proposition: Lemma is_functor_ptd_forget : is_functor ptd_forget_data.
proof: Proof. split; intros; red; intros; apply idpath. Qed.
imports: UniMath.Foundations.PartD UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Functors UniMath.CategoryTheory.Core.NaturalTransformations UniMath.CategoryTheory.FunctorCategory