theorem extracted_formal_statement_12.{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.1) : Measurable fun c => c.1 := 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.1
⊢ Measurable fun c => c.1

Tactic: exact measurable_fst

State after:

Relevant lemmas:
measurable_fst : ∀ {α : Type u_1} {β : Type u_2} {x : MeasurableSpace α} {x_1 : MeasurableSpace β}, Measurable Prod.fst