int TNtuple_example () {
  Float_t energy , time, dEdx;

  TFile f ("tntuple.root", "RECREATE");
  TNtuple n ("tntuple", "My ntuple", "Time:En:dEdx" );

  TRandom3 r; r.SetSeed ();
  for (int i=0; i<100; i++) {
    time   = r.Rndm() * 20.;
    energy = r.Rndm() * 30.;
    dEdx   = r.Rndm() * 0.5;
    n.Fill (time, energy, dEdx);
  }
  n.Write();
  return 0;
}