# ay < # ax, unique_al(ax), ∀ a. a ∈ ax → a ∈ ay ⊦ ax = []
Used simplifier rules: ⊦ # ax = 0 ↔ ax = [];% ⊦ m < n → n ≠ 0; (forward)% ⊦ ¬ m ≤ n ↔ n < m;