“…The point was to show that the 15 functions, and thus all their further combinations, could be defined in terms of the four primitive operations of join, meet, implication and nand. 9 The fourteen non-zero operations occur in natural pairs: ⇒ and , ⇐ and , ≡ and ≡, ∨ and ∨, and ∧ and | in addition to σ and ¬σ , and τ and ¬τ . Except in the case of the join ∨ (and, of course, σ and τ ), the second operation in the pair is not the negation of the first.…”