∀ {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]
  (S : J.MayerVietorisSquare), CategoryTheory.Epi S.shortComplex.g