∀ {C : Type u_1} [inst : CategoryTheory.Category.{u_2, u_1} C] {D : Type u_3}
  [inst_1 : CategoryTheory.Category.{u_4, u_3} D] (R : CategoryTheory.Functor D C) (L : CategoryTheory.Functor C D)
  (a : C),
  Subsingleton
    ((CategoryTheory.Functor.id (CategoryTheory.ThinSkeleton C)).obj ⟦a⟧ ⟶
      ((CategoryTheory.ThinSkeleton.map L).comp (CategoryTheory.ThinSkeleton.map R)).obj ⟦a⟧)