∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {e f : C ≌ D} (η : e.functor ⟶ f.functor),
  CategoryTheory.Equivalence.mkHom (CategoryTheory.Equivalence.asNatTrans η) = η