public override void Remove(PhanTu ptu)
 {
     throw new Exception();
 }
 public override void Add(PhanTu ptu)
 {
     throw new Exception();
 }
 public override void Remove(PhanTu ptu)
 {
     this._phanTuCon.Remove(ptu);
 }
 public override void Add(PhanTu ptu)
 {
     this._phanTuCon.Add(ptu);
 }
 public abstract void Remove(PhanTu ptu);
 public abstract void Add(PhanTu ptu);