∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {f g h i j : C} (α : f ≅ g) (η : g ⟶ h) {ηs : h ⟶ i}
  {θ : i ⟶ j} {ι : h ⟶ j},
  CategoryTheory.CategoryStruct.comp ηs θ = ι →
    CategoryTheory.CategoryStruct.comp
        (CategoryTheory.CategoryStruct.comp α.hom (CategoryTheory.CategoryStruct.comp η ηs)) θ =
      CategoryTheory.CategoryStruct.comp α.hom (CategoryTheory.CategoryStruct.comp η ι)