∀ {C : Type u_3} [inst : CategoryTheory.Category.{u_2, u_3} C] {J : CategoryTheory.GrothendieckTopology C}
  [inst_1 : CategoryTheory.HasWeakSheafify J (Type u_2)] [inst_2 : CategoryTheory.HasSheafify J AddCommGrpCat]
  [inst_3 : CategoryTheory.HasExt (CategoryTheory.Sheaf J AddCommGrpCat)] (S : J.MayerVietorisSquare)
  (F : CategoryTheory.Sheaf J AddCommGrpCat) (n₀ n₁ : ℕ) (h : n₀ + 1 = n₁),
  CategoryTheory.CategoryStruct.comp ((S.sequence F n₀ n₁ h).map' 4 5 _proof_445✝ _proof_446✝)
      (CategoryTheory.Iso.refl ((S.sequence F n₀ n₁ h).obj' 5 _proof_446✝¹)).hom =
    CategoryTheory.CategoryStruct.comp
      ((F.H' n₁ S.X₂).biprodIsoProd (F.H' n₁ S.X₃) ≪≫
          CategoryTheory.Abelian.Ext.biprodAddEquiv.symm.toAddCommGrpIso).hom
      ((CategoryTheory.Abelian.Ext.contravariantSequence ⋯ F n₀ n₁ ⋯).map' 4 5 _proof_445✝¹ _proof_446✝²)