∀ {X Y Z : TopCat}
  {x : ↑(CategoryTheory.MonoidalCategoryStruct.tensorObj X (CategoryTheory.MonoidalCategoryStruct.tensorObj Y Z))},
  ((CategoryTheory.ConcreteCategory.hom (CategoryTheory.MonoidalCategoryStruct.associator X Y Z).inv) x).1.2 = x.2.1