∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {P : CategoryTheory.Functor Cᵒᵖ (Type w)} {X Y : C}
  {f : X ⟶ Y},
  CategoryTheory.Presieve.IsSeparatedFor P (CategoryTheory.Presieve.singleton f) ↔ Function.Injective (P.map f.op)