>> I ask to implement the same for the PDF format (and probably for
>> Info, too) to help with very large indices – for example,
>> LilyPond's main index has a length of almost 30 pages.
>
> [...] PDF readers also have search commands, so I'm not sure what
> exactly is missing here.
Well, a big problem is that normal PDF readers don't have regex
search. Additionally, the index is also used by people who don't know
exactly what they are looking for. As an example, in music there are
appoggiaturas and accacciaturas, which are very similar but not
identical things – (a) you have to know these words, and (b) you have
to know how to spell them correctly. However, the iser might remember
that the word start with letter 'a', so it's quite natural IMHO to
look up this letter in the index.
A similar situation is to look up keywords, functions, variables,
properties, etc., that are very similar, for example, `foo`, `\foo`,
`\Foo`, `\foobar`, `\fooBar`. Within an index you can identify them
easily, however, it's not so easy to search a single one of them
without hitting zillions of unwanted matches.
Werner
