proposition: Lemma Zsgn_2 : forall x : Z, Z.sgn x = 0%Z -> x = 0%Z.
proof: Proof. intro x. case x. intro H. reflexivity. intros p H. inversion H. intros p H. inversion H. Qed.
imports: Coq.Arith.Compare_dec CoRN.logic.CornBasics Coq.ZArith.ZArith Coq.setoid_ring.ZArithRing Coq.Arith.Wf_nat Coq(Lia)