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 Zabs_Zmult : forall z1 z2 : Z, Zabs (z1 * z2) = (Zabs z1 * Zabs z2)%Z.