proposition: Lemma nmule_rgt0 x y : x < 0 -> (x * y > 0) = (y < 0).
proof: Proof. by rewrite -oppe_gt0 => /pmule_rlt0<-; rewrite mulNe oppe_lt0. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap) mathcomp(mathcomp_extra) signed