class TEntryListBlock: public TObject

 TEntryListBlock

 Used internally in TEntryList to store the entry numbers.

 There are 2 ways to represent entry numbers in a TEntryListBlock:
 1) as bits, where passing entry numbers are assigned 1, not passing - 0
 2) as a simple array of entry numbers
 In both cases, a UShort_t* is used. The second option is better in case
 less than 1/16 of entries passes the selection, and the representation can be
 changed by calling OptimizeStorage() function.
 When the block is being filled, it's always stored as bits, and the OptimizeStorage()
 function is called by TEntryList when it starts filling the next block. If
 Enter() or Remove() is called after OptimizeStorage(), representation is
 again changed to 1).

 Operations on blocks (see also function comments):
 - Merge() - adds all entries from one block to the other. If the first block
             uses array representation, it's changed to bits representation only
             if the total number of passing entries is still less than kBlockSize
 - GetEntry(n) - returns n-th non-zero entry.
 - Next()      - return next non-zero entry. In case of representation 1), Next()
                 is faster than GetEntry()