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
  x0 : V.t
  x1 : V.t
  ============================
   (VSet.Equal (VSet.add x0 (VSet.add x1 _z_))
      (VSet.add x1 (VSet.add x0 _z_)))


DEBUG:Going to execute:
repeat
            match goal with
            | H:context [ ?x = ?y :> ?T ]
              |- _ =>
                  tryif change T with V.t in H then 
                   fail else
                   tryif change T with VSet.t in H then fail else clear H  
            | H:?P
              |- _ =>
                  tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P)
                   holds by auto  100 with MSet_Prop then 
                   fail else clear H 
            end