%% An attempt to have several index files
%%
%% Defines      \altindex{filename}{word to index}
%% and          \makealtindex{filename}
%%
%% It is possible to define a macro for each index as follows:
%%              \newcommand{\myindex}{\altindex{myindexfile}}
%%
%% This code is not really clean, there are still a number of things
%% that I don't understand... but it works.

%% \makealtindex{filename} opens filename.idx for writing.

\def\makealtindex#1{\if@filesw
  \expandafter\newwrite\csname @#1altindexfile\endcsname
  \immediate\openout\expandafter\csname @#1altindexfile\endcsname=#1.idx
  \typeout{Writing alternate index file #1.idx}\fi}

%% \@wraltindex makes the assumes that a trailing `\fi' will get bound
%% to #2. So, it `eats' it as second parameter and reinserts it.
%% Quick and dirty, I know...
%% Writes the index entry #3 into #1.

\def\@wraltindex#1#2#3{\let\thepage\relax
   \xdef\@gtempa{\write#1{\string
      \indexentry{#3}{\thepage}}}\fi\endgroup\@gtempa
   \if@nobreak \ifvmode\nobreak\fi\fi\@esphack}

%% \altindex{filename}{index entry} does nothing if
%% \@<filename>altindexfile is \relax (i.e. filename.idx not open).
%% Otherwise, writes the index entry, and closes the whole stuff (some
%% groups, and some \if).

\def\altindex#1{\@bsphack\begingroup
   \def\protect##1{\string##1\space}\@sanitize
   \@ifundefined{@#1altindexfile}%
          {\endgroup\@esphack}%
          {\@wraltindex{\expandafter\csname @#1altindexfile\endcsname}}
}