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