-- Lean 4 proof step from Mathlib.MeasureTheory.Measure.Decomposition.Lebesgue (goal)
Goal:
α : Type u_1
m : MeasurableSpace α
μ ν : Measure α
inst : IsFiniteMeasure μ
inst_1 : IsFiniteMeasure ν
h : ¬μ ⟂ₘ ν
f : (n : ℕ) → Set α
hf₁ : ∀ (n : ℕ), MeasurableSet (f n)
hf₂ :
  ∀ (n : ℕ) (t : Set α),
    MeasurableSet t →
      LE.le (α := ℝ≥0∞) (((1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) • ν : (a : Set α) → ℝ≥0∞) (t ∩ f n) : ℝ≥0∞)
        ((μ : (a : Set α) → ℝ≥0∞) (t ∩ f n) : ℝ≥0∞)
hf₃ :
  ∀ (n : ℕ) (t : Set α),
    MeasurableSet t →
      LE.le (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (t ∩ (f n)ᶜ) : ℝ≥0∞)
        (((1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) • ν : (a : Set α) → ℝ≥0∞) (t ∩ (f n)ᶜ) : ℝ≥0∞)
A : Set α := ⋂ (n : ℕ), (f n)ᶜ
hAmeas : MeasurableSet A
hA₂ :
  ∀ (n : ℕ) (t : Set α) (ht : MeasurableSet t),
    LE.le (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (t ∩ A) : ℝ≥0∞)
      (((1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) • ν : (a : Set α) → ℝ≥0∞) (t ∩ A) : ℝ≥0∞)
hA₃ :
  ∀ (n : ℕ),
    (μ : (a : Set α) → ℝ≥0∞) A ≤
      HMul.hMul (β := ℝ≥0∞) (↑(1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) A : ℝ≥0∞)
hμ : Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) A : ℝ≥0∞) (0 : ℝ≥0∞)
⊢ ∃ (ε : ℝ≥0),
    0 < ε ∧
      ∃ (E : Set α),
        MeasurableSet E ∧
          LT.lt (α := ℝ≥0∞) (0 : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) E : ℝ≥0∞) ∧
            ∀ (A : Set α),
              MeasurableSet A →
                HMul.hMul (β := ℝ≥0∞) (↑ε : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) (A ∩ E) : ℝ≥0∞) ≤
                  (μ : (a : Set α) → ℝ≥0∞) (A ∩ E)
have rw : ∀
  (h :
    ¬∃ (s : Set α),
        MeasurableSet s ∧
          Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) s : ℝ≥0∞) (0 : ℝ≥0∞) ∧
            Eq (α := ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) sᶜ : ℝ≥0∞) (0 : ℝ≥0∞))
  (f : (n : ℕ) → Set α) (hf₁ : ∀ (n : ℕ), MeasurableSet (f n))
  (hf₂ :
    ∀ (n : ℕ) (t : Set α),
      MeasurableSet t →
        LE.le (α := ℝ≥0∞) (((1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) • ν : (a : Set α) → ℝ≥0∞) (t ∩ f n) : ℝ≥0∞)
          ((μ : (a : Set α) → ℝ≥0∞) (t ∩ f n) : ℝ≥0∞))
  (hf₃ :
    ∀ (n : ℕ) (t : Set α),
      MeasurableSet t →
        LE.le (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (t ∩ (f n)ᶜ) : ℝ≥0∞)
          (((1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) • ν : (a : Set α) → ℝ≥0∞) (t ∩ (f n)ᶜ) : ℝ≥0∞)),
  let A : Set α := ⋂ (n : ℕ), (f n)ᶜ;
  ∀ (hAmeas : MeasurableSet A)
    (hA₂ :
      ∀ (n : ℕ) (t : Set α) (ht : MeasurableSet t),
        LE.le (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) (t ∩ A) : ℝ≥0∞)
          (((1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) • ν : (a : Set α) → ℝ≥0∞) (t ∩ A) : ℝ≥0∞))
    (hA₃ :
      ∀ (n : ℕ),
        (μ : (a : Set α) → ℝ≥0∞) A ≤
          HMul.hMul (β := ℝ≥0∞) (↑(1 / HAdd.hAdd (α := ℝ≥0) (↑n : ℝ≥0) 1) : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) A : ℝ≥0∞))
    (hμ : Eq (α := ℝ≥0∞) ((μ : (a : Set α) → ℝ≥0∞) A : ℝ≥0∞) (0 : ℝ≥0∞)),
    ∃ (ε : ℝ≥0),
      0 < ε ∧
        ∃ (E : Set α),
          MeasurableSet E ∧
            LT.lt (α := ℝ≥0∞) (0 : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) E : ℝ≥0∞) ∧
              ∀ (A : Set α),
                MeasurableSet A →
                  HMul.hMul (β := ℝ≥0∞) (↑ε : ℝ≥0∞) ((ν : (a : Set α) → ℝ≥0∞) (A ∩ E) : ℝ≥0∞) ≤
                    (μ : (a : Set α) → ℝ≥0∞) (A ∩ E)
Tactic: rw [MutuallySingular] at h
Cleared hypotheses: h, f, hf₁, hf₂, hf₃, A, hAmeas, hA₂, hA₃, hμ