Definition Zmax : forall x_ x_ : Z, Z :=
  fun n m : Z =>
  match (n ?= m)%Z with
  | Datatypes.Eq => m
  | Datatypes.Lt => m
  | Datatypes.Gt => n
  end.

Theorem absolu_Zs_neg : forall z : Z, (z < 0)%Z -> S (Zabs_nat (Zsucc z)) = Zabs_nat z.