Goal:
  
  G : t
  l : labelling
  Gl := relabel G l : t
  HGl : correct_labelling G l
  x : V.t
  p : PathOf Gl x x
  acG : acyclic_no_loop G
  xx0 : lsp G x x = Some 0
  correct_labelling_lsp0 : Z.of_nat (l x) + 0 <= Z.of_nat (l x)
  ============================
   (Z.of_nat (l x) - Z.of_nat (l x) <= 0)


DEBUG:Going to execute:
zify_op