proposition: Theorem mag_opp : forall x, mag (-x) = mag x :> Z.
proof: Proof. intros x. destruct (Req_dec x 0) as [Hx|Hx]. now rewrite Hx, Ropp_0. destruct (mag x) as (e, He). simpl. specialize (He Hx). apply mag_unique. now rewrite Rabs_Ropp. Qed.
imports: Coq(Psatz Reals ZArith) Zaux