Module: Mathlib.Probability.ProductMeasure

Goal state before:
pp: ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ ∏ i ∈ s, (μ i) (if h : i ∈ s then t ⟨i, h⟩ else Set.univ) = ∏ i, (μ ↑i) (t i)

pp: case a
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ ∀ (i : ι), MeasurableSet (if h : i ∈ s then t ⟨i, h⟩ else Set.univ)

pp: case hf
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ Measurable s.restrict

pp: case hs
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ MeasurableSet (Set.univ.pi t)

Tactic:
← prod_attach,

Goal state after:
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ ∏ x ∈ s.attach, (μ ↑x) (if h : ↑x ∈ s then t ⟨↑x, h⟩ else Set.univ) = ∏ i, (μ ↑i) (t i)

case a
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ ∀ (i : ι), MeasurableSet (if h : i ∈ s then t ⟨i, h⟩ else Set.univ)

case hf
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ Measurable s.restrict

case hs
ι : Type u_1
X : ι → Type u_2
mX : (i : ι) → MeasurableSpace (X i)
μ : (i : ι) → Measure (X i)
hμ : ∀ (i : ι), IsProbabilityMeasure (μ i)
ν : Measure ((i : ι) → X i)
hν : ∀ (s : Finset ι) (t : (i : ι) → Set (X i)), (∀ (i : ι), MeasurableSet (t i)) → ν ((↑s).pi t) = ∏ i ∈ s, (μ i) (t i)
s : Finset ι
t : (i : ↥s) → Set (X ↑i)
ht : ∀ (i : ↥s), MeasurableSet (t i)
⊢ MeasurableSet (Set.univ.pi t)