(2) From properties (P-3) and (P-2) of proposition 2.3 and axiom (KKM-1), we have x∗(y∗x) = y∗(x∗x) = y∗0 = 0. Then, if we assume that (y∗x)∗x = 0, we have from axiom (KK-3) that y∗x = x. To prove the converse, we assume that x = y∗x and then from property (P-2) of proposition 2.3, we have (y∗x)∗x = (y∗x)∗(y∗x) = 0.
(3) From property (P-1) of proposition 2.3, we have