| ¬ dups(x), ¬ dups(y), ∀ a. a ∈ x → a ∈ y ⊦ # x ≤ # y |
# y < # x, ∀ a. a ∈ x → a ∈ y ⊦ x = [], dups(x), dups(y)
Used simplifier rules: l: ⊦ # x = 0 ↔ x = []; from specification list-data%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1%lels-01: ⊦ ¬ m ≤ n ↔ n < m; from specification nat