have ab_disjoint : Disjoint A B := by
  intro _C hca hcb c hc
  exact Set.not_mem_of_mem_diff (hca hc) (Set.image_subset f sdiff_le (hcb hc))
case intro
m : ℕ
f : ℕ → ℕ
hf : ∀ (n : ℕ), f (f n) = n + (2 * m + 1)
f_injective : Function.Injective f
NN : Set ℕ := Set.univ
A : Set ℕ := NN \ f '' NN
B : Set ℕ := f '' A
hid : B = f '' NN \ f '' (f '' NN)
⊢ False
case intro
m : ℕ
f : ℕ → ℕ
hf : ∀ (n : ℕ), f (f n) = n + (2 * m + 1)
f_injective : Function.Injective f
NN : Set ℕ := Set.univ
A : Set ℕ := NN \ f '' NN
B : Set ℕ := f '' A
hid : B = f '' NN \ f '' (f '' NN)
ab_disjoint : Disjoint A B
⊢ False