[tex-k] Mismatch between makeindex and the included styles under makeindex/bst

[Karl Berry]

    Looking at the styles in
    /usr/local/texlive/2021/texmf-dist/makeindex/base, I noticed that
    they use directives like include and merge_rule that are, in fact,
    not actually implemented in makeindex. Any clues as to
    what’s going on there?

I never noticed. I see the math.ist etc. files on
CTAN:indexing/makeindex/ist. Thus I surmise that Sebastian simply copied
them in when creating TeX Live without thinking about it. As for where
they came from, I don't know Gabor Herr, but I suppose Joachim Schrod
could have been working on a project (previous to xindy) that extended
makeindex. All ~30 years ago, it seems.

I guess I should remove them. --thanks, karl.