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


DEBUG:Going to execute:
tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P) holds by
            auto  100 with MSet_Prop then fail else clear H