Proof. (i) Trivially follows from Proposition 2.2 (iii) and the fact that 0 = yy. (ii) If 0x = 0y, then
0 = xx = (xx)0 = x(0(0x)) = x(0(0y)) = (xy)0 = xy,
and hence x = y by (i). (iii) For any x ∈ X, since 0x = (0x)0 = 0(0(0x)) by (ii), we have x = 0(0x).