⊦ (∃ d. a × d ∈ ax ∨ a × d ∈ ay) ↔ a ∈ ax ∨ a ∈ ay
Used simplifier rules: ⊦ (ax + ax0) + ax1 = ax + ax0 + ax1;% ⊦ p ∈ ax + ax0 ↔ ¬ ¬(p ∈ ax ∨ p ∈ ax0);