∀ {X Y Z : SSet} {A : X.Subcomplex} {B : Y.Subcomplex} {φ : A.toSSet ⟶ B.toSSet} {f g : SSet.RelativeMorphism A B φ}
  {C : Z.Subcomplex} {ψ : B.toSSet ⟶ C.toSSet} (h : f.Homotopy g) (f' : SSet.RelativeMorphism B C ψ)
  {φψ : A.toSSet ⟶ C.toSSet} (fac : CategoryTheory.CategoryStruct.comp φ ψ = φψ),
  (h.postcomp f' fac).h = CategoryTheory.CategoryStruct.comp h.h f'.map