∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {F : CategoryTheory.Functor C C}
  (X : CategoryTheory.Endofunctor.Algebra F),
  (CategoryTheory.Endofunctor.Algebra.functorOfNatTransId.inv.app X).f = CategoryTheory.CategoryStruct.id X.a