Definition 3.1. By a generalized BCK-algebra (gBCK-algebra, for short) we mean a
triplet (G, ∗, 0), where G is a nonempty set, ∗ is a binary operation on G and 0 ∈ G is
a nullary operation, called zero element, such that
(G1) x ∗0 = x,
(G2) x ∗ x = 0,
(G3) (x ∗ y) ∗ z = (x ∗ z) ∗ y,
(G4) (x ∗ y) ∗ z = (x ∗ z) ∗ (y ∗ z).