proposition: Lemma sgrN x : sg (- x) = - sg x.
proof: Proof. by rewrite -mulrN1 sgrM sgrN1 mulrN1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)