theorem extracted_formal_statement_51.{w, v, u} {Z : Type w} {ι : Type v} {f : ι → Z → ℝ} {Ω : Type u}
  [inst : MeasurableSpace Ω] {X : Ω → Z} [inst_1 : Countable ι] (h𝓕 : ∀ (I : ι), Measurable (f I ∘ X)) {n : ℕ} {I : ι}
  (i : Fin n) (a : i ∈ Finset.univ) : Measurable fun a => f I (X (a i).1) - f I (X (a i).2) := sorry

State before:
Z : Type w
ι : Type v
f : ι → Z → ℝ
Ω : Type u
inst✝¹ : MeasurableSpace Ω
X : Ω → Z
inst✝ : Countable ι
h𝓕 : ∀ (I : ι), Measurable (f I ∘ X)
n : ℕ
I : ι
i : Fin n
a✝ : i ∈ Finset.univ
⊢ Measurable fun a => f I (X (a i).1) - f I (X (a i).2)

Tactic: exact measurable_sub_part h𝓕

State after:

Relevant lemmas:
measurable_sub_part : ∀ {Z : Type w} {ι : Type v} {f : ι → Z → ℝ} {Ω : Type u} [inst : MeasurableSpace Ω] {X : Ω → Z}
  [inst_2 : Countable ι],
  (∀ (I : ι), Measurable (f I ∘ X)) →
    ∀ {n : ℕ} {I : ι} {i : Fin n}, Measurable fun ω => f I (X (ω i).1) - f I (X (ω i).2)