theorem extracted_formal_statement_100.{w, v, u} {Z : Type w} {ι : Type v} {f : ι → Z → ℝ} {Ω : Type u}
  [inst : MeasurableSpace Ω] {X : Ω → Z} {μ : Measure Ω} [inst_1 : IsProbabilityMeasure μ] {n : ℕ}
  [inst_2 : Countable ι] [inst_3 : Nonempty ι] (h𝓕 : ∀ (I : ι), Measurable (f I ∘ X)) {b : ℝ}
  (h𝓕' : ∀ (I : ι) (z : Z), |f I z| ≤ b) {c : ι → ℝ} {C : ℝ} (hC : ∀ (I : ι), |c I| ≤ C)
  (σ : Fin n → { x // x ∈ {-1, 1} }) (a : σ ∈ Finset.univ) (I : ι) (i : Fin n) (a_1 : i ∈ Finset.univ)
  (h_original_goal : Measurable fun a => ↑↑(σ i) * (f I (X (a i).1) - f I (X (a i).2))) :
  Measurable fun x => f I (X (x i).1) - f I (X (x i).2) := sorry

State before:
case left.hf.hf.hf.hf
Z : Type w
ι : Type v
f : ι → Z → ℝ
Ω : Type u
inst✝³ : MeasurableSpace Ω
X : Ω → Z
μ : Measure Ω
inst✝² : IsProbabilityMeasure μ
n : ℕ
inst✝¹ : Countable ι
inst✝ : Nonempty ι
h𝓕 : ∀ (I : ι), Measurable (f I ∘ X)
b : ℝ
h𝓕' : ∀ (I : ι) (z : Z), |f I z| ≤ b
c : ι → ℝ
C : ℝ
hC : ∀ (I : ι), |c I| ≤ C
σ : Fin n → { x // x ∈ {-1, 1} }
a✝¹ : σ ∈ Finset.univ
I : ι
i : Fin n
a✝ : i ∈ Finset.univ
⊢ Measurable fun a => ↑↑(σ i) * (f I (X (a i).1) - f I (X (a i).2))

Tactic: apply Measurable.const_mul

State after:
[case left.hf.hf.hf.hf.hf
 Z : Type w
 ι : Type v
 f : ι → Z → ℝ
 Ω : Type u
 inst✝³ : MeasurableSpace Ω
 X : Ω → Z
 μ : Measure Ω
 inst✝² : IsProbabilityMeasure μ
 n : ℕ
 inst✝¹ : Countable ι
 inst✝ : Nonempty ι
 h𝓕 : ∀ (I : ι), Measurable (f I ∘ X)
 b : ℝ
 h𝓕' : ∀ (I : ι) (z : Z), |f I z| ≤ b
 c : ι → ℝ
 C : ℝ
 hC : ∀ (I : ι), |c I| ≤ C
 σ : Fin n → { x // x ∈ {-1, 1} }
 a✝¹ : σ ∈ Finset.univ
 I : ι
 i : Fin n
 a✝ : i ∈ Finset.univ
 ⊢ Measurable fun x => f I (X (x i).1) - f I (X (x i).2)]

Relevant lemmas:
Measurable.const_mul : ∀ {M : Type u_2} {α : Type u_3} [inst : MeasurableSpace M] [inst_4 : Mul M]
  {m : MeasurableSpace α} {f : α → M} [inst_5 : MeasurableMul M], Measurable f → ∀ (c : M), Measurable fun x => c * f x