Tim Peters writes:
 > [Ethan Furman <et...@stoneleaf.us>]

 > > .. 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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