∀ {C : Type u_1} [inst : CategoryTheory.Category.{v, u_1} C] [inst_1 : CategoryTheory.Preadditive C]
  {F G : CochainComplex C ℤ} (φ : F ⟶ G) [inst_2 : HomologicalComplex.HasHomotopyCofiber φ] {K : CochainComplex C ℤ}
  (α : CochainComplex.HomComplex.Cochain F K (-1)) (β : G ⟶ K)
  (eq :
    CochainComplex.HomComplex.δ (-1) 0 α =
      CochainComplex.HomComplex.Cochain.ofHom (CategoryTheory.CategoryStruct.comp φ β)),
  CochainComplex.mappingCone.desc φ α β eq =
    (CochainComplex.mappingCone.descCocycle φ α (CochainComplex.HomComplex.Cocycle.ofHom β)
        CochainComplex.mappingCone.desc._proof_2 ⋯).homOf