void genera()
{
  gROOT->Reset();

  TFile* f = (TFile*)gROOT->FindObject("myGenera.root"); if (f) f->Close();
  f = new TFile ("MyGenera.root","RECREATE","Esempio scrittura");

  TTree * t = new TTree("t", "Nuovo Tree");
  double var1;
  double var2;
  double var3;

  t->Branch("var1", &var1, "var1/D");
  t->Branch("var2", &var2, "var2/D");
  t->Branch("var3", &var3, "var3/D");

  var1=5.1;
  var2=7.3;
  var3=11.6;

  t->Fill();

  var1=8.3;
  var2=6.8;
  var3=2.1;
  t->Fill();

  f->cd();
  t->Write();
  f->Close();
}