Goal:
  
  G : t
  l : labelling
  Hl : correct_labelling l
  x : V.t
  cstr : 0 <= Z.of_nat (l x)
  ============================
   (Z.of_nat (l x) + 0 <= Z.of_nat (l x))


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