// part1 : model building
   RooRealVar ST("ST", "ST", 500, 400, 1000);  // observable
   RooRealVar nSignal("nSignal",   "signal events",     50,   0, 100);
   nSignal.setConstant(true);
   RooRealVar nTTbar("nTTbar",    "ttbar events",       85,  40, 160);  // nuisance parameter
   RooRealVar nOtherBack("nOtherBack",   "other back events",   5,   0, 10);
   nOtherBack.setConstant(true);

   RooRealVar mu("mu", "signal strength", 1, 0, 1); // parameter of interest
   RooProduct muTimesSignal("muTimesSignal",   "mu x signal", RooArgSet(mu, nSignal));
   
   // pdfSignalST, pdfTTbarST etc. are fixed shapes from histograms
   RooExtendPdf * epdfSignalST     = new RooExtendPdf("epdfSignalST",     "signal x PDF(ST)",     *pdfSignalST,     muTimesSignal);
   RooExtendPdf * epdfTTbarST      = new RooExtendPdf("epdfTTbarST",      "ttbar x PDF(ST)",      *pdfTTbarST,      nTTbar);
   RooExtendPdf * epdfOtherBackST  = new RooExtendPdf("epdfOtherBackST",  "other back x PDF(ST)",  *pdfOtherBackST,  nOtherBack);

   RooRealVar nTTbarMean("nTTbarMean",     "mean nTTbar",   85,   70, 110);
   nTTbarMean.setConstant(true);
   RooRealVar nTTbarSigma("nTTbarSigma",  "sigma nTTbar",   5,   1, 10);
   nTTbarSigma.setConstant(true);
   RooGaussian * prior_nTTbar = new RooGaussian("prior_nTTbar", "prior_nTTbar", nTTbar, nTTbarMean, nTTbarSigma);
   RooProdPdf * epdfTTbarSTFinal = new RooProdPdf("epdfTTbarSTFinal", "conditional ttbar PDF", *prior_nTTbar, Conditional(*epdfTTbarST, ST));
   RooAddPdf * model     = new RooAddPdf("model",       "model",      RooArgList(*epdfTTbarSTFinal, *epdfOtherBackST, *epdfSignalST));


   // part2, fitting and plotting
   RooFitResult * result = model->fitTo(*data, Save());
   nll->SetName("nll");

   RooMinuit minuit(*nll);
   minuit.migrad();
   result = minuit.save();
   const double nllMin = nll->getVal();

   canvas1->cd(1);
   RooPlot * plot1 = mu->frame(Title("-ln(L) vs mu"));
   nll->plotOn(plot1, ShiftToZero());
   plot1->Draw();

   canvas1->cd(2);
   RooPlot * plot2 = nTTbar->frame(Title("-ln(L) vs nTTbar"));
   nll->plotOn(plot2, ShiftToZero());
   plot2->Draw();

   canvas1->cd(3);
   TH1 * h_nll = nll->createHistogram("h_nll", *mu, Binning(50, 0, 0.2), YVar(*nTTbar, Binning(20, 40, 120)));
   h_nll->SetTitle("-ln(L) vs (mu, nTTbar)");
   h_nll->Draw("surf");

   canvas1->cd(4);
   TString tmp;
   tmp += nllMin;
   tmp += " )";
   RooFormulaVar * likl = new RooFormulaVar("likl", "L", "exp(-nll  + "+tmp, RooArgList(*nll));
   TH1 * hPDF = model->createHistogram("hPDF", *mu, Binning(50, 0, 1), YVar(*nTTbar, Binning(20, 70, 100)) ,  ConditionalObservables(*nTTbar));
   hPDF->SetTitle("L vs (mu, nTTbar)");
   hPDF->Draw("SURF1");

   canvas1->cd(5);
   TH2D * manL = new TH2D("manL", "manual calc. of L", 50, 0, 1, 20, 70, 100);
   manL->SetXTitle(mu->GetTitle());
   manL->SetYTitle(nTTbar->GetTitle());
   for(int i = 1; i <= manL->GetNbinsX(); i++)
   {  double x = manL->GetXaxis()->GetBinCenter(i);
      mu->setVal(x);
      for(int j = 1; j <= manL->GetNbinsY(); j++)
      {  double y = manL->GetYaxis()->GetBinCenter(j);
         nTTbar->setVal(y);
         manL->SetBinContent(i, j, likl->getVal());
      }
   }
   manL->Draw("SURF1");

   canvas1->cd(6);
   TH1 * h_likl = likl->createHistogram("h_likl", *mu, Binning(50, 0, 1), YVar(*nTTbar, Binning(20, 70, 100)));
   h_likl->SetTitle("L vs (mu, nTTbar)");
   h_likl->Draw();

   canvas1->cd(7);
   mu->setVal(muBest);
   nTTbar->setVal(nTTbarBest);
   RooPlot * plot3 = minuit.contour(*mu, *nTTbar, 1, 2, 3) ;
   plot3->SetTitle("RooMinuit contour plot");
   plot3->Draw();