// ----------------------------------------------------------------------
double f_expo(double *x, double *par) {
  return par[0]*TMath::Exp(-x[0]*par[1]);
}

// ----------------------------------------------------------------------
void bla() {



TFile *f = TFile::Open("QCD_macro_167_fast.root");

  TH1D *h0 = new TH1D("h0", "h0", 50, 0., 50.);
  TF1  *fE = new TF1("fE", f_expo, 0., 50.0, 2);
  fE->SetParameters(10., 0.1);


/* 
 // -- Create histogram with random Exp contents
  int NEVT(1000);
  for (int i = 0; i < NEVT; ++i) {
    h0->Fill(fE->GetRandom());
  }
*/



  // -- patch errors for empty bins
  for (int i = 0; i <invmumugamma->GetNbinsX(); ++i) {
    if (0 == invmumugamma->GetBinContent(i+1)) invmumugamma->SetBinError(i+1, 1);
  }


  // -- Fit histogram
  fE->SetParameters(invmumugamma->GetBinContent(1), invmumugamma->GetMean()/100.);  // the second parameter should be computed...
       

//invmumugamma->Draw();
  // -- Use "L" for low-statistics!
  invmumugamma->Fit("fE", "L");
//invmumugamma->Fit("fE","LR","",2.5,6.0);



/*fE->GetParameter(1);
fE->GetParError(1);
*/

}