proposition: Lemma gtrN x : 0 < x -> - x < x.
proof: Proof. by move=> n0; rewrite -subr_lt0 -opprD oppr_lt0 addr_gt0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)