void test_write(){
  gSystem->Load("libMyTrack");
  MyTrack* a=new MyTrack();
  TFile f1("test.root","recreate");
  TTree t1("t1","test");
  t1.Branch("my","MyTrack",&a);
  TRandom* b=new TRandom();
  for (Int_t i=0 ; i < 100 ; i++) {
    a.SetTrack(b);
    t1.Fill();
  }
  f1.Write();
}