. next x_ge_n =>
  exfalso
  have h_range_size : (Array.range n).size = n := Array.range_size
  simp only [Array.mkEmpty_eq, Array.range, flip] at h_range_size
  simp only [Array.mkEmpty_eq, h_range_size] at x_ge_n
  exact x_ge_n x_lt_n
case inl.isFalse
x n : Nat
h : x < n + 1
x_lt_n : x < n
ih : (Nat.fold (flip Array.push) n (Array.mkEmpty n))[x] = x
h✝ : ¬x < (Nat.fold (flip Array.push) n (Array.mkEmpty (n + 1))).size
⊢ n = x
no goals