-- Lean 4 proof step from Mathlib.MeasureTheory.Measure.Decomposition.Hahn (hypothesis)
Goal:
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
inst : IsFiniteMeasure μ
inst_1 : IsFiniteMeasure ν
d : Set α → ℝ :=
  fun (s : Set α) ↦
    (↑(ENNReal.toNNReal ((μ : (a : Set α) → ℝ≥0∞) s)) : ℝ) - (↑(ENNReal.toNNReal ((ν : (a : Set α) → ℝ≥0∞) s)) : ℝ)
c : Set ℝ := d '' {s : Set α | MeasurableSet s}
γ : ℝ := sSup c
hμ : ∀ (s : Set α), Ne (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (∞ : ℝ≥0∞)
hν : ∀ (s : Set α), Ne (α := ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (∞ : ℝ≥0∞)
to_nnreal_μ : ∀ (s : Set α), (↑(ENNReal.toNNReal ((μ : (a : Set α) → ℝ≥0∞) s)) : ℝ≥0∞) = (μ : (a : Set α) → ℝ≥0∞) s
to_nnreal_ν : ∀ (s : Set α), (↑(ENNReal.toNNReal ((ν : (a : Set α) → ℝ≥0∞) s)) : ℝ≥0∞) = (ν : (a : Set α) → ℝ≥0∞) s
d_split : ∀ (s t : Set α) (ht : MeasurableSet t), d s = d (s \ t) + d (s ∩ t)
d_Union : ∀ (s : ℕ → Set α) (hm : Monotone s), Tendsto (fun (n : ℕ) ↦ d (s n)) atTop (𝓝 (d (⋃ (n : ℕ), s n)))
d_Inter :
  ∀ (s : ℕ → Set α) (hs : ∀ (n : ℕ), MeasurableSet (s n)) (hm : ∀ (n m : ℕ), n ≤ m → s m ⊆ s n),
    Tendsto (fun (n : ℕ) ↦ d (s n)) atTop (𝓝 (d (⋂ (n : ℕ), s n)))
bdd_c : BddAbove c
c_nonempty : c.Nonempty
d_le_γ : ∀ (s : Set α), MeasurableSet s → d s ≤ γ
this : ∀ (n : ℕ), ∃ (s : Set α), MeasurableSet s ∧ γ - (1 / 2) ^ n < d s
e : ℕ → Set α
he : ∀ (x : ℕ), MeasurableSet (e x) ∧ γ - (1 / 2) ^ x < d (e x)
he₁ : ∀ (n : ℕ), MeasurableSet (e n)
he₂ : ∀ (n : ℕ), γ - (1 / 2) ^ n < d (e n)
f : ℕ → ℕ → Set α := fun (n m : ℕ) ↦ (Finset.Ico n (m + 1)).inf e
hf : ∀ (n m : ℕ), MeasurableSet (f n m)
f_subset_f : ∀ {a b c d : ℕ} (hab : a ≤ b) (hcd : c ≤ d), f a d ⊆ f b c
f_succ : ∀ (n m : ℕ) (hnm : n ≤ m), f n (m + 1) = f n m ∩ e (m + 1)
⊢ ∃ (s : Set α),
    MeasurableSet s ∧
      (∀ (t : Set α),
          MeasurableSet t →
            t ⊆ s → LE.le (α := ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) t : ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) t : ℝ≥0∞)) ∧
        ∀ (t : Set α),
          MeasurableSet t →
            t ⊆ sᶜ → LE.le (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) t : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) t : ℝ≥0∞)
have le_d_f : ∀ (n m : ℕ) (h : m ≤ n), γ - 2 * (1 / 2) ^ m + (1 / 2) ^ n ≤ d (f m n)
Tactic: have le_d_f n m (h : m ≤ n) : γ - 2 * (1 / 2) ^ m + (1 / 2) ^ n ≤ d (f m n) := by
    refine Nat.le_induction ?_ ?_ n h
    · have := he₂ m
      simp_rw [f, Nat.Ico_succ_singleton, Finset.inf_singleton]
      linarith
    · intro n (hmn : m ≤ n) ih
      refine le_of_add_le_add_left (a := γ) ?_
      calc
        γ + (γ - 2 * (1 / 2) ^ m + (1 / 2) ^ (n + 1)) =
            γ + (γ - 2 * (1 / 2) ^ m + ((1 / 2) ^ n - (1 / 2) ^ (n + 1))) := by
          rw [pow_succ, mul_one_div, _root_.sub_half]
        _ = γ - (1 / 2) ^ (n + 1) + (γ - 2 * (1 / 2) ^ m + (1 / 2) ^ n) := by
          simp only [sub_eq_add_neg]; abel
        _ ≤ d (e (n + 1)) + d (f m n) := add_le_add (le_of_lt <| he₂ _) ih
        _ ≤ d (e (n + 1)) + d (f m n \ e (n + 1)) + d (f m (n + 1)) := by
          rw [f_succ _ _ hmn, d_split (f m n) (e (n + 1)) (he₁ _), add_assoc]
        _ = d (e (n + 1) ∪ f m n) + d (f m (n + 1)) := by
          rw [d_split (e (n + 1) ∪ f m n) (e (n + 1)), union_diff_left, union_inter_cancel_left]
          · abel
          · exact he₁ _
        _ ≤ γ + d (f m (n + 1)) := by grw [d_le_γ _ <| (he₁ _).union (hf _ _)]