This is makeindex, version 2.17 [TeX Live 2023] (kpathsea + Thai support).
Scanning input file refman.idx....done (362 entries accepted, 0 rejected).
Sorting entries......done (3590 comparisons).
Generating output file refman.ind....done (396 lines written, 0 warnings).
Output written in refman.ind.
Transcript written in refman.ilg.
