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


DEBUG:Going to execute:
t; easy