void test_TTreeFormulaAfterFill() {

  TTree * t_new = new TTree("t_new", "Create new tree");
  TTreeFormula * tf_new = new TTreeFormula("tf_new", "var1/var2", t_new);
  
  float var1;
  float var2;
  t_new->Branch("var1",  &var1, "var1/F");
  t_new->Branch("var2", &var2, "var2/F");

  TRandom * rand = new TRandom;
  for (int ievt=0; ievt<100; ievt++) {

    var1 = rand->Rndm();
    var2 = rand->Gaus();

    printf("Before filling: ");
    printf("  var1: %f  var2: %f \n", var1, var2);
    
    t_new->Fill();
    t_new->GetEntry(ievt);
    tf_new = new TTreeFormula("tf_new", "var1/var2", t_new); //I have to define tf_new per entry
    printf("var1/var2 from formular: %10.3f <=>%10.3f\n\n", (float)tf_new->EvalInstance(), (float)var1/var2);
    delete tf_new;
  }

//   //I have to wait until the tree was filled and re-define a TTreeFormular. 
//   tf_new = new TTreeFormula("tf_new", "var1/var2", t_new); //comment this line, tf_new->EvalInstance(ievt) return 0.
//   for (int ievt=0; ievt<t_new->GetEntries(); ievt++) {
//     t_new->GetEntry(ievt);
//     //printf("After filling: var1/var2 from formular: %10.3f <=>%10.3f\n\n", (float)tf_new->EvalInstance(ievt), (float)var1/var2);
//   }
  
}