proposition: Lemma normr_sg x : `|sgr 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 choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)