has a greatest element, denoted by a + b, is called a BCK-algebra with condition
(S). Every such BCK-algebra is a commutative semigroup with respect
to the operation + and 0 is its zero element. If it satisfies also the condition
xz · yz = xy · z, then it is equivalent to implicative semilattice (cf. [3] or
[21]). Moreover, as proved J. Meng (cf. [20]), BCK-algebras with condition (S),
commutative residual pomonoids with the identity as the greatest element and
implicative commutative semigroups are categorically equivalent to each other.
As a simple consequence of the above axioms system we obtain