“…Since the partial order ≤ is determined by either of the two "arrows", we can eliminate ≤ from the signature and denote a pseudo BCK-algebra by B = (X, →, , 1). An equivalent definition of a pseudo BCK-algebra is given in [21]. The structure B = (B, →, , 1) of the type (2, 2, 0) is a pseudo BCK-algebra iff it satisfies the following identities and quasi-identity, for all x, y, z ∈ B:…”