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 (e0..s, diff l e0..s e0..t, e0..t)..s (V Gl) /\
    VSet.In (e0..s, diff l e0..s e0..t, e0..t)..t (V Gl))


DEBUG:Going to execute:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.