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


DEBUG:Going to execute:
prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P) holds by auto 
100 with MSet_Prop