For any BCK-algebra X, the relation ≤ defined by x ≤ y if and only if x∗y = 0 is a partial
order on X. A BCK-algebra X is said to be positive implicative if (x∗y)∗z = (x∗z)∗(y ∗z)
for all x, y, z ∈ X. A nonempty subset I of a BCK-algebra X is called an ideal of X if it
satisfies
• 0 ∈ I,
• x ∗ y ∈ I and y ∈ I imply x ∈ I for all x, y ∈ X.