Tim Peters writes:
> [Ethan Furman <[email protected]>]
> > .. on the other hand, it seems that collections of related flags
> > are often treated as in set theory, where the empty set is a
> > member of any non-empty set.
>
> Not how any set theory I've ever seen works: a set S contains the
> empty set if and only if some member of S _is_ the empty set. Which
> is also how Python's frozenset.__contains__ works (a plain Python set
> can never contain the empty set, because only a frozenset can contain
> a set (empty or not)).
Maybe Ethan's thinking of one of the standard set-theoretic
constructions of natural numbers? Ie,
0 = {}, 1 = {0} = {{}}, 2 = {0, 1} = {{}, {{}}}, etc.
Steve
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