proposition: Fact hsgpN x : q.[x] < 0 -> sgr p.[x] = - sp'c.
proof: Proof. by rewrite -sgr_cp0 sgp => /eqP->; rewrite mulrN1. Qed.
imports: mathcomp.ssreflect.ssreflect mathcomp(ssrfun ssrbool eqtype ssrnat seq choice fintype) mathcomp(bigop order ssralg poly polydiv ssrnum zmodp) mathcomp(polyorder path interval ssrint)