Proof. Let x ∈ X and a ∈ Iw. Then we have w ∗ a ∈ I, and so w ∗ (x ∗ a) =
(w ∗ x) ∗ (w ∗ a) ∈ X ∗ I ⊆ I from (I1). This implies x ∗ a ∈ Iw. Now let
a, b ∈ Iw and x ∈ X. Then we obtain w ∗ a ∈ I and w ∗ b ∈ I. Thus we get
w∗((a∗(b∗x))∗x) = (w∗((a∗(b∗x))))∗(w∗x) = ((w∗a)∗(w∗(b∗x)))∗(w∗x) =
((w∗ a) ∗ ((w∗ b) ∗ (w∗ x))) ∗ (w∗ x) = ((w∗ a) ∗ ((w∗ b) ∗ (w∗ x))) ∗ (w∗ x) ∈ I
by (I2). This implies (a ∗ (b ∗ x)) ∗ x ∈ Iw. This completes the proof.