Goal:
  
  G : t
  l : labelling
  Hl : correct_labelling l
  x : V.t
  p : PathOf x x
  H : Z.of_nat (l x) + weight p <= Z.of_nat (l x)
  cstr : 0 <= Z.of_nat (l x)
  ============================
   (weight p <= 0)


DEBUG:Going to execute:
idtac