∀ (X : TopCat),
  CategoryTheory.CategoryStruct.comp { app := fun U => CategoryTheory.eqToHom ⋯, naturality := ⋯ }
      { app := fun U => CategoryTheory.eqToHom ⋯, naturality := ⋯ } =
    CategoryTheory.CategoryStruct.id (TopologicalSpace.Opens.map (CategoryTheory.CategoryStruct.id X))