∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {F : CategoryTheory.Functor Cᵒᵖ (Type u_1)} {X Y : C}
  {f : X ⟶ Y} [inst_1 : CategoryTheory.Limits.HasPullback f f],
  CategoryTheory.Presieve.IsSheafFor F (CategoryTheory.Presieve.singleton f) ↔
    Nonempty (CategoryTheory.Limits.IsLimit (CategoryTheory.Limits.Fork.ofι (F.map f.op) ⋯))