proposition: Lemma sgr_gt0 x : (sg x > 0) = (x > 0).
proof: Proof. by rewrite -[LHS]sgr_cp0 sgr_id sgr_cp0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)