∀ {m : Type u_1 → Type u_2} {α : Type u_1} [inst : Monad m] (n : ℕ) (f : Fin n → α → m α)
  (_x : (_ : { i // i ≤ n }) ×' α),
  Fin.foldrM.loop._unary✝ n f _x =
    PSigma.casesOn _x fun a a_1 =>
      match a, a_1 with
      | ⟨0, property⟩, x => pure x
      | ⟨i.succ, h⟩, x => do
        let a ← f ⟨i, h⟩ x
        Fin.foldrM.loop._unary✝¹ n f ⟨⟨i, ⋯⟩, a⟩