-- Lean 4 proof step from Mathlib.Probability.Kernel.Disintegration.CDFToKernel (goal)
Goal:
α : Type u_1
β : Type u_2
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel.{u_1, u_2} α (β × ℝ)
ν : Kernel α β
f : α × β → StieltjesFunction ℝ
inst : IsFiniteKernel κ
hf : IsCondKernelCDF f κ ν
a : α
s : Set β
hs : MeasurableSet s
t : Set ℝ
ht : MeasurableSet t
empty :
  ∫⁻ (b : β) in s,
      (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
        ∅ ∂(ν : (a : α) → Measure β) a =
    (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞) (SProd.sprod (β := Set ℝ) s ∅)
basic :
  ∀ (t : Set ℝ) (ht : Membership.mem (γ := Set (Set ℝ)) (range Iic) t),
    ∫⁻ (b : β) in s,
        (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
          t ∂(ν : (a : α) → Measure β) a =
      (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞) (s ×ˢ t)
compl :
  ∀ (t : Set ℝ) (ht : MeasurableSet t)
    (iht :
      ∫⁻ (b : β) in s,
          (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
            t ∂(ν : (a : α) → Measure β) a =
        (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞) (s ×ˢ t)),
    ∫⁻ (b : β) in s,
        (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
          tᶜ ∂(ν : (a : α) → Measure β) a =
      (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞) (s ×ˢ tᶜ)
⊢ ∫⁻ (b : β) in s,
      (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
        t ∂(ν : (a : α) → Measure β) a =
    (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞) (s ×ˢ t)
have iUnion : ∀ (f_1 : ℕ → Set ℝ) (hf_disj : Pairwise (Function.onFun Disjoint f_1)) (hf_meas : ∀ (i : ℕ), MeasurableSet (f_1 i))
  (ihf :
    ∀ (i : ℕ),
      ∫⁻ (b : β) in s,
          (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
            (f_1 i) ∂(ν : (a : α) → Measure β) a =
        (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞) (s ×ˢ f_1 i)),
  ∫⁻ (b : β) in s,
      (((IsCondKernelCDF.toKernel f hf : (a : α × β) → Measure ℝ) (a, b) : Measure ℝ) : (a : Set ℝ) → ℝ≥0∞)
        (⋃ (i : ℕ), f_1 i) ∂(ν : (a : α) → Measure β) a =
    (((κ : (a : α) → Measure (β × ℝ)) a : Measure (β × ℝ)) : (a : Set (β × ℝ)) → ℝ≥0∞)
      (SProd.sprod (β := Set ℝ) s (⋃ (i : ℕ), f_1 i))
Tactic: induction t, ht
    using MeasurableSpace.induction_on_inter (borel_eq_generateFrom_Iic ℝ) isPiSystem_Iic with
  | empty => simp only [measure_empty, lintegral_const, zero_mul, prod_empty]
  | basic t ht =>
    obtain ⟨q, rfl⟩ := ht
    exact setLIntegral_toKernel_Iic hf a _ hs
  | compl t ht iht =>
    calc ∫⁻ b in s, hf.toKernel f (a, b) tᶜ ∂(ν a)
      = ∫⁻ b in s, hf.toKernel f (a, b) univ - hf.toKernel f (a, b) t ∂(ν a) := by
          congr with x; rw [measure_compl ht (measure_ne_top (hf.toKernel f (a, x)) _)]
    _ = ∫⁻ b in s, hf.toKernel f (a, b) univ ∂(ν a)
          - ∫⁻ b in s, hf.toKernel f (a, b) t ∂(ν a) := by
        rw [lintegral_sub]
        · exact (Kernel.measurable_coe (hf.toKernel f) ht).comp measurable_prodMk_left
        · rw [iht]
          exact measure_ne_top _ _
        · exact Eventually.of_forall fun a ↦ measure_mono (subset_univ _)
    _ = κ a (s ×ˢ univ) - κ a (s ×ˢ t) := by
        rw [setLIntegral_toKernel_univ hf a hs, iht]
    _ = κ a (s ×ˢ tᶜ) := by
        rw [← measure_diff _ (hs.prod ht).nullMeasurableSet (measure_ne_top _ _)]
        · rw [prod_diff_prod, compl_eq_univ_diff]
          simp only [diff_self, empty_prod, union_empty]
        · rw [prod_subset_prod_iff]
          exact Or.inl ⟨subset_rfl, subset_univ t⟩
  | iUnion f hf_disj hf_meas ihf =>
    simp_rw [measure_iUnion hf_disj hf_meas]
    rw [lintegral_tsum, prod_iUnion, measure_iUnion]
    · simp_rw [ihf]
    · exact hf_disj.mono fun i j h ↦ h.set_prod_right _ _
    · exact fun i ↦ MeasurableSet.prod hs (hf_meas i)
    · exact fun i ↦
        ((Kernel.measurable_coe _ (hf_meas i)).comp measurable_prodMk_left).aemeasurable.restrict
Cleared hypotheses: ht