Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  y : VSet.elt
  Vy : VSet.In y (V G)
  Hle : leq_vertices G n x y
  Vx : VSet.In x (V G)
  ly : Z
  Hly : lsp G (s G) y = Some ly
  Hpos : 0 <= ly
  Hlab : Z.of_nat (to_label (lsp G (s G) y)) = ly
  lx : Z
  Hlx : lsp G (s G) x = Some lx
  poskx : 0 <= lx
  Hlab' : Z.of_nat (to_label (lsp G (s G) x)) = lx
  xs : lsp G x (s G) = None
  K := if n <=? 0 then Z.max lx (Z.succ ly - n) else Z.max lx (Z.succ ly) : Z
  ============================
   (Some n <= lsp G x y)%nbar


DEBUG:Going to execute:
unshelve
            (epose proof (correct_labelling_lsp_G' _ _ _ _ xs K) as XX); tas
DEBUG LTAC Evaluated term: (if n <=? 0
                            then Z.max lx (Z.succ ly - n)
                            else Z.max lx (Z.succ ly))