Hi Anh Toa`n, Please let us know about the process of thinking. I heard that you are expert. What do you think about " Kie^'n Tri Ha`nh"? I don't understand the concept of Quye^'t thu+'c ( Kie^'n thu+'c, Tri Thu+'c, yes) It is Anh Vu Ho^`ng La^m, who advised me to ask you such questions. Cheers Aiviet