∀ {k : ℕ} {α : Type u_1} {n : ℕ} {m : Type u_1 → Type u_2} [inst : Monad m] [LawfulMonad m] {f : Fin (n + k) → m α},
  Array.ofFnM f = do
    let as ← Array.ofFnM fun i => f (Fin.castLE ⋯ i)
    let bs ← Array.ofFnM fun i => f (Fin.natAdd n i)
    pure (as ++ bs)