proposition: Lemma mulz_sg x : sgz x * sgz x = (x != 0)%:~R.
proof: Proof. by case: sgzP; rewrite ?(mulr0, mulr1, mulrNN). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)