proposition: Lemma Qcabs_triangle_reverse x y : [x] - [y] <= [x - y].
proof: Proof. unfold Qcle, Qcabs, Qcminus, Qcplus, Qcopp, Q2Qc, this; fold (this x) (this y) (this (-x)) (this (-y)). case Qred_abs; rewrite !Qred_le, !Qred_opp, Qred_abs. apply Qabs_triangle_reverse. Qed.
imports: Qabs Qcanon