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
  ============================
   (forall G : t, well_founded (SPath_subterm G))


DEBUG:Going to execute:
intros; apply Transitive_Closure.wf_clos_trans; red; intros;
            CoreTactics.simp_sigmas;
            CoreTactics.on_last_hyp ltac:(fun H => DepElim.depind H);
            constructor