proposition: Lemma filternat_notempty : isfilter_notempty filternat.
proof: Proof. intros A. apply hinhfun. intros N. exists (pr1 N). apply (pr2 N (pr1 N)). now apply isreflnatleh. Qed.
imports: UniMath.Foundations.Preamble UniMath.MoreFoundations.Tactics UniMath.Topology.Prelim UniMath.MoreFoundations.PartA