Class

[[category:There are several definitions of a class in use. The following definition is given by Copi. An ordered 4-tuple (C,(∩,∪),~,(1,0)) is called a class if it satisfies the following axioms.
1. (C, ∩, 1) is an abelian monoid
2. (C, ∪, 0) is an abelian monoid
3. 0 ≠ 1
4. for every a in C,

• a ∩ ~a = 0
• a ∪ ~a = 1

5. and for every a, b, and c in C,

• a ∪ (b ∩ c) = (a ∪ b) ∩ (a ∪ c)
• a ∪ (b ∩ c) = (a ∩ b) ∪ (a ∩ c).