Goal:
  
  G : t
  x : V.t
  y : V.t
  n : Z
  p : SPath (V G) x y
  Hp : sweight p = n
  l : labelling
  Hl : correct_labelling l
  ============================
   (Z.of_nat (l x) + n <= Z.of_nat (l y))


DEBUG:Going to execute:
eapply correct_labelling_PathOf with (p := to_pathOf p) in Hl