-- Mathlib/Probability/Independence/Kernel.lean
/-- The probability of an intersection of preimages conditioning on another intersection factors
into a product. -/
lemma iIndepFun.cond_iInter [Finite ι] (hY : ∀ i, Measurable (Y i))
    (hindep : iIndepFun (fun _ ↦ mα.prod mβ) (fun i ω ↦ (X i ω, Y i ω)) κ μ)
    (hf : ∀ i ∈ s, MeasurableSet[mα.comap (X i)] (f i))
    (hy : ∀ᵐ a ∂μ, ∀ i ∉ s, κ a (Y i ⁻¹' t i) ≠ 0) (ht : ∀ i, MeasurableSet (t i)) :
    ∀ᵐ a ∂μ, (κ a)[⋂ i ∈ s, f i | ⋂ i, Y i ⁻¹' t i] = ∏ i ∈ s, (κ a)[f i | Y i in t i]

Proof state:
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
i : ι
hi : i ∈ s
A : Set α
hA : MeasurableSet A
hA' : X i ⁻¹' A = f i
⊢ (fun ω => (X i ω, Y i ω)) ⁻¹' A ×ˢ univ = f i

Tactic:
ext

Result:
case h
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
i : ι
hi : i ∈ s
A : Set α
hA : MeasurableSet A
hA' : X i ⁻¹' A = f i
x✝ : Ω
⊢ x✝ ∈ (fun ω => (X i ω, Y i ω)) ⁻¹' A ×ˢ univ ↔ x✝ ∈ f i

Proof state:
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy✝ : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt✝ : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
hg✝ : ∀ (i : ι), MeasurableSet (g i)
a : α
hy : ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
a✝ : IsProbabilityMeasure (κ a)
hYt : (κ a) (⋂ i, Y i ⁻¹' t i) = ∏ i : ι, (κ a) (Y i ⁻¹' t i)
hg : (κ a) (⋂ i, g i) = ∏ i : ι, (κ a) (g i)
⊢ ((⋂ i, Y i ⁻¹' t i) ∩ ⋂ i, if i ∈ s then f i else univ) = ⋂ i, Y i ⁻¹' t i ∩ if i ∈ s then f i else univ

Tactic:
rw [Set.iInter_inter_distrib]

Result:
no goals

Proof state:
case pos
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
i : ι
hi : i ∈ s
⊢ MeasurableSet (Y i ⁻¹' t i ∩ f i)

Tactic:
obtain ⟨A, hA, hA'⟩ := hf i hi

Result:
case pos.intro.intro
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
i : ι
hi : i ∈ s
A : Set α
hA : MeasurableSet A
hA' : X i ⁻¹' A = f i
⊢ MeasurableSet (Y i ⁻¹' t i ∩ f i)

Proof state:
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy✝ : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt✝ : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
hg✝ : ∀ (i : ι), MeasurableSet (g i)
a : α
hy : ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
a✝ : IsProbabilityMeasure (κ a)
hYt : (κ a) (⋂ i, Y i ⁻¹' t i) = ∏ i : ι, (κ a) (Y i ⁻¹' t i)
hg : (κ a) (⋂ i, g i) = ∏ i : ι, (κ a) (g i)
⊢ (↑Finset.univ).Pairwise fun i j => (κ a) (Y i ⁻¹' t i) ≠ 0 ∨ (κ a) (Y j ⁻¹' t j) ≠ ⊤

Tactic:
exact fun _ _ i _ _ ↦ .inr <| measure_ne_top _ _

Result:
no goals

Proof state:
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy✝ : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt✝ : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
hg✝ : ∀ (i : ι), MeasurableSet (g i)
a : α
hy : ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
a✝ : IsProbabilityMeasure (κ a)
hYt : (κ a) (⋂ i, Y i ⁻¹' t i) = ∏ i : ι, (κ a) (Y i ⁻¹' t i)
hg : (κ a) (⋂ i, g i) = ∏ i : ι, (κ a) (g i)
⊢ (⋂ i, Y i ⁻¹' t i) ∩ ⋂ i ∈ s, f i = (⋂ i, Y i ⁻¹' t i) ∩ ⋂ i, if i ∈ s then f i else univ

Tactic:
congr

Result:
case e_a
ι : Type u_4
Ω : Type u_5
α : Type u_6
β : Type u_7
mΩ : MeasurableSpace Ω
mα : MeasurableSpace α
mβ : MeasurableSpace β
κ : Kernel α Ω
μ : Measure α
X : ι → Ω → α
Y : ι → Ω → β
f : ι → Set Ω
t : ι → Set β
s : Finset ι
inst✝ : Finite ι
hY : ∀ (i : ι), Measurable (Y i)
hindep : iIndepFun (fun x => mα.prod mβ) (fun i ω => (X i ω, Y i ω)) κ μ
hf : ∀ i ∈ s, MeasurableSet (f i)
hy✝ : ∀ᵐ (a : α) ∂μ, ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
ht : ∀ (i : ι), MeasurableSet (t i)
val✝ : Fintype ι
g : ι → Set Ω := fun i' => if i' ∈ s then Y i' ⁻¹' t i' ∩ f i' else Y i' ⁻¹' t i'
hYt✝ : ∀ (i : ι), MeasurableSet (Y i ⁻¹' t i)
hg✝ : ∀ (i : ι), MeasurableSet (g i)
a : α
hy : ∀ i ∉ s, (κ a) (Y i ⁻¹' t i) ≠ 0
a✝ : IsProbabilityMeasure (κ a)
hYt : (κ a) (⋂ i, Y i ⁻¹' t i) = ∏ i : ι, (κ a) (Y i ⁻¹' t i)
hg : (κ a) (⋂ i, g i) = ∏ i : ι, (κ a) (g i)
⊢ ⋂ i ∈ s, f i = ⋂ i, if i ∈ s then f i else univ