proposition: Lemma adde_ss_eq0 x y : (0 <= x) && (0 <= y) || (x <= 0) && (y <= 0) -> x + y == 0 = (x == 0) && (y == 0).
proof: Proof. by move=> /orP[|] /andP[]; [exact: padde_eq0|exact: nadde_eq0]. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap) mathcomp(mathcomp_extra) signed