Chris, (01)
Sorry, I should have added more qualifications. (02)
> John, I know you know that there's no complement
> operation in set theory, at least, not in ZF. That
> said, in the class theories underlying, e.g., OWL
> and SUMO, and I'm thinking perhaps your own account,
> it does appear that classes form a Boolean algebra;
> so I'm guessing that's what lies behind your remark here. (03)
In my theories.htm paper, I was using the example of
a lattice of all subsets of a designated set U as the top
and the empty set as bottom. That's what I was thinking of. (04)
In fact, I should have mentioned that example as an excellent
reason why the empty set is needed: In order for every element
of that lattice to have a complement, you need the empty set
as the complement of U. (05)
John (06)
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