∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {E : Type u₃} [inst_2 : CategoryTheory.Category.{v₃, u₃} E] [Quiver.IsThin C] (F : CategoryTheory.Functor E D)
  (G : CategoryTheory.Functor D C),
  CategoryTheory.ThinSkeleton.map (F.comp G) =
    (CategoryTheory.ThinSkeleton.map F).comp (CategoryTheory.ThinSkeleton.map G)