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
  x : V.t
  y : V.t
  vsub : VSet.Subset (V G1) (V G2)
  p : PathOf G1 x y
  xin : VSet.In x (V G1)
  yin : VSet.In y (V G1)
  q : SPath G1 (V G1) x y
  w : Z
  Elsp : lsp Gl x y = Some w
  ub : w <= diff l x y
  _lb_ : diff l x y <= w
  ============================
   (w = diff l x y)


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