∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {J : CategoryTheory.GrothendieckTopology C}
  [inst_1 : CategoryTheory.HasWeakSheafify J (Type v)] [inst_2 : CategoryTheory.HasSheafify J AddCommGrpCat]
  [inst_3 : CategoryTheory.HasExt (CategoryTheory.Sheaf J AddCommGrpCat)] (S : J.MayerVietorisSquare)
  (F : CategoryTheory.Sheaf J AddCommGrpCat) (n : ℕ),
  S.fromBiprod F n =
    CategoryTheory.Limits.biprod.desc ((F.cohomologyPresheaf n).map S.f₁₂.op) (-(F.cohomologyPresheaf n).map S.f₁₃.op)