proposition: Lemma normr_sg x : `|sg x| = (x != 0)%:R.
proof: Proof. by rewrite sgr_def -mulr_natr normrMsign normr_nat. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)