Professor Rasiowa [HR49] considers implication algebras (A,⇒, V ) such that ⇒ is a binary operation on the universe A and V ∈ A. In particular, there are studied implicative algebras, positive implication algebras and implication algebras; the second kind corresponds to the logic of positive implication, and the third one to the logic of classical implication. In each case, the distinguished el...