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
  e : EdgeSet.elt
  e0 : Edge.t
  _a_ : VSet.In e0..s (V G2)
  _b_ : VSet.In e0..t (V G2)
  ============================
   (VSet.In (relabel_map G1 l e0)..s (V Gl) /\
    VSet.In (relabel_map G1 l e0)..t (V Gl))


DEBUG:Going to execute:
unfold relabel_map
DEBUG LTAC Evaluated term: edge_map_spec2
DEBUG LTAC Evaluated term: (iffLR
                              (Q:=exists e0 : Edge.t,
                                    e = relabel_map G1 l e0 /\
                                    EdgeSet.In e0 (E G2))
                              (edge_map_spec2 (relabel_map G1 l) (E G2) e)
                              _view_subject_)
DEBUG LTAC Evaluated term: (edges_vertices G2)
DEBUG LTAC Evaluated term: (edges_vertices G2 e0 _view_subject_)