∀ {X Y : CategoryTheory.Cat} {F G : X ⟶ Y} (e : F ≅ G),
  { toNatTrans := CategoryTheory.CategoryStruct.comp e.inv.toNatTrans e.hom.toNatTrans }.toNatTrans =
    { toNatTrans := CategoryTheory.CategoryStruct.id G.toFunctor }.toNatTrans