proposition: Lemma invr_sg x : (sg x)^-1 = sgr x.
proof: Proof. by rewrite !(fun_if GRing.inv) !(invr0, invrN, invr1). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)