proposition: Lemma has_homsets_Under : has_homsets Undercategory.
proof: Proof. intros X Y. apply (isaset_Under_mor X Y). Qed.
imports: UniMath.Foundations.PartD UniMath.Foundations.Propositions UniMath.Foundations.Sets UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Functors