∀ {α : Type u_1} {M : Type u_2} [inst : CommMonoid M] {f : α → M} {a b : α} [inst_1 : PartialOrder α]
  [inst_2 : LocallyFiniteOrder α], a ≤ b → (∏ x ∈ Finset.Ico a b, f x) * f b = ∏ x ∈ Finset.Icc a b, f x