Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  y : V.t
  Vy : VSet.mem y (V G) = false
  e : n <= 0
  Hxs : (Some n <= lsp G x (s G))%nbar
  l : labelling
  Hl : correct_labelling G l
  ============================
   (lsp G x (s G) = None -> Z.of_nat (l x) + n <= Z.of_nat (l y))


DEBUG:Going to execute:
intro H; rewrite H in   Hxs; inversion Hxs