-- Mathlib/Probability/Independence/Kernel.lean
theorem iIndepSets.iIndep (m : ι → MeasurableSpace Ω)
    (h_le : ∀ i, m i ≤ _mΩ) (π : ι → Set (Set Ω)) (h_pi : ∀ n, IsPiSystem (π n))
    (h_generate : ∀ i, m i = generateFrom (π i)) (h_ind : iIndepSets π κ μ) :
    iIndep m κ μ

Proof state:
case inr.intro.intro
α : Type u_1
Ω : Type u_2
ι : Type u_3
_mα : MeasurableSpace α
_mΩ : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
m : ι → MeasurableSpace Ω
h_le : ∀ (i : ι), m i ≤ _mΩ
π : ι → Set (Set Ω)
h_pi : ∀ (n : ι), IsPiSystem (π n)
h_generate : ∀ (i : ι), m i = generateFrom (π i)
h_ind : iIndepSets π κ μ
hμ : μ ≠ 0
η : Kernel α Ω
η_eq : ⇑κ =ᶠ[ae μ] ⇑η
hη : IsMarkovKernel η
s : Finset ι
f : ι → Set Ω
⊢ (∀ i ∈ s, f i ∈ (fun x => {s | MeasurableSet s}) i) → ∀ᵐ (a : α) ∂μ, (η a) (⋂ i ∈ s, f i) = ∏ i ∈ s, (η a) (f i)

Tactic:
refine Finset.induction ?_ ?_ s

Result:
case inr.intro.intro.refine_1
α : Type u_1
Ω : Type u_2
ι : Type u_3
_mα : MeasurableSpace α
_mΩ : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
m : ι → MeasurableSpace Ω
h_le : ∀ (i : ι), m i ≤ _mΩ
π : ι → Set (Set Ω)
h_pi : ∀ (n : ι), IsPiSystem (π n)
h_generate : ∀ (i : ι), m i = generateFrom (π i)
h_ind : iIndepSets π κ μ
hμ : μ ≠ 0
η : Kernel α Ω
η_eq : ⇑κ =ᶠ[ae μ] ⇑η
hη : IsMarkovKernel η
s : Finset ι
f : ι → Set Ω
⊢ (∀ i ∈ ∅, f i ∈ (fun x => {s | MeasurableSet s}) i) → ∀ᵐ (a : α) ∂μ, (η a) (⋂ i ∈ ∅, f i) = ∏ i ∈ ∅, (η a) (f i)

case inr.intro.intro.refine_2
α : Type u_1
Ω : Type u_2
ι : Type u_3
_mα : MeasurableSpace α
_mΩ : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
m : ι → MeasurableSpace Ω
h_le : ∀ (i : ι), m i ≤ _mΩ
π : ι → Set (Set Ω)
h_pi : ∀ (n : ι), IsPiSystem (π n)
h_generate : ∀ (i : ι), m i = generateFrom (π i)
h_ind : iIndepSets π κ μ
hμ : μ ≠ 0
η : Kernel α Ω
η_eq : ⇑κ =ᶠ[ae μ] ⇑η
hη : IsMarkovKernel η
s : Finset ι
f : ι → Set Ω
⊢ ∀ ⦃a : ι⦄ {s : Finset ι},
    a ∉ s →
      ((∀ i ∈ s, f i ∈ (fun x => {s | MeasurableSet s}) i) →
          ∀ᵐ (a : α) ∂μ, (η a) (⋂ i ∈ s, f i) = ∏ i ∈ s, (η a) (f i)) →
        (∀ i ∈ insert a s, f i ∈ (fun x => {s | MeasurableSet s}) i) →
          ∀ᵐ (a_3 : α) ∂μ, (η a_3) (⋂ i ∈ insert a s, f i) = ∏ i ∈ insert a s, (η a_3) (f i)

Proof state:
α : Type u_1
Ω : Type u_2
ι : Type u_3
_mα : MeasurableSpace α
_mΩ : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
m : ι → MeasurableSpace Ω
h_le : ∀ (i : ι), m i ≤ _mΩ
π : ι → Set (Set Ω)
h_pi : ∀ (n : ι), IsPiSystem (π n)
h_generate : ∀ (i : ι), m i = generateFrom (π i)
h_ind : iIndepSets π κ μ
hμ : μ ≠ 0
η : Kernel α Ω
η_eq : ⇑κ =ᶠ[ae μ] ⇑η
hη : IsMarkovKernel η
s : Finset ι
f : ι → Set Ω
a : ι
S : Finset ι
ha_notin_S : a ∉ S
h_rec : (∀ i ∈ S, f i ∈ (fun x => {s | MeasurableSet s}) i) → ∀ᵐ (a : α) ∂μ, (η a) (⋂ i ∈ S, f i) = ∏ i ∈ S, (η a) (f i)
hf_m : ∀ i ∈ insert a S, f i ∈ (fun x => {s | MeasurableSet s}) i
hf_m_S : ∀ x ∈ S, MeasurableSet (f x)
p : Set (Set Ω) := piiUnionInter π ↑S
m_p : MeasurableSpace Ω := generateFrom p
hS_eq_generate : m_p = generateFrom p
⊢ Indep m_p (m a) η μ

Tactic:
have hp : IsPiSystem p := isPiSystem_piiUnionInter π h_pi S

Result:
α : Type u_1
Ω : Type u_2
ι : Type u_3
_mα : MeasurableSpace α
_mΩ : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
m : ι → MeasurableSpace Ω
h_le : ∀ (i : ι), m i ≤ _mΩ
π : ι → Set (Set Ω)
h_pi : ∀ (n : ι), IsPiSystem (π n)
h_generate : ∀ (i : ι), m i = generateFrom (π i)
h_ind : iIndepSets π κ μ
hμ : μ ≠ 0
η : Kernel α Ω
η_eq : ⇑κ =ᶠ[ae μ] ⇑η
hη : IsMarkovKernel η
s : Finset ι
f : ι → Set Ω
a : ι
S : Finset ι
ha_notin_S : a ∉ S
h_rec : (∀ i ∈ S, f i ∈ (fun x => {s | MeasurableSet s}) i) → ∀ᵐ (a : α) ∂μ, (η a) (⋂ i ∈ S, f i) = ∏ i ∈ S, (η a) (f i)
hf_m : ∀ i ∈ insert a S, f i ∈ (fun x => {s | MeasurableSet s}) i
hf_m_S : ∀ x ∈ S, MeasurableSet (f x)
p : Set (Set Ω) := piiUnionInter π ↑S
m_p : MeasurableSpace Ω := generateFrom p
hS_eq_generate : m_p = generateFrom p
hp : IsPiSystem p
⊢ Indep m_p (m a) η μ

Proof state:
α : Type u_1
Ω : Type u_2
ι : Type u_3
_mα : MeasurableSpace α
_mΩ : MeasurableSpace Ω
κ : Kernel α Ω
μ : Measure α
m : ι → MeasurableSpace Ω
h_le : ∀ (i : ι), m i ≤ _mΩ
π : ι → Set (Set Ω)
h_pi : ∀ (n : ι), IsPiSystem (π n)
h_generate : ∀ (i : ι), m i = generateFrom (π i)
h_ind : iIndepSets π κ μ
hμ : μ ≠ 0
η : Kernel α Ω
η_eq : ⇑κ =ᶠ[ae μ] ⇑η
hη : IsMarkovKernel η
s : Finset ι
f : ι → Set Ω
a : ι
S : Finset ι
ha_notin_S : a ∉ S
h_rec : (∀ i ∈ S, f i ∈ (fun x => {s | MeasurableSet s}) i) → ∀ᵐ (a : α) ∂μ, (η a) (⋂ i ∈ S, f i) = ∏ i ∈ S, (η a) (f i)
hf_m : ∀ i ∈ insert a S, f i ∈ (fun x => {s | MeasurableSet s}) i
x : ι
hx : x ∈ S
⊢ x ∈ insert a S

Tactic:
simp [hx]

Result:
no goals