proposition: Lemma openN (R : numFieldType) (A : set R) : open A -> open [set - x | x in A].
proof: Proof. move=> Aop; rewrite openE => _ [x /Aop x_A <-]. by rewrite /interior nbhsNimage; exists A. Qed.
imports: HB(structures) mathcomp(all_ssreflect ssralg ssrint ssrnum finmap matrix) mathcomp(rat interval zmodp vector fieldext falgebra) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality set_interval Rstruct) ereal reals signed topology prodnormedzmodule