∀ {X : Type u_1} [inst : TopologicalSpace X] (U : TopologicalSpace.Opens X) {X_1 Y Z : TopologicalSpace.Opens ↥U}
  (f : X_1 ⟶ Y) (g : Y ⟶ Z),
  CategoryTheory.Over.homMk (CategoryTheory.homOfLE ⋯) ⋯ =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.Over.homMk (CategoryTheory.homOfLE ⋯) ⋯)
      (CategoryTheory.Over.homMk (CategoryTheory.homOfLE ⋯) ⋯)