Goal:
  
  G : t
  HI : invariants
  z : Z
  H : 0 <= z
  cstr : 0 <= Z.of_nat (to_label None)
  z0 : Z
  H0 : z < Z.of_nat (to_label None) /\ z0 = Z.of_nat (to_label None) \/
       Z.of_nat (to_label None) <= z /\ z0 = z
  ============================
   (z = z0)


DEBUG:Going to execute:
constr:(false)