proposition: Lemma open_closedC (D : set T) : open D -> closed (~` D).
proof: Proof. by rewrite openE => Dop p clNDp /Dop /clNDp [? []]. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed