∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.CartesianMonoidalCategory C]
  (X Y : C) {X_1 Y_1 Z : CategoryTheory.Over Y} (f : X_1 ⟶ Y_1) (g : Y_1 ⟶ Z),
  CategoryTheory.Over.homMk
      (CategoryTheory.MonoidalCategoryStruct.whiskerLeft X (CategoryTheory.CategoryStruct.comp f g).left) ⋯ =
    CategoryTheory.CategoryStruct.comp
      (CategoryTheory.Over.homMk (CategoryTheory.MonoidalCategoryStruct.whiskerLeft X f.left) ⋯)
      (CategoryTheory.Over.homMk (CategoryTheory.MonoidalCategoryStruct.whiskerLeft X g.left) ⋯)