# x < n1, a ∈ (n0 ≤ # x ⊃ firstn(n0, x);x), n0 ≤ n1 ⊦ a ∈ x
Used simplifier rules: ⊦ m < n → n ≠ 0; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic% ⊦ m ≤ n ↔ ¬ n < m; (ss)