Goal:
    G : t  HI : invariants  z : Z  z0 : Z  H : 0 <= z
  ============================
   (Z.of_nat (to_label (Some (Z.max z z0))) =
    Z.max (Z.of_nat (to_label (Some z))) (Z.of_nat (to_label (Some z0))))


DEBUG:Going to execute:
rewrite Z_of_to_label_pos