proposition: Lemma doppeD x y : x +? y -> - (x + y) = - x - y.
proof: Proof. by move: x y => [x| |] [y| |] //= _; rewrite opprD. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap) mathcomp(mathcomp_extra) signed