void sumw2_test() {

  Info("sumw2_test","I. Fill histo to 100 by different methods:\n");

  // 1. Sumw2 histo filled with Fill(x,100)
  TH1F* h = new TH1F("h","Sumw2 histo filled with Fill(x,100)",1,0,1);
  h->Sumw2(kTRUE);
  h->Fill(0.5,100);
  Info("sumw2_test","1. Sumw2 histo filled with Fill(x,100): %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;
  
  // 2. None-sumw2 histo filled with Fill(x,100)
  h = new TH1F("h","None-sumw2 histo filled with Fill(x,100)",1,0,1);
  h->Sumw2(kFALSE);
  h->Fill(0.5,100);
  Info("sumw2_test","2. None-sumw2 histo filled with Fill(x,100): %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // 3. Sumw2 histo filled 100 times with Fill(x[,1])
  h = new TH1F("h","Sumw2 histo filled 100 times with Fill(x[,1])",1,0,1);
  h->Sumw2(kTRUE);
  for (int i=0; i<100; i++) 
    h->Fill(0.5);
  Info("sumw2_test","3. Sumw2 histo filled 100 times with Fill(x[,1]): %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // 4. Non-Sumw2 histo filled with Fill(x[,1])
  h = new TH1F("h","None-sumw2 histo filled 100 times with Fill(x[,1])",1,0,1);
  h->Sumw2(kFALSE);
  for (int i=0; i<100; i++) 
    h->Fill(0.5);
  Info("sumw2_test","4. None-sumw2 histo filled 100 times with Fill(x[,1]): %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // 5. None-sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","None-sumw2 histo filled with SetBinContent=100",1,0,1);
  h->Sumw2(kFALSE);
  h->SetBinContent(1,100);
  Info("sumw2_test","5. None-sumw2 histo filled with SetBinContent=100: %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // 6. None-sumw2 histo filled with AddBinContent=100
  h = new TH1F("h","Sumw2 histo filled with AddBinContent=100",1,0,1);
  h->Sumw2(kFALSE);
  h->AddBinContent(1,100);
  Info("sumw2_test","6. None-sumw2 histo filled with AddBinContent=100: %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;  
  
  // 7. Sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","Sumw2 histo filled with SetBinContent=100 and zero Entries",1,0,1);
  h->Sumw2(kTRUE);
  h->SetBinContent(1,100);
  Info("sumw2_test","7. Sumw2 histo filled with SetBinContent=100 and zero Entries: %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // 8. Sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","Sumw2 histo filled with AddBinContent=100 and zero Entries",1,0,1);
  h->Sumw2(kTRUE);
  h->AddBinContent(1,100);
  Info("sumw2_test","8. Sumw2 histo filled with AddBinContent=100 and zero Entries: %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // it was claimed here https://root-forum.cern.ch/t/sumw2-with-setbincontent/19329/1
  // that Sumw2 works correctly onlye if histo have number of entries > 0.
  
  // 9. Sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","Sumw2 histo filled with SetBinContent=100 and none-zero Entries(=100)",1,0,1);
  h->Sumw2(kTRUE);
  h->SetEntries(100);
  h->SetBinContent(1,100);
  Info("sumw2_test","9. Sumw2 histo filled with SetBinContent=100 and none-zero Entries(=100): %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  // 10.Sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","Sumw2 histo filled with AddBinContent=100 and none-zero Entries(=100)",1,0,1);
  h->Sumw2(kTRUE);
  h->SetEntries(100);
  h->AddBinContent(1,100);
  Info("sumw2_test","10.Sumw2 histo filled with AddBinContent=100 and none-zero Entries(=100): %g +/- %g\n",
       h->GetBinContent(1),h->GetBinError(1));
  delete h;

  Info("sumw2_test","II. Divide same histos with errors:\n");

  // 11. None-sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","histo",1,0,1);
  h->Sumw2(kFALSE);
  h->SetBinContent(1,100);
  TH1* h1 = (TH1*) h->Clone("ht");
  h->Divide(h1);
  Info("sumw2_test","11. None-sumw2 histo: 100+/-10 divide on 100+/-10: %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h; delete h1;

  // 12. Sumw2 histo filled with SetBinContent=100
  h = new TH1F("h","histo",1,0,1);
  h->Sumw2(kFALSE);
  h->SetBinContent(1,100);
  h1 = (TH1*) h->Clone("ht");
  h->Sumw2(kTRUE); h1->Sumw2(kTRUE);
  h->Divide(h1);
  Info("sumw2_test","12. Sumw2 histo: 100+/-10 divide on 100+/-10: %g +/- %g",
       h->GetBinContent(1),h->GetBinError(1));
  delete h; delete h1;

}