Back in June 2003, we discussed methods for reliably determining if an
Info node is an index node.  This is to make the ][ commands work
better, among other reasons.  See this message from Luc and the
containing thread for background:
http://mail.gnu.org/archive/html/emacs-devel/2003-06/msg00657.html

Our conclusion was that the best approach would be for makeinfo to write
a magic cookie indicating "this is an index" into index nodes.

We are now approaching the next Texinfo release.  Here is my question:
is the collective wisdom that I should write this cookie now, before an
Emacs release has been made that supports it?  Or should I wait?

If  a user reads an Info file with the cookie when Emacs doesn't support
it, it will be displayed, thus:
^@^H[index^@^H]
(except with real control characters).  This does no harm, but I can see
users being confused or thinking it is a bug if they see it.

It doesn't matter to me.

I'll attach an example info file with the cookie.  It also shows the
(line NNN) feature for precisely locating index entries that we
discussed in another thread.  I hope someone will be able to work on
using the line numbers in the Emacs Info reader.

Thanks,
k