theorem extracted_formal_statement_78.{u_1, w, u} {Ω : Type u} [inst : MeasurableSpace Ω] {Z : Type w} {n : ℕ}
  {ι : Type u_1} {f : ι → Z → ℝ} {μ : Measure Ω} [inst_1 : IsProbabilityMeasure μ] {X : Ω → Z} [inst_2 : Nonempty ι]
  [inst_3 : Countable ι] (hf : ∀ (i : ι), Measurable (f i ∘ X)) {b : ℝ} (hb : b ≥ 0)
  (hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b) (hn : ¬n = 0)
  (hr : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), y ≤ C)
  (hr' : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), C ≤ y)
  (hq : ∀ (ω : Fin n → Ω) (k : Fin n), BddAbove (Set.range fun i => f i (X (ω k))))
  (hq' : ∀ (ω : Fin n → Ω) (k : Fin n), BddBelow (Set.range fun i => f i (X (ω k)))) :
  let mp := measurePreserving_arrowProdEquivProdArrow Ω Ω (Fin n) (fun x => μ) fun x => μ;
  ∀ (i : ι),
    ∀ i_1 ∈ Finset.univ,
      (Measurable fun x => f i (X (x.1 i_1)) - f i (X (x.2 i_1))) → Measurable fun a => f i (X (a.2 i_1)) := sorry

State before:
case hf.hf.hg.hf
Ω : Type u
inst✝³ : MeasurableSpace Ω
Z : Type w
n : ℕ
ι : Type u_1
f : ι → Z → ℝ
μ : Measure Ω
inst✝² : IsProbabilityMeasure μ
X : Ω → Z
inst✝¹ : Nonempty ι
inst✝ : Countable ι
hf : ∀ (i : ι), Measurable (f i ∘ X)
b : ℝ
hb : b ≥ 0
hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
hn : ¬n = 0
hr : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), y ≤ C
hr' : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), C ≤ y
hq : ∀ (ω : Fin n → Ω) (k : Fin n), BddAbove (Set.range fun i => f i (X (ω k)))
hq' : ∀ (ω : Fin n → Ω) (k : Fin n), BddBelow (Set.range fun i => f i (X (ω k)))
mp : MeasurePreserving (⇑(MeasurableEquiv.arrowProdEquivProdArrow Ω Ω (Fin n))) (Measure.pi fun i => μ.prod μ)
  ((Measure.pi fun i => μ).prod (Measure.pi fun i => μ)) :=
  measurePreserving_arrowProdEquivProdArrow Ω Ω (Fin n) (fun x => μ) fun x => μ
i : ι
i_1 : Fin n
hi_1 : i_1 ∈ Finset.univ
⊢ Measurable fun x => f i (X (x.1 i_1)) - f i (X (x.2 i_1))

Tactic: apply Measurable.sub
    apply (hf i).comp
    refine Measurable.eval ?_

State after:
[case hf.hf.hg.hf.hf
 Ω : Type u
 inst✝³ : MeasurableSpace Ω
 Z : Type w
 n : ℕ
 ι : Type u_1
 f : ι → Z → ℝ
 μ : Measure Ω
 inst✝² : IsProbabilityMeasure μ
 X : Ω → Z
 inst✝¹ : Nonempty ι
 inst✝ : Countable ι
 hf : ∀ (i : ι), Measurable (f i ∘ X)
 b : ℝ
 hb : b ≥ 0
 hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
 hn : ¬n = 0
 hr : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), y ≤ C
 hr' : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), C ≤ y
 hq : ∀ (ω : Fin n → Ω) (k : Fin n), BddAbove (Set.range fun i => f i (X (ω k)))
 hq' : ∀ (ω : Fin n → Ω) (k : Fin n), BddBelow (Set.range fun i => f i (X (ω k)))
 mp : MeasurePreserving (⇑(MeasurableEquiv.arrowProdEquivProdArrow Ω Ω (Fin n))) (Measure.pi fun i => μ.prod μ)
   ((Measure.pi fun i => μ).prod (Measure.pi fun i => μ)) :=
   measurePreserving_arrowProdEquivProdArrow Ω Ω (Fin n) (fun x => μ) fun x => μ
 i : ι
 i_1 : Fin n
 hi_1 : i_1 ∈ Finset.univ
 ⊢ Measurable Prod.fst,
 case hf.hf.hg.hf.hg
 Ω : Type u
 inst✝³ : MeasurableSpace Ω
 Z : Type w
 n : ℕ
 ι : Type u_1
 f : ι → Z → ℝ
 μ : Measure Ω
 inst✝² : IsProbabilityMeasure μ
 X : Ω → Z
 inst✝¹ : Nonempty ι
 inst✝ : Countable ι
 hf : ∀ (i : ι), Measurable (f i ∘ X)
 b : ℝ
 hb : b ≥ 0
 hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
 hn : ¬n = 0
 hr : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), y ≤ C
 hr' : ∃ C, ∀ (ω : Fin n → Ω) (k : Fin n), ∀ y ∈ Set.range fun i => f i (X (ω k)), C ≤ y
 hq : ∀ (ω : Fin n → Ω) (k : Fin n), BddAbove (Set.range fun i => f i (X (ω k)))
 hq' : ∀ (ω : Fin n → Ω) (k : Fin n), BddBelow (Set.range fun i => f i (X (ω k)))
 mp : MeasurePreserving (⇑(MeasurableEquiv.arrowProdEquivProdArrow Ω Ω (Fin n))) (Measure.pi fun i => μ.prod μ)
   ((Measure.pi fun i => μ).prod (Measure.pi fun i => μ)) :=
   measurePreserving_arrowProdEquivProdArrow Ω Ω (Fin n) (fun x => μ) fun x => μ
 i : ι
 i_1 : Fin n
 hi_1 : i_1 ∈ Finset.univ
 ⊢ Measurable fun a => f i (X (a.2 i_1))]

Relevant lemmas:
Measurable.eval : ∀ {α : Type u_1} {δ : Type u_4} {X : δ → Type u_6} [inst : MeasurableSpace α]
  [inst_4 : (a : δ) → MeasurableSpace (X a)] {a : δ} {g : α → (a : δ) → X a}, Measurable g → Measurable fun x => g x a
Measurable.sub : ∀ {G : Type u_2} {α : Type u_3} [inst : MeasurableSpace G] [inst_4 : Sub G] {m : MeasurableSpace α}
  {f g : α → G} [inst_5 : MeasurableSub₂ G], Measurable f → Measurable g → Measurable fun a => f a - g a