(** If [a] divides [b] and [b] divides [a] then [a] is [b] or [-b]. *)

(** If [a] divides [b] and [b<>0] then [|a| <= |b|]. *)
Lemma Zabs_ind :
forall (P : Z -> Prop) (x : Z),
(x >= 0 -> P x) -> (x <= 0 -> P (- x)) -> P (Zabs x).