(* Succ and Pred on Z *)

(**************)
Definition succZ (x : Z) :=
  match x return Z with
  | OZ =>
      (* OZ *)  IZ 
      (* pos n *) 
  | pos n => pos (S n)
      (* neg n *) 
  | neg n =>
      match n return Z with
      | O =>
          (* O *)  OZ
          (* S m *) 
      | S m => neg m
      end
  end.(**************)
Definition predZ (x : Z) :=
  match x return Z with
  | OZ =>
      (* OZ *)  neg 0
      (* pos n *) 
  | pos n =>
      match n return Z with
      | O =>
          (* O *)  OZ
          (* S m *) 
      | S m => pos m
      end
      (* neg n *) 
  | neg n => neg (S n)
  end.

Lemma pred_succZ : forall x : Z, predZ (succZ x) = x.