∀ {C : Type u_1} [inst : CategoryTheory.Category.{v, u_1} C] [inst_1 : CategoryTheory.Preadditive C]
  (S : CategoryTheory.ShortComplex (CochainComplex C ℤ))
  (σ : (n : ℤ) → (S.map (HomologicalComplex.eval C (ComplexShape.up ℤ) n)).Splitting),
  (CochainComplex.triangleOfDegreewiseSplit S σ).mor₃ = CochainComplex.homOfDegreewiseSplit S σ