void simple()
{
  // create a random number generator
  gRandom = new TRandom3();

  // create a histogram
  TH1D * cthTrue  = new TH1D("cthTrue", "cos(theta*) true", 10, -1, 1);
  TH1D * cthSeen  = new TH1D("cthSeen", "cos(theta*) seen", 10, -1, 1);
  TH1D * cthEff  = new TH1D("cthEff", "cos(theta*) efficiency", 10, -1, 1);

  for (int i = 0; i < 20000; ++i) {
    Double_t ctht = -1+1e-4*i;
    cthTrue->Fill(ctht);

    // Emulate some efficiency
    Double_t x = 0.3 + (ctht+1)*0.1;
    if ( gRandom->Rndm() < x )
      cthSeen->Fill(ctht);
  }

  TCanvas * c1= new TCanvas("c1","c1",20,20,800,800);
  c1->Divide(1,2);
  c1->cd(1); cthTrue->Draw("hist e");
  c1->cd(2); cthSeen->Draw("hist e");

  cthEff->Divide(cthSeen,cthTrue,1.,1.,"B");

  TCanvas * c2= new TCanvas("c2","c2",60,60,800,800);
  cthEff->Draw();
}