∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] [inst_1 : CategoryTheory.MonoidalCategory C] {M X : C}
  [inst_2 : CategoryTheory.MonObj M] (e : M ≅ X),
  CategoryTheory.MonObj.mul =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.MonoidalCategoryStruct.tensorHom e.inv e.inv)
      (CategoryTheory.CategoryStruct.comp CategoryTheory.MonObj.mul e.hom)