-- Mathlib/Probability/Kernel/Disintegration/Density.lean
lemma densityProcess_fst_univ_ae (κ : Kernel α (γ × β)) [IsFiniteKernel κ] (n : ℕ) (a : α) :
    ∀ᵐ x ∂(fst κ a), densityProcess κ (fst κ) n a x univ = 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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
s : Set γ
hs : s ∈ countablePartition γ n
h : (κ.fst a) s = 0
⊢ MeasurableSet (⋃ (_ : (κ.fst a) s = 0), s)

Tactic:
simp [h, measurableSet_countablePartition n hs]

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
hx : ¬(if (κ.fst a) (countablePartitionSet n x) = 0 then 0 else 1) = 1
⊢ (κ.fst a) (countablePartitionSet n x) = 0

Tactic:
simpa using hx

Result:
no goals

Proof state:
case hd
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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
⊢ (countablePartition γ n).PairwiseDisjoint fun u => ⋃ (_ : (κ.fst a) u = 0), u

Tactic:
intro s hs t ht hst

Result:
case hd
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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
s : Set γ
hs : s ∈ countablePartition γ n
t : Set γ
ht : t ∈ countablePartition γ n
hst : s ≠ t
⊢ Function.onFun Disjoint (fun u => ⋃ (_ : (κ.fst a) u = 0), u) s t

Proof state:
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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
s : Set γ
hs : s ∈ countablePartition γ n
⊢ MeasurableSet (⋃ (_ : (κ.fst a) s = 0), s)

Tactic:
by_cases h : fst κ a s = 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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
s : Set γ
hs : s ∈ countablePartition γ n
h : (κ.fst a) s = 0
⊢ MeasurableSet (⋃ (_ : (κ.fst a) s = 0), s)

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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
s : Set γ
hs : s ∈ countablePartition γ n
h : ¬(κ.fst a) s = 0
⊢ MeasurableSet (⋃ (_ : (κ.fst a) s = 0), s)

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
this : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
⊢ (κ.fst a) {x | (κ.fst a) (countablePartitionSet n x) = 0} = 0

Tactic:
have : {x | fst κ a (countablePartitionSet n x) = 0}
    ⊆ ⋃ (u) (_ : u ∈ countablePartition γ n) (_ : fst κ a u = 0), u := by
  intro t ht
  simp only [mem_setOf_eq, mem_iUnion, exists_prop] at ht ⊢
  exact ⟨countablePartitionSet n t, countablePartitionSet_mem _ _, ht,
    mem_countablePartitionSet _ _⟩

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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
⊢ (κ.fst a) {x | (κ.fst a) (countablePartitionSet n x) = 0} = 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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
⊢ (κ.fst a) {x | (κ.fst a) (countablePartitionSet n x) = 0} = 0

Tactic:
refine measure_mono_null this ?_

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
this✝ : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
this : {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u
⊢ (κ.fst a) (⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u) = 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
this : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
⊢ {x | (κ.fst a) (countablePartitionSet n x) = 0} ⊆ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u

Tactic:
intro t ht

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
this : {x | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}
t : γ
ht : t ∈ {x | (κ.fst a) (countablePartitionSet n x) = 0}
⊢ t ∈ ⋃ u ∈ countablePartition γ n, ⋃ (_ : (κ.fst a) u = 0), u

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 | ¬κ.densityProcess κ.fst n a x univ = 1} ⊆ {x | (κ.fst a) (countablePartitionSet n x) = 0}

Tactic:
intro x hx

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
hx : x ∈ {x | ¬κ.densityProcess κ.fst n a x univ = 1}
⊢ x ∈ {x | (κ.fst a) (countablePartitionSet n x) = 0}