proposition: Lemma mulNrNz n x : (-x) *~ (-n) = x *~ n.
proof: Proof. by rewrite mulNrz mulrNz opprK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)