 documents will
   not be un-inverted... it will be skipped while building the un-inverted field structure,
   and will use a set intersection method during faceting.

   To further save memory, the terms (the actual string values) are not all stored in
   memory, but a TermIndex is used to convert term numbers to term values only
   for the terms needed after faceting has completed.  Only every 128th term value
   is stored, along with it's corresponding term number, and this is used as an
   index to find