∀ {C : Type u_1} [inst : CategoryTheory.Category.{u_4, u_1} C] {D : Type u_2}
  [inst_1 : CategoryTheory.Category.{u_5, u_2} D] {E : Type u_3} [inst_2 : CategoryTheory.Category.{u_6, u_3} E]
  (F : CategoryTheory.Functor C (CategoryTheory.Functor D E)) (a b : C),
  Subsingleton
    ((⟦a⟧ ⟶ ⟦b⟧) → (CategoryTheory.ThinSkeleton.map₂Functor F ⟦a⟧ ⟶ CategoryTheory.ThinSkeleton.map₂Functor F ⟦b⟧))