void treew() {
  TFile f("junk.root","recreate");
  TNtuple *ntuple = new TNtuple("ntuple","Demo","px:py:pz:random:i");
  Float_t px, py, pz;
  for ( Int_t i=0; i<10000000; i++) {
     gRandom->Rannor(px,py);
     pz = px*px + py*py;
     Float_t random = gRandom->Rndm(1);
     ntuple->Fill(px,py,pz,random,i);
     if (i%1000 == 1) ntuple->AutoSave("SaveSelf");
  }
}