simp only [Array.mkEmpty_eq, Array.range, flip] at h_range_size
x n : Nat
h : x < n + 1
x_lt_n : x < n
ih : (Nat.fold (flip Array.push) n (Array.mkEmpty n))[x] = x
x_ge_n : ¬x < (Nat.fold (flip Array.push) n (Array.mkEmpty (n + 1))).size
h_range_size : (Array.range n).size = n
⊢ False
x n : Nat
h : x < n + 1
x_lt_n : x < n
ih : (Nat.fold (flip Array.push) n (Array.mkEmpty n))[x] = x
x_ge_n : ¬x < (Nat.fold (flip Array.push) n (Array.mkEmpty (n + 1))).size
h_range_size : (Nat.fold (flip Array.push) n #[]).size = n
⊢ False