Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  xs : 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:
tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P) holds by
            auto  100 with MSet_Prop then fail else clear H