∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.MonoidalCategory C] (W : C)
  {X Y Z : C} (f : X ⟶ Y) (g : Y ⟶ Z),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.whiskerLeft W f)
      (CategoryTheory.MonoidalCategoryStruct.whiskerLeft W g) =
    CategoryTheory.MonoidalCategoryStruct.whiskerLeft W (CategoryTheory.CategoryStruct.comp f g)