Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  y_0 : V.t
  x_0 : V.t
  Vx : VSet.In x_0 (V G)
  Vy : VSet.In y_0 (V G)
  K : Z
  Hxs : lsp G x_0 y_0 = Some K
  x : VSet.elt
  Hx : VSet.In x (V G')
  H : (Some 0 <= lsp0 G' (V G) x x)%nbar
  ============================
   (lsp0 G' (V G) x x = None -> lsp G' x x = Some 0)


DEBUG:Going to execute:
intro e; rewrite e in   H; cbn in H; lia