-- Lean 4 proof step from Mathlib.Probability.Independence.Kernel.Indep (goal)
Goal:
α : Type u_1
Ω : Type u_2
_mα : MeasurableSpace α
m₂ m : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
inst : IsZeroOrMarkovKernel κ
p1 p2 : Set (Set Ω)
h2 : m₂ ≤ m
hp2 : IsPiSystem p2
hpm2 : m₂ = generateFrom p2
hyp : IndepSets p1 p2 κ μ
t1 t2 : Set Ω
ht1 : t1 ∈ p1
ht1m : MeasurableSet t1
ht2m : MeasurableSet t2
h : IsMarkovKernel κ
inr :
  ∀ᵐ (a : α) ∂μ,
    (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ ∅) =
      HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
        ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) ∅ : ℝ≥0∞)
inr₁ :
  ∀ (u : Set Ω) (hu : u ∈ p2),
    ∀ᵐ (a : α) ∂μ,
      (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ u) =
        HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
          ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) u : ℝ≥0∞)
inr₂ :
  ∀ (u : Set Ω) (hu : MeasurableSet u)
    (ihu :
      ∀ᵐ (a : α) ∂μ,
        (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ u) =
          HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
            ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) u : ℝ≥0∞)),
    ∀ᵐ (a : α) ∂μ,
      (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ uᶜ) =
        HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
          ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) uᶜ : ℝ≥0∞)
⊢ ∀ᵐ (a : α) ∂μ,
    (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ t2) =
      HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
        ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t2 : ℝ≥0∞)
have inr₃ : ∀ (f : ℕ → Set Ω) (hfd : Pairwise (Function.onFun Disjoint f)) (hfm : ∀ (i : ℕ), MeasurableSet (f i))
  (ihf :
    ∀ (i : ℕ),
      ∀ᵐ (a : α) ∂μ,
        (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ f i) =
          HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
            ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (f i) : ℝ≥0∞)),
  ∀ᵐ (a : α) ∂μ,
    (((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (t1 ∩ ⋃ (i : ℕ), f i) =
      HMul.hMul (α := ℝ≥0∞) (β := ℝ≥0∞) ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) t1 : ℝ≥0∞)
        ((((κ : (a : α) → Measure Ω) a : Measure Ω) : (a : Set Ω) → ℝ≥0∞) (⋃ (i : ℕ), f i) : ℝ≥0∞)
Tactic: induction t2, ht2m using induction_on_inter hpm2 hp2 with
  | empty => simp
  | basic u hu => exact hyp t1 u ht1 hu
  | compl u hu ihu =>
    filter_upwards [ihu] with a ha
    rw [← Set.diff_eq, ← Set.diff_self_inter,
      measure_diff inter_subset_left (ht1m.inter (h2 _ hu)).nullMeasurableSet (measure_ne_top _ _),
      ha, measure_compl (h2 _ hu) (measure_ne_top _ _), measure_univ, ENNReal.mul_sub, mul_one]
    exact fun _ _ ↦ measure_ne_top _ _
  | iUnion f hfd hfm ihf =>
    rw [← ae_all_iff] at ihf
    filter_upwards [ihf] with a ha
    rw [inter_iUnion, measure_iUnion, measure_iUnion hfd fun i ↦ h2 _ (hfm i)]
    · simp only [ENNReal.tsum_mul_left, ha]
    · exact hfd.mono fun i j h ↦ (h.inter_left' _).inter_right' _
    · exact fun i ↦ .inter ht1m (h2 _ <| hfm i)
Cleared hypotheses: ht2m