∀ {X Y Z : TopCat} (f : X ⟶ Y) (g : Y ⟶ Z) (U : TopologicalSpace.Opens ↑Z),
  (TopologicalSpace.Opens.map f).obj ((TopologicalSpace.Opens.map g).obj U) =
    (TopologicalSpace.Opens.map (CategoryTheory.CategoryStruct.comp f g)).obj U