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