theorem extracted_formal_statement_54.{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} (hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b)
  (h_original_goal :
    AEStronglyMeasurable (fun x => ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (x.2 k))|)
      ((Measure.pi fun x => μ).prod (Measure.pi fun x => μ)))
  (i : Signs n) : i ∈ Finset.univ → Measurable fun x => ⨆ i_1, |∑ k, ↑↑(i k) * f i_1 (X (x.2 k))| := sorry

State before:
case left.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
hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
⊢ AEStronglyMeasurable (fun x => ∑ σ, ⨆ i, |∑ k, ↑↑(σ k) * f i (X (x.2 k))|)
    ((Measure.pi fun x => μ).prod (Measure.pi fun x => μ))

Tactic: apply AEMeasurable.aestronglyMeasurable
    apply Measurable.aemeasurable
    refine Finset.measurable_sum Finset.univ ?left.hf.hf.h.hf

State after:
[case left.hf.hf.h.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
 hf' : ∀ (i : ι) (z : Z), |f i z| ≤ b
 ⊢ ∀ i ∈ Finset.univ, Measurable fun x => ⨆ i_1, |∑ k, ↑↑(i k) * f i_1 (X (x.2 k))|]

Relevant lemmas:
AEMeasurable.aestronglyMeasurable : ∀ {α : Type u_1} {β : Type u_2} [inst : TopologicalSpace β] {m₀ : MeasurableSpace α}
  {μ : Measure α} {f : α → β} [inst_4 : MeasurableSpace β] [inst_5 : TopologicalSpace.PseudoMetrizableSpace β]
  [inst_6 : OpensMeasurableSpace β] [inst_7 : SecondCountableTopology β], AEMeasurable f μ → AEStronglyMeasurable f μ
Finset.measurable_sum : ∀ {M : Type u_2} {ι : Type u_3} {α : Type u_4} [inst : AddCommMonoid M]
  [inst_4 : MeasurableSpace M] [inst_5 : MeasurableAdd₂ M] {m : MeasurableSpace α} {f : ι → α → M} (s : Finset ι),
  (∀ i ∈ s, Measurable (f i)) → Measurable fun a => ∑ i ∈ s, f i a
Measurable.aemeasurable : ∀ {α : Type u_1} {β : Type u_2} {m : MeasurableSpace α} [inst : MeasurableSpace β] {f : α → β}
  {μ : Measure α}, Measurable f → AEMeasurable f μ