Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  xs : V.t
  _b_ : Z
  xt : V.t
  ys : V.t
  _b1_ : Z
  yt : V.t
  _z_ : VSet.t
  ============================
   (VSet.Equal (VSet.add (xs, _b_, xt)..s (VSet.add (ys, _b1_, yt)..s _z_))
      (VSet.add (ys, _b1_, yt)..s (VSet.add (xs, _b_, xt)..s _z_)))


DEBUG:Going to execute:
repeat
            match goal with
            | H:context [ ?P -> False ] |- _ => fold (~ P) in H
            | |- context [ ?P -> False ] => fold (~ P)
            end