∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.CartesianMonoidalCategory C]
  (X Y Z : C),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.associator X Y Z).hom
      (CategoryTheory.CategoryStruct.comp
        (CategoryTheory.SemiCartesianMonoidalCategory.snd X (CategoryTheory.MonoidalCategoryStruct.tensorObj Y Z))
        (CategoryTheory.SemiCartesianMonoidalCategory.fst Y Z)) =
    CategoryTheory.CategoryStruct.comp
      (CategoryTheory.SemiCartesianMonoidalCategory.fst (CategoryTheory.MonoidalCategoryStruct.tensorObj X Y) Z)
      (CategoryTheory.SemiCartesianMonoidalCategory.snd X Y)