Goal:
    x : VSet.elt  y : VSet.elt  p : VSet.t
  ============================
   (VSet.Equal (VSet.add x (VSet.add y p)) (VSet.add y (VSet.add x p)))


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