imperfect support in texindex for @ as escape character (was multilevel indexing for TeX)

[Gavin Smith]

On Sun, Mar 03, 2019 at 08:51:05PM +0000, Gavin Smith wrote:
> As far as I know the only barrier to changing the escape character is
> that old texindex is still installed.

texindex 6.6 doesn't appear to handle @ properly as an index initial.  With 
the input

@entry{\, address@hidden @backslashcurfont }, backslash}
@address@hidden }, open address@hidden {}, open brace}
@address@hidden }, close address@hidden {}, close brace}
@entry{@@, at sign}{1}{@@, at sign}
@entry{@@, at address@hidden {}, at sign}

it gives the output

@initial address@hidden
@entry {@@, at sign}{1}
@initial {\}
@entry address@hidden @backslashcurfont }, backslash}{1}
@initial address@hidden }}
@entry address@hidden {}, open brace}{1}
@initial address@hidden }}
@entry address@hidden {}, close brace}{1}

- it should be @initial{@@} instead.

It doesn't work with \\ in an index entry either (outputing \initial{\} 
if \ is the escape character), but that doesn't matter because 
texinfo.tex is not putting \\ in the index files.