exfalso
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
⊢ n = x
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
⊢ False