Goal:
  
  G1 : t
  G2 : t
  l : labelling
  Gl := relabel_on G1 G2 l : t
  H : invariants G1
  preserves_edges : forall e : EdgeSet.elt,
                    EdgeSet.In e (E G1) -> EdgeSet.In e (E G2)
  HGl : correct_labelling G1 l
  H0 : invariants G2
  H1 : acyclic_no_loop G2
  embed : forall v1 v2 : VSet.elt,
          VSet.In v1 (V G1) ->
          VSet.In v2 (V G1) -> (lsp G2 v1 v2 <= lsp G1 v1 v2)%nbar
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  d : DisjointAdd x s0 s'
  e : EdgeOf G1 x y
  p : SPath G1 s0 y z
  IHp : forall (V : VSet.t) (vsub : VSet.Subset s0 V),
        let q := SPath_sub Gl vsub (map_spath from1 p) in
        sweight q = diff l y z
  s1 : VSet.t
  vsub : VSet.Subset s' s1
  vsub' : forall H : VSet.elt, VSet.In H s0 -> VSet.In H (VSet.remove x s1)
  ============================
   (Z.of_nat (l y) - Z.of_nat (l x) + (Z.of_nat (l z) - Z.of_nat (l y)) =
    Z.of_nat (l z) - Z.of_nat (l x))


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