∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C]
  (F : CategoryTheory.Pseudofunctor (CategoryTheory.LocallyDiscrete Cᵒᵖ) CategoryTheory.Cat) {S : C}
  (R : CategoryTheory.Presieve S), F.IsPrestackFor R ↔ Nonempty (F.toDescentData fun f => f.obj.hom).FullyFaithful