int df015_LazyDataSource()
{
   using namespace ROOT::RDF;

   // Let's first create a RDF that will read from the CSV file.
   // See the tutorial (https://root.cern/doc/master/df014__CSVDataSource_8C.html) on CSV data sources for more details!
   auto fileUrl = "http://root.cern/files/tutorials/df014_CsvDataSource_MuRun2010B.csv";
   auto csv_rdf = FromCSV(fileUrl);

   // Now we take out two columns: px and py of the first muon in the muon pair
   std::string px1Name = "px1";
   auto px1 = csv_rdf.Take<double>(px1Name);
   std::string py1Name = "py1";
   auto py1 = csv_rdf.Take<double>(py1Name);

   // Now we create a new dataframe built on top of the columns above. Note that up to now, no event loop
   // has been carried out!
   auto df = MakeLazyDataFrame(std::make_pair(px1Name, px1), std::make_pair(py1Name, py1));

   // We build a histogram of the transverse momentum of the muons.
   auto ptFormula = [](double px, double py) { return sqrt(px * px + py * py); };
   auto pt_h = df.Define("pt", ptFormula, {"px1", "py1"})
                 .Histo1D<double>({"pt", "Muon p_{T};p_{T} [GeV/c];", 128, 0, 128}, "pt");

   auto can = new TCanvas();
   can->SetLogy();
   pt_h->DrawCopy();

   return 0;
}