This is MakeIndex, version 3.0.8 [31 Dec 91].
Scanning style file bibtool.ist...

** Input style error (file = bibtool.ist, line = 1):
   -- Unknown style specifier: 'lethead_prefix'.** Input style error (file = bibtool.ist, line = 2):
   -- Unknown style specifier: 'lethead_suffix'.** Input style error (file = bibtool.ist, line = 3):
   -- Unknown style specifier: 'lethead_flag'.Scanning input file c_lib.idx....
done (237 entries accepted, 0 rejected).
Sorting entries.....done (2115 comparisons).
Generating output file c_lib.ind....
done (330 lines written, 0 warnings).
Output written in c_lib.ind.
Transcript written in c_lib.ilg.