∀ {T : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} T] {D : Type u_4}
  [inst_1 : CategoryTheory.Category.{u_3, u_4} D] {X : T} (F : CategoryTheory.Functor T D)
  {X_1 Y Z : CategoryTheory.Over X} (f : X_1 ⟶ Y) (g : Y ⟶ Z),
  CategoryTheory.Over.homMk (F.map (CategoryTheory.CategoryStruct.comp f g).left) ⋯ =
    CategoryTheory.CategoryStruct.comp (CategoryTheory.Over.homMk (F.map f.left) ⋯)
      (CategoryTheory.Over.homMk (F.map g.left) ⋯)