n < # ar ⊦ # ar = 0, ∃ n0. n0 < # ar ∧ ar[n0] = ar[n]
Used simplifier rules: in: ⊦ a ∈ ar ↔ (∃ n. n < # ar ∧ ar[n] = a);%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1