TEntryList

Class Description

 TEntryList

 Stores entry numbers. 

 There are two types of entry lists:
 - for a TTree (fBlocks data member is non-zero)
   Entry numbers are stored in TEntryListBlocks, which, in their turn, are stored
   in the TObjArray fBlocks. The range of the entry numbers is cut into intervals
   of kBlockSize entries (currently 64000), so that the first block contains
   information which entries out of the first 64000 pass the selection, the second
   block - which entries out of the 64000-127999 interval pass the selection, etc.
   Some blocks, obviously, might be empty. The internal representation of entry
   numbers in the blocks is described in the TEntryListBlock class description, and
   this representation might be changed by calling OptimizeStorage() function
   (when the list is filled via the Enter() function, this is done automatically).
   Individual entry lists can be merged (functions Merge() and Add())
   to make an entry list for a TChain of corresponding TTrees.
  
 - for a TChain (fLists data member is non-zero)
   It contains a TList of sub-lists (TEntryList objects, corresponding to each TTree)
   Trees and lists are matched by the TTree name and its file name (full path). 
   All sub-lists are returned by the GetLists() function and individual lists are
   returned by GetEntryList() function. Such lists are no different from the lists for
   TTrees, described above.

 Operations on entry lists (see also function comments):
 - Add() - if the lists are for the same tree, adds all the entries of the second list
           to the first list. If the lists are for different trees, creates a TEntryList
           with 2 sublists for each TTree. If the lists are for TChains, merges the ones
           for the same trees and adds new sublists for the TTrees that were not included
           in the first TEntryList
 - Subtract() - if the lists are for the same TTree, removes the entries of the second
                list from the first list. If the lists are for TChains, loops over all
                sub-lists
 - GetEntry(n) - returns the n-th non-zero entry number 
 - Next()      - returns next non-zero entry number. Note, that this function is 
                 much faster than GetEntry, and it's called when GetEntry() is called
                 for 2 or more indices in a row.

 TEntryList objects are added to the list of objects in the current directory

 TTree::Draw() and TChain::Draw():
   tree->Draw(">>elist", "x<0 && y>0");
   TEntryList *elist = (TEntryList*)gDirectory->Get("elist");

 TSelectors:
   To fill an TEntryList from a TSelector correctly, one must add the TEntryList object
   to the output list of the selector (TSelector::fOutput). This is the only way to 
   make the sub-lists of the TEntryList switch when the current tree of the TChain is
   changed. 

 Using a TEntryList as input (TTree::SetEntryList() and TChain::SetEntryList())
   - while the TTree::SetEntryList() function is only setting the TTree::fEntryList
     data member, the same function in TChain also finds correspondance between
     the TTrees of this TChain and the sub-lists of this TEntryList.

 TEntryList and the current directory:
   - TEntryList objects are automatically added to the current directory (like TTrees).
     However, in case of a TEntryList for a chain, only the top-level entry list is added,
     not the sub-lists for specific trees. Placing entry lists in the current directory
     allows calling them as a part of a TTreeFormula expression, so if the user wants 
     to extract a sublist from a TChain entry list via the GetEntryList() or some other
     function, he has to add it to the current directory to be able to use it in 
     TTreeFormula expressions.