Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  x : VSet.elt
  y : VSet.elt
  inx : VSet.In x (V G)
  iny : VSet.In y (V G)
  zl : Z
  eq : lsp (s G) x = Some zl
  pos : 0 <= zl
  zof : Z.of_nat (to_label (lsp (s G) x)) = zl
  z : Z
  yx : lsp y x = Some z
  ly : Z
  lspy : lsp (s G) y = Some ly
  neg : 0 <= ly
  H : ly + z <= zl
  cstr : 0 <= Z.of_nat (to_label (lsp (s G) x))
  ============================
   (z <= zl)


DEBUG:Going to execute:
Zify.zify_post_hook