-- Mathlib/Probability/Kernel/Disintegration/Density.lean
lemma densityProcess_fst_univ [IsFiniteKernel κ] (n : ℕ) (a : α) (x : γ) :
    densityProcess κ (fst κ) n a x univ
      = if fst κ a (countablePartitionSet n x) = 0 then 0 else 1

Proof state:
case pos
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : (κ.fst a) (countablePartitionSet n x) = 0
⊢ ((κ a) (countablePartitionSet n x ×ˢ univ) / 0).toReal = 0

Tactic:
by_cases h' : κ a (countablePartitionSet n x ×ˢ univ) = 0

Result:
case pos
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : (κ.fst a) (countablePartitionSet n x) = 0
h' : (κ a) (countablePartitionSet n x ×ˢ univ) = 0
⊢ ((κ a) (countablePartitionSet n x ×ˢ univ) / 0).toReal = 0

case neg
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : (κ.fst a) (countablePartitionSet n x) = 0
h' : ¬(κ a) (countablePartitionSet n x ×ˢ univ) = 0
⊢ ((κ a) (countablePartitionSet n x ×ˢ univ) / 0).toReal = 0

Proof state:
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : ¬(κ.fst a) (countablePartitionSet n x) = 0
⊢ countablePartitionSet n x ×ˢ univ = {p | p.1 ∈ countablePartitionSet n x}

Tactic:
ext x

Result:
case h
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x✝ : γ
h : ¬(κ.fst a) (countablePartitionSet n x✝) = 0
x : γ × β
⊢ x ∈ countablePartitionSet n x✝ ×ˢ univ ↔ x ∈ {p | p.1 ∈ countablePartitionSet n x✝}

Proof state:
case neg.h0
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : ¬(κ.fst a) (countablePartitionSet n x) = 0
this : countablePartitionSet n x ×ˢ univ = {p | p.1 ∈ countablePartitionSet n x}
⊢ (κ a) {p | p.1 ∈ countablePartitionSet n x} ≠ 0

Tactic:
rwa [fst_apply' _ _ (measurableSet_countablePartitionSet _ _)] at h

Result:
no goals

Proof state:
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
⊢ κ.densityProcess κ.fst n a x univ = if (κ.fst a) (countablePartitionSet n x) = 0 then 0 else 1

Tactic:
rw [densityProcess]

Result:
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
⊢ ((κ a) (countablePartitionSet n x ×ˢ univ) / (κ.fst a) (countablePartitionSet n x)).toReal =
    if (κ.fst a) (countablePartitionSet n x) = 0 then 0 else 1

Proof state:
case pos
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : (κ.fst a) (countablePartitionSet n x) = 0
⊢ ((κ a) (countablePartitionSet n x ×ˢ univ) / (κ.fst a) (countablePartitionSet n x)).toReal = 0

Tactic:
simp only [h]

Result:
case pos
α : Type u_1
β : Type u_2
γ : Type u_3
mα : MeasurableSpace α
mβ : MeasurableSpace β
mγ : MeasurableSpace γ
inst✝¹ : CountablyGenerated γ
κ : Kernel α (γ × β)
inst✝ : IsFiniteKernel κ
n : ℕ
a : α
x : γ
h : (κ.fst a) (countablePartitionSet n x) = 0
⊢ ((κ a) (countablePartitionSet n x ×ˢ univ) / 0).toReal = 0