∀ {C : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} C] [inst_1 : CategoryTheory.Preadditive C] {ι : Type u_2}
  {c : ComplexShape ι} {F G : HomologicalComplex C c} (φ : F ⟶ G) [inst_2 : HomologicalComplex.HasHomotopyCofiber φ]
  [inst_3 : DecidableRel c.Rel] (i j : ι) (hij : c.Rel i j),
  HomologicalComplex.homotopyCofiber.fstX φ i j hij =
    CategoryTheory.CategoryStruct.comp (HomologicalComplex.homotopyCofiber.XIsoBiprod φ i j hij).hom
      CategoryTheory.Limits.biprod.fst