proposition: Lemma sgzN x : sgz (- x) = - sgz x.
proof: Proof. by rewrite /sgz oppr_eq0 oppr_lt0; case: ltrgtP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)