate way to need to spell "get the
first" if a FIFO queue _is_ a prime dynamic use case.  And there's no
efficient way at all to access the other end (although I suppose
set.pop() would - like dict.popitem() - change to give a _destructive_
way to get at "the other end").