proposition: Lemma sgrV x : sgr x^-1 = sgr x.
proof: Proof. by rewrite /sgr invr_eq0 invr_lt0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)