∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Abelian C] {X Y Z : C}
  (f : X ⟶ Z) (g : Y ⟶ Z) (s : CategoryTheory.Limits.Fork (CategoryTheory.Limits.biprod.desc f (-g)) 0),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp s.ι CategoryTheory.Limits.biprod.fst) f =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp s.ι CategoryTheory.Limits.biprod.snd) g