Goal:
  
  G : t
  HI : invariants
  z : Z
  H : 0 <= z
  cstr : 0 <= Z.of_nat (to_label None)
  ============================
   (z = Z.max z (Z.of_nat (to_label None)))


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