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
  s : VSet.t
  p : SPath Gl s x x
  wp : sweight p > 0
  y : VSet.elt
  yinp : VSet.In y (snodes Gl p)
  yin1 : VSet.In y (V G1)
  q : SPath Gl s y y
  wq : sweight q = sweight p
  _x_ : Z
  a : lsp G2 y y = Some _x_
  b : sweight q <= _x_ + diff l y y
  ============================
   False


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym