docs: def load_HEP_data(
      ROOT_filename            = "output.root",
      tree_name                = "nominal",
      maximum_number_of_events = None
      ):
      """
      Load HEP data and return dataset.
      """
      ROOT_file        = open_ROOT_file(ROOT_filename)
      tree             = ROOT_file.Get(tree_name)
      number_of_events = tree.GetEntries()
      data             = datavision.Dataset()
  
      progress = shijian.Progress()
      progress.engage_quick_calculation_mode()
      # counters
      number_of_events_loaded = 0
  
      log.info("")
  
      index = 0
      for event in tree:
  
          if maximum_number_of_events is not None and\
              number_of_events_loaded >= int(maximum_number_of_events):
              log.info(
                  "loaded maximum requested number of events " +
                  "({maximum_number_of_events})\r".format(
                      maximum_number_of_events = maximum_number_of_events
                  )
              )
              break
          print progress.add_datum(fraction = (index + 2) / number_of_events),
  
          if select_event(event):
              index += 1
              #event.GetReadEntry()
              #data.variable(index = index, name = "eventNumber",        value = event.eventNumber)
              data.variable(index = index, name = "el_1_pt",            value = event.el_pt[0])
              #data.variable(index = index, name = "el_1_eta",           value = event.el_eta[0])
              #data.variable(index = index, name = "el_1_phi",           value = event.el_phi[0])
              ##data.variable(index = index, name = "jet_1_pt",           value = event.jet_pt[0])
              #data.variable(index = index, name = "jet_1_eta",          value = event.jet_eta[0])
              #data.variable(index = index, name = "jet_1_phi",          value = event.jet_phi[0])
              ##data.variable(index = index, name = "jet_1_e",            value = event.jet_e[0])
              ##data.variable(index = index, name = "jet_2_pt",           value = event.jet_pt[1])
              #data.variable(index = index, name = "jet_2_eta",          value = event.jet_eta[1])
              #data.variable(index = index, name = "jet_2_phi",          value = event.jet_phi[1])
              ##data.variable(index = index, name = "jet_2_e",            value = event.jet_e[1])
              #data.variable(index = index, name = "nJets",              value = event.nJets)
              ##data.variable(index = index, name = "nBTags",             value = event.nBTags)
              ##data.variable(index = index, name = "nLjets",             value = event.nLjets)
              ##data.variable(index = index, name = "ljet_1_m",           value = event.ljet_m[0])
              #data.variable(index = index, name = "met",                value = event.met_met)
              #data.variable(index = index, name = "met_phi",            value = event.met_phi)
              #data.variable(index = index, name = "Centrality_all",     value = event.Centrality_all)
              #data.variable(index = index, name = "Mbb_MindR",          value = event.Mbb_MindR)
              #data.variable(index = index, name = "ljet_tau21",         value = event.ljet_tau21),
              #data.variable(index = index, name = "ljet_tau32",         value = event.ljet_tau32),
              #data.variable(index = index, name = "Aplan_bjets",        value = event.Aplan_bjets),
              #data.variable(index = index, name = "H4_all",             value = event.H4_all),
              #data.variable(index = index, name = "NBFricoNN_6jin4bin", value = event.NBFricoNN_6jin4bin),
              #data.variable(index = index, name = "NBFricoNN_6jin3bex", value = event.NBFricoNN_6jin3bex),
              #data.variable(index = index, name = "NBFricoNN_5jex4bin", value = event.NBFricoNN_5jex4bin),
              #data.variable(index = index, name = "NBFricoNN_3jex3bex", value = event.NBFricoNN_3jex3bex),
              #data.variable(index = index, name = "NBFricoNN_4jin3bex", value = event.NBFricoNN_4jin3bex),
              #data.variable(index = index, name = "NBFricoNN_4jin4bin", value = event.NBFricoNN_4jin4bin)
  
              number_of_events_loaded += 1
  
      log.info("")
  
      return data