Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  xs : V.t
  _z_ : VSet.t
  x0 : V.t
  x1 : V.t
  x2 : V.t
  ============================
   (VSet.Equal (VSet.add x0 (VSet.add x1 (VSet.add x2 _z_)))
      (VSet.add x1 (VSet.add x2 (VSet.add x0 _z_))))


DEBUG:Going to execute:
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