theorem extracted_formal_statement_111.{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_3 : Countable ι]
  (hf : ∀ (i : ι), Measurable (f i ∘ X)) (i : ι)
  (h_original_goal : StronglyMeasurable (Function.uncurry fun x y => ∑ k, (f i (X (y k)) - f i (X (x k))))) :
  Measurable (Function.uncurry fun x y => ∑ k, (f i (X (y k)) - f i (X (x k)))) := sorry

State before:
case hf.left.hf.hf.hg.hf.hf.hf
Ω : Type u
inst✝¹ : MeasurableSpace Ω
Z : Type w
n : ℕ
ι : Type u_1
f : ι → Z → ℝ
μ : Measure Ω
inst✝ : IsProbabilityMeasure μ
X : Ω → Z
inst_3 : Countable ι
hf : ∀ (i : ι), Measurable (f i ∘ X)
i : ι
⊢ StronglyMeasurable (Function.uncurry fun x y => ∑ k, (f i (X (y k)) - f i (X (x k))))

Tactic: apply Measurable.stronglyMeasurable

State after:
[case hf.left.hf.hf.hg.hf.hf.hf.hf
 Ω : Type u
 inst✝¹ : MeasurableSpace Ω
 Z : Type w
 n : ℕ
 ι : Type u_1
 f : ι → Z → ℝ
 μ : Measure Ω
 inst✝ : IsProbabilityMeasure μ
 X : Ω → Z
 inst_3 : Countable ι
 hf : ∀ (i : ι), Measurable (f i ∘ X)
 i : ι
 ⊢ Measurable (Function.uncurry fun x y => ∑ k, (f i (X (y k)) - f i (X (x k))))]

Relevant lemmas:
Measurable.stronglyMeasurable : ∀ {α : Type u_1} {β : Type u_2} {f : α → β} {mα : MeasurableSpace α}
  [inst : MeasurableSpace β] [inst_2 : TopologicalSpace β] [inst_4 : TopologicalSpace.PseudoMetrizableSpace β]
  [inst_5 : SecondCountableTopology β] [inst_6 : OpensMeasurableSpace β], Measurable f → StronglyMeasurable f