∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {J : CategoryTheory.GrothendieckTopology C}
  {F F' : CategoryTheory.Functor Cᵒᵖ (Type w)} (G : CategoryTheory.Subfunctor F),
  CategoryTheory.Presieve.IsSheaf J F' →
    ∀ (l₁ l₂ : (CategoryTheory.Subfunctor.sheafify J G).toFunctor ⟶ F'),
      CategoryTheory.CategoryStruct.comp (CategoryTheory.Subfunctor.homOfLe ⋯) l₁ =
          CategoryTheory.CategoryStruct.comp (CategoryTheory.Subfunctor.homOfLe ⋯) l₂ →
        l₁ = l₂