∀ {Ω : Type u_1} {ι : Type u_3} {m : MeasurableSpace Ω} [inst : PartialOrder ι] {τ : Ω → WithTop ι}
  {f : MeasureTheory.Filtration ι m} [Countable ι],
  MeasureTheory.IsStoppingTime f τ → ∀ (i : ι), MeasurableSet {ω | τ ω = ↑i}