Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  y : V.t
  Vy : VSet.mem y (V G) = false
  e : n <= 0
  l : labelling
  Hl : correct_labelling G l
  ============================
   (Z.of_nat (l y) + n <= Z.of_nat (l y))


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op