∀ {α : Type u_1} {β : Type u_2} {γ : Type u_3} {xs : Array α} {f : (i : ℕ) → α → i < xs.size → β}
  {g : (i : ℕ) → β → i < (xs.mapFinIdx f).size → γ},
  (xs.mapFinIdx f).mapFinIdx g = xs.mapFinIdx fun i a h => g i (f i a h) ⋯