Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  y_0 : V.t
  x_0 : V.t
  K : Z
  Hxs : lsp G x_0 y_0 = Some K
  x : V.t
  y : V.t
  z : V.t
  e : EdgeOf G' x y
  q : PathOf G' y z
  q1 : PathOf G y y_0
  q2 : PathOf G x_0 z
  IHq : weight q <= weight q1 - K + weight q2
  XX : (y_0, - K, x_0) <> (x, e.π1, y)
  ============================
   (e.π1 + weight q <= e.π1 + weight q1 - K + weight q2)


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op;
            Zify.zify_iter_specs; Zify.zify_to_euclidean_division_equations;
            Zify.zify_post_hook