proposition: Lemma sgz_smul x y : sgz (y *~ (sgz x)) = (sgz x) * (sgz y).
proof: Proof. by rewrite -mulrzl sgzM -sgrEz sgz_sgr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)