proposition: Lemma closedC (D : set T) : closed (~` D) = open D.
proof: Proof. by apply/propext; split=> [/closed_openC|]; [rewrite setCK|exact: open_closedC]. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed