Goal:
  
  G : t
  HI : invariants
  z : Z
  z0 : Z
  H : 0 <= z
  cstr : 0 <= Z.of_nat (to_label (Some z))
  cstr0 : 0 <= Z.of_nat (to_label (Some z0))
  ============================
   (Z.max z z0 =
    Z.max (Z.of_nat (to_label (Some z))) (Z.of_nat (to_label (Some z0))))


DEBUG:Going to execute:
fun => <coq-core.plugins.zify::ITER@0>