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'
  p : SPath Gl s0 y z
  IHp : Disjoint (snodes Gl p) (V G1) ->
        (Some (sweight p) <= lsp G2 y z)%nbar
  disj : Disjoint (VSet.add x (snodes Gl p)) (V G1)
  w : Z
  s : V.t
  w' : Z
  t : V.t
  sin : VSet.In (s, w', t)..s (V G1)
  _b_ : VSet.In (s, w', t)..t (V G1)
  _Hyp_ : x = (s, w', t)..s
  _Hyp1_ : w = diff l (s, w', t)..s t
  _Hyp2_ : y = t
  ============================
   (EdgeSet.In (s, w', t) (E G2) -> (Some w <= lsp G2 x y)%nbar)


DEBUG:Going to execute:
subst; exfalso
DEBUG LTAC Evaluated term: E0
DEBUG LTAC Evaluated term: EdgeSet.mem_spec
DEBUG LTAC Evaluated term: (iffLR (Q:=EdgeSet.In (s, w', t) (E G1))
                              (EdgeSet.mem_spec (E G1) (s, w', t))
                              _view_subject_)
DEBUG LTAC Evaluated term: (edges_vertices G1 ?e)
DEBUG LTAC Evaluated term: (edges_vertices G1 (s, w', t)
                              (iffLR (Q:=EdgeSet.In (s, w', t) (E G1))
                                 (EdgeSet.mem_spec (E G1) (s, w', t))
                                 _view_subject_))