void cl() {
  TH1F* h = new TH1F("abc", "abc", 3,0,3);
  for (int i = 1; i <= 3; ++i) {
    for (int j = 0; j < i; ++j) {
      h->Fill(i-.5);
    }
  }

  TFitResultPtr r = h->Fit("pol1", "S");

  double point[1] = {1.5};

  double interval[1];

  r->GetConfidenceIntervals(1, 1, 1, point, interval, 0.95);
  std::cout << "confidence interval at x=1.5 at 95% CL: " << interval[0] << std::endl;

  // f = p0 + p1 * x
  // df / dp0 = 1
  // df / dp1 = x
  double myCalculation = sqrt(r->CovMatrix(0,0) + r->CovMatrix(0,1)*1.5 + r->CovMatrix(1,0)*1.5 + r->CovMatrix(1,1)*1.5*1.5);

  std::cout << "my calculation at x=1.5: " << myCalculation << std::endl;
}