class AliPhysicsSelection: public AliAnalysisCuts

                      Implementation of   Class AliPhysicsSelection
 This class selects collision candidates from data runs, applying selection cuts on triggers
 and background rejection based on the content of the ESD

 Usage:

 Create the object:
   fPhysicsSelection = new AliPhysicsSelection;

 For MC data, call
   fPhysicsSelection->SetAnalyzeMC()

 To check if an event is a collision candidate, use:
   fPhysicsSelection->IsCollisionCandidate(fESD)

 After processing save the resulting histograms to a file with (a folder physics_selection
   will be created that contains the histograms):
   fPhysicsSelection->SaveHistograms("physics_selection")

 To print statistics after processing use:
   fPhysicsSelection->Print();

 The BX ids corresponding to real bunches crossings p2 are
 automatically selected. You cannot process runs with different
 filling schemes if this option is set. If you want to disable this,
 use:
   fPhysicsSelection->SetUseBXNumbers(0);


 If you are analizing muons and you want to keep the muon triggers
 besides the CINT1B you can set:
   fPhysicsSelection->SetUseMuonTriggers();


 To compute the Background automatically using the control triggers
 use:
   fPhysicsSelection->SetComputeBG();
 this will show the value of the Beam Gas, accidentals and good
 events as additional rows in the statistic tables, but it will NOT
 subtract the background automatically.
 This option will only work for runs taken with the CINT1
 suite. This options enables automatically also the usage of BX
 numbers. You can only process one run at a time if you require this
 options, because it uses the bunch intensity estimated run by run.

 The BG will usually be more important in the so-called "bin 0": the
 class can also compute the statistics table for events in this
 bin. Since the definition of bin 0 may in general change from
 analysis to analysis, the user needs to provide a callback
 implementing the definition of bin zero. The callback should be
 implemented as a method in the analysis task and should override
 the IsEventInBinZero method of AliAnalysisTaskSE, and should thus
 have the the following prototype:
   Bool_t IsEventInBinZero();
 It should return true if the event is in the bin 0 and it is set by
 passing to the physics selection the NAME of the task where the
 callback is implemented:
   fPhysicsSelection->SetBin0Callback("MyTask").


 Usually the class selects the trigger scheme by itself depending on the run number.
 Nevertheless, you can do that manually by calling AddCollisionTriggerClass() and AddBGTriggerClass()
 Example:
 To define the class CINT1B-ABCE-NOPF-ALL as collision trigger (those will be accepted as
 collision candidates when they pass the selection):
   AddCollisionTriggerClass("+CINT1B-ABCE-NOPF-ALL #769 #3119");
 To select on bunch crossing IDs in addition, use:
   AddCollisionTriggerClass("+CINT1B-ABCE-NOPF-ALL #769 #3119");
 To define the class CINT1A-ABCE-NOPF-ALL as a background trigger (those will only be counted
 for the control histograms):
   AddBGTriggerClass("+CINT1A-ABCE-NOPF-ALL");
 You can also specify more than one trigger class in a string or you can require that some are *not*
 present. The following line would require CSMBA-ABCE-NOPF-ALL, but CSMBB-ABCE-NOPF-ALL is not allowed
 to be present:
   AddBGTriggerClass("+CSMBA-ABCE-NOPF-ALL -CSMBB-ABCE-NOPF-ALL");

 The class also supports the triggers used in heavy ion runs

   Origin: Jan Fiete Grosse-Oetringhaus, CERN
           Michele Floris, CERN