⊦ n ≤ # x ∧ ¬ dups(x + y) → ¬ dups(firstn(n, x) + y)
Used simplifier rules: ⊦ m ≤ n ↔ ¬ n < m; (ss)% ⊦ m < n → ¬ n < m; (s) Used terms: y, x, n