# y < # x, ∀ a. a ∈ x → a ∈ y ⊦ x = [], dups(x), dups(y)
Used simplifier rules: ⊦ # x = 0 ↔ x = []; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic% ⊦ m ≤ n ↔ ¬ n < m; (ss)