This is makeindex, version 2.15 [TeX Live 2021] (kpathsea + Thai support).
Scanning input file refman.idx......done (2810 entries accepted, 0 rejected).
Sorting entries.............................done (38198 comparisons).
Generating output file refman.ind......done (3668 lines written, 0 warnings).
Output written in refman.ind.
Transcript written in refman.ilg.