theorem extracted_formal_statement_9.{u} {Ω : Type u} [inst : MeasurableSpace Ω] {n : ℕ} (i : Fin (n + 1)) (h : ↑i < n)
  (this : (fun c => if h : ↑i < n then c.2 (i.castLT h) else c.1) = fun c => c.2 (i.castLT h)) :
  Measurable fun c => c.2 (i.castLT h) := sorry

State before:
Ω : Type u
inst✝ : MeasurableSpace Ω
n : ℕ
i : Fin (n + 1)
h : ↑i < n
this : (fun c => if h : ↑i < n then c.2 (i.castLT h) else c.1) = fun c => c.2 (i.castLT h)
⊢ Measurable fun c => c.2 (i.castLT h)

Tactic: exact Measurable.eval measurable_snd

State after:

Relevant lemmas:
Measurable.eval : ∀ {α : Type u_1} {δ : Type u_4} {X : δ → Type u_6} [inst : MeasurableSpace α]
  [inst_1 : (a : δ) → MeasurableSpace (X a)] {a : δ} {g : α → (a : δ) → X a}, Measurable g → Measurable fun x => g x a
measurable_snd : ∀ {α : Type u_1} {β : Type u_2} {x : MeasurableSpace α} {x_1 : MeasurableSpace β}, Measurable Prod.snd