∀ {C : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} C] (J : CategoryTheory.GrothendieckTopology C)
  {A : Type u_2} [inst_1 : CategoryTheory.Category.{v_2, u_2} A] [inst_2 : CategoryTheory.HasWeakSheafify J A]
  {P₁ P₂ : CategoryTheory.Functor Cᵒᵖ A} (f : P₁ ⟶ P₂),
  J.W f ↔ CategoryTheory.IsIso ((CategoryTheory.presheafToSheaf J A).map f)