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