Fix checksum.
[mdwtools] / at.dtx
1 % \begin{meta-comment}
2 %
3 % $Id: at.dtx,v 1.1 2002/02/03 20:49:02 mdw Exp $
4 %
5 % Allow @-commands
6 %
7 % (c) 1995 Mark Wooding
8 %
9 %----- Revision history -----------------------------------------------------
10 %
11 % $Log: at.dtx,v $
12 % Revision 1.1 2002/02/03 20:49:02 mdw
13 % Checkin for new build system.
14 %
15 % Revision 1.3 1996/11/19 20:46:55 mdw
16 % Entered into RCS
17 %
18 %
19 % \end{meta-comment}
20 %
21 % \begin{meta-comment} <general public licence>
22 %%
23 %% at package -- support for `@' commands'
24 %% Copyright (c) 1996 Mark Wooding
25 %%
26 %% This program is free software; you can redistribute it and/or modify
27 %% it under the terms of the GNU General Public License as published by
28 %% the Free Software Foundation; either version 2 of the License, or
29 %% (at your option) any later version.
30 %%
31 %% This program is distributed in the hope that it will be useful,
32 %% but WITHOUT ANY WARRANTY; without even the implied warranty of
33 %% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
34 %% GNU General Public License for more details.
35 %%
36 %% You should have received a copy of the GNU General Public License
37 %% along with this program; if not, write to the Free Software
38 %% Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
39 %%
40 % \end{meta-comment}
41 %
42 % \begin{meta-comment} <Package preamble>
43 %<+package>\NeedsTeXFormat{LaTeX2e}
44 %<+package>\ProvidesPackage{at}
45 %<+package> [1996/05/02 1.3 @-command support (MDW)]
46 % \end{meta-comment}
47 %
48 % \CheckSum{355}
49 %% \CharacterTable
50 %% {Upper-case \A\B\C\D\E\F\G\H\I\J\K\L\M\N\O\P\Q\R\S\T\U\V\W\X\Y\Z
51 %% Lower-case \a\b\c\d\e\f\g\h\i\j\k\l\m\n\o\p\q\r\s\t\u\v\w\x\y\z
52 %% Digits \0\1\2\3\4\5\6\7\8\9
53 %% Exclamation \! Double quote \" Hash (number) \#
54 %% Dollar \$ Percent \% Ampersand \&
55 %% Acute accent \' Left paren \( Right paren \)
56 %% Asterisk \* Plus \+ Comma \,
57 %% Minus \- Point \. Solidus \/
58 %% Colon \: Semicolon \; Less than \<
59 %% Equals \= Greater than \> Question mark \?
60 %% Commercial at \@ Left bracket \[ Backslash \\
61 %% Right bracket \] Circumflex \^ Underscore \_
62 %% Grave accent \` Left brace \{ Vertical bar \|
63 %% Right brace \} Tilde \~}
64 %%
65 %
66 % \begin{meta-comment} <driver>
67 %
68 %<*driver>
69 \input{mdwtools}
70 \describespackage{at}
71 \aton
72 \atlet p=\package
73 \atdef at{\package{at}}
74 \atdef={\mbox{-}}
75 \atdef-{@@@=}
76 \atlet.=\syntax
77 \mdwdoc
78 %</driver>
79 %
80 % \end{meta-comment}
81 %
82 % \section{User guide}
83 %
84 % The @at\ package is an attempt to remove a lot of tedious typing that
85 % ends up in \LaTeX\ documents, by expanding the number of short command
86 % names available. The new command names begin with the `|@|' character,
87 % rather than the conventional `|\|', so you can tell them apart.
88 %
89 % The package provides some general commands for defining @-commands, and
90 % then uses them to define some fairly simple ones which will be useful to
91 % most people.
92 %
93 % The rules for @-command names aren't terribly complex:
94 % \begin{itemize}
95 % \item If the first character of the name is a letter, then the command name
96 % consists of all characters up to, but not including, the first
97 % nonletter. Spaces following the command name are ignored.
98 % \item If the first character of the name is a backslash, then the @-command
99 % name consists of the control sequence introduced by the backslash.
100 % \item Otherwise, the command name consists only of that first character.
101 % Spaces following the name are not ignored, unless that character
102 % was itself a space character.
103 % \end{itemize}
104 %
105 % Usually, digits are not considered to be letters. However, the
106 % \package{at} package will consider digits to be letters if you give it the
107 % \textsf{digits} option in the |\usepackage| command. (Note that this
108 % only affects the \package{at} package; it won't change the characters
109 % allowed in normal command names.)
110 %
111 % \DescribeMacro{\atallowdigits}
112 % \DescribeMacro{\atdisallowdigits}
113 % You can enable and disable digits being considered as letters dynamically.
114 % The |\atallowdigits| command allows digits to be used as letters;
115 % |\atdisallowdigits| prevents this. Both declarations follow \LaTeX's
116 % usual scoping rules. Both of these commands have corresponding
117 % environments with the same names (without the leading `|\|', obviously).
118 %
119 % \subsection{Defining @-commands}
120 %
121 % \DescribeMacro{\newatcommand}
122 % \DescribeMacro{\renewatcommand}
123 % The |\newatcommand| command will define a new @-command using a syntax
124 % similar to |\newcommand|. For example, you could define
125 % \begin{listing}
126 %\newatcommand c[1]{\chapter{#1}}
127 % \end{listing}
128 % to make @.{"@c{"<name>"}"} equivalent to @.{"\\chapter{"<name>"}"}.
129 %
130 % A |\renewatcommand| is also provided to redefine existing commands, should
131 % the need arise.
132 %
133 % \DescribeMacro{\atdef}
134 % For \TeX\ hackers, the |\atdef| command defines @-commands using a syntax
135 % similar to \TeX's built-in |\def|.
136 %
137 % As an example, the following command makes @.{"@/"<text>"/"} write its
138 % argument \<text> in italics:
139 % \begin{listing}
140 %\atdef/#1/{\textit{#1}}
141 % \end{listing}
142 % The real implementation of the |@/|\dots|/| command is a bit more
143 % complex, and is given in the next section.
144 %
145 % You can use all of \TeX's features for defining the syntax of your
146 % command. (See chapter~20 of @/The \TeX book/ for more details.)
147 %
148 % \DescribeMacro{\atlet}
149 % Since |\atdef| is provided to behave similarly to |\def|, @at\ provides
150 % |\atlet| which works similarly to |\let|. For example you can say
151 % \begin{listing}
152 %\atlet!=\index
153 % \end{listing}
154 % to allow the short |@!| to behave exactly like |\index|.
155 %
156 % Note that all commands defined using these commands are robust even if you
157 % use fragile commands in their definitions. Unless you start doing very
158 % strange things, @-commands never need |\protect|ing.
159 %
160 % \subsection{Predefined @-commands}
161 %
162 % A small number of hopefully useful commands are provided by default.
163 % These are described in the table below:
164 %
165 % \bigskip \begin{center} \begin{tabular}{lp{3in}} \hline
166 % \bf Command & \bf Meaning \\ \hline
167 % @.{"@@"} & Typesets an `@@' character. \\
168 % @.{"@/"<text>"/"} & In text (LR or paragraph) mode, typesets its
169 % argument emphasised. In maths mode, it
170 % always chooses italics. \\
171 % @.{"@*"<text>"*"} & Typesets its argument \<text> in bold. \\
172 % @.{"@i{"<text>"}"} & Equivalent to `@.{"\\index{"<text>"}"}'. \\
173 % @.{"@I{"<text>"}"} & As for |@i|, but also writes its argument
174 % to the document. \\ \hline
175 % \end{tabular} \end{center} \bigskip
176 %
177 % Package writers should not rely on any predefined @-commands -- they're
178 % provided for users, and users should be able to redefine them without
179 % fear of messing anything up. (This includes the `standard' commands
180 % provided by the @at\ package, by the way. They're provided in the vague
181 % hope that they might be useful, and as examples.)
182 %
183 % \implementation
184 %
185 % \section{Implementation}
186 %
187 % \begin{macrocode}
188 %<*package>
189 % \end{macrocode}
190 %
191 % \subsection{Options handling}
192 %
193 % We need a switch to say whether digits should be allowed. Since this
194 % is a user thing, I'll avoid |\newif| and just define the thing by hand.
195 %
196 % \begin{macrocode}
197 \def\atallowdigits{\let\ifat@digits\iftrue}
198 \def\atdisallowdigits{\let\ifat@digits\iffalse}
199 % \end{macrocode}
200 %
201 % Now define the options.
202 %
203 % \begin{macrocode}
204 \DeclareOption{digits}{\atallowdigits}
205 \DeclareOption{nodigits}{\atdisallowdigits}
206 \ExecuteOptions{nodigits}
207 \ProcessOptions
208 % \end{macrocode}
209 %
210 % \subsection{How the commands work}
211 %
212 % Obviously we make the `@@' character active. It inspects the next
213 % character (or argument, actually -- it can be enclosed in braces for
214 % longer commands, although this is a bit futile), and builds the command
215 % name from that.
216 %
217 % The |\at| command is equivalent to the active `@@' character always.
218 %
219 %
220 % \subsection{Converting command names}
221 %
222 % We need to be able to read an @-command name, and convert it to a normal
223 % \TeX\ control sequence. First, we declare some control sequences for
224 % braces, which we need later.
225 %
226 % \begin{macrocode}
227 \begingroup
228 \catcode`\<1
229 \catcode`\>2
230 \catcode`\{12
231 \catcode`\}12
232 \gdef\at@lb<{>
233 \gdef\at@rb<}>
234 \gdef\at@spc< >
235 \endgroup
236 % \end{macrocode}
237 %
238 % I'll set up some helper routines now, to help me read the command
239 % names. The way this works is that we |\futurelet| the token into
240 % |\@let@token|. These routines will then sort out what to do next.
241 %
242 % \begin{macro}{\at@test}
243 %
244 % Given an |\if|\dots\ test, does its first or second argument.
245 %
246 % \begin{macrocode}
247 \def\at@test#1\then{%
248 #1\expandafter\@firstoftwo\else\expandafter\@secondoftwo\fi%
249 }
250 % \end{macrocode}
251 %
252 % \end{macro}
253 %
254 % \begin{macro}{\at@ifcat}
255 %
256 % Checks the category code of the current character. If it matches the
257 % argument, it does its second argument, otherwise it does the third.
258 %
259 % \begin{macrocode}
260 \def\at@ifcat#1{\at@test\ifcat#1\noexpand\@let@token\then}
261 % \end{macrocode}
262 %
263 % \end{macro}
264 %
265 % \begin{macro}{\at@ifletter}
266 %
267 % This routine tests the token to see if it's a letter, and if so adds
268 % it to the token list and does the first argument; otherwise it does the
269 % second argument. It accepts digits as letters if the switch is turned
270 % on.
271 %
272 % There's some fun later, so I'll describe this slowly. First, we compare
273 % the category code to a letter, and if we have a match, we know we're done;
274 % we need to pick up the letter as an argument. If the catcode is `other',
275 % we must compare with numbers to see if it's in range.
276 %
277 % \begin{macrocode}
278 \def\at@ifletter#1#2{%
279 \at@ifcat x%
280 {\at@ifletter@ii{#1}}%
281 {\at@ifcat 0%
282 {\at@ifletter@i{#1}{#2}}%
283 {#2}%
284 }%
285 }
286 % \end{macrocode}
287 %
288 % Right. It's `other' (so it's safe to handle as a macro argument) and we
289 % need to know if it's a digit. This is a little tricky: I use |\if| to
290 % compare two characters. The first character is~`1' or~`0' depending on the
291 % `digit' switch; the second is~`1' or~`x' depending on whether it's actually
292 % a digit. They'll only match if everything's worked out OK.
293 %
294 % \begin{macrocode}
295 \def\at@ifletter@i#1#2#3{%
296 \at@test\if%
297 \ifat@digits1\else0\fi%
298 \ifnum`#3<`0x\else\ifnum`#3>`9x\else1\fi\fi%
299 \then%
300 {\at@ifletter@ii{#1}{#3}}%
301 {#2#3}%
302 }
303 % \end{macrocode}
304 %
305 % Right; we have the character, so add it to the list and carry on.
306 %
307 % \begin{macrocode}
308 \def\at@ifletter@ii#1#2{\toks@\expandafter{\the\toks@#2}#1}
309 % \end{macrocode}
310 %
311 % \end{macro}
312 %
313 % Now we define the command name reading routines. We have @/almost/ the
314 % same behaviour as \TeX, although we can't support `|%|' characters for
315 % reasons to do with \TeX's tokenising algorithm.
316 %
317 % \begin{macro}{\at@read@name}
318 %
319 % The routine which actually reads the command name works as follows:
320 % \begin{enumerate}
321 % \item Have a peek at the next character. If it's a left or right brace,
322 % then use the appropriate character.
323 % \item If the character is not a letter, just use the character (or whole
324 % control sequence.
325 % \item Finally, if it's a letter, keep reading letters until we find one
326 % that wasn't.
327 % \end{enumerate}
328 %
329 % First, we do some setting up and read the first character
330 %
331 % \begin{macrocode}
332 \def\at@read@name#1{%
333 \let\at@next=#1%
334 \toks@{}%
335 \futurelet\@let@token\at@rn@i%
336 }
337 % \end{macrocode}
338 %
339 % Next, sort out what to do, based on the category code.
340 %
341 % \begin{macrocode}
342 \def\at@rn@i{%
343 \def\@tempa{\afterassignment\at@rn@iv\let\@let@token= }%
344 \at@ifletter%
345 {\futurelet\@let@token\at@rn@iii}%
346 {\at@ifcat\bgroup%
347 {\toks@\expandafter{\at@lb}\@tempa}%
348 {\at@ifcat\egroup%
349 {\toks@\expandafter{\at@rb}\@tempa}%
350 {\at@ifcat\at@spc%
351 {\toks@{ }\@tempa}%
352 {\at@rn@ii}%
353 }%
354 }%
355 }%
356 }
357 % \end{macrocode}
358 %
359 % Most types of tokens can be fiddled using |\string|.
360 %
361 % \begin{macrocode}
362 \def\at@rn@ii#1{%
363 \toks@\expandafter{\string#1}%
364 \at@rn@iv%
365 }
366 % \end{macrocode}
367 %
368 % We've found a letter, so we should check for another one.
369 %
370 % \begin{macrocode}
371 \def\at@rn@iii{%
372 \at@ifletter%
373 {\futurelet\@let@token\at@rn@iii}%
374 {\@ifnextchar.\at@rn@iv\at@rn@iv}%
375 }
376 % \end{macrocode}
377 %
378 % Finally, we need to pass the real string, as an argument, to the
379 % macro. We make |\@let@token| relax, since it might be something which will
380 % upset \TeX\ later, e.g., a |#| character.
381 %
382 % \begin{macrocode}
383 \def\at@rn@iv{%
384 \let\@let@token\relax%
385 \expandafter\at@next\csname at.\the\toks@\endcsname%
386 }
387 % \end{macrocode}
388 %
389 % \end{macro}
390 %
391 % \begin{macro}{\at@cmdname}
392 %
393 % Given a control sequence, work out which @-command it came from.
394 %
395 % \begin{macrocode}
396 \def\at@cmdname#1{\expandafter\at@cmdname@i\string#1\@@foo}
397 % \end{macrocode}
398 %
399 % Now extract the trailing bits.
400 %
401 % \begin{macrocode}
402 \def\at@cmdname@i#1.#2\@@foo{#2}
403 % \end{macrocode}
404 %
405 % \end{macro}
406 %
407 % \begin{macro}{\at@decode}
408 %
409 % The |\at@decode| macro takes an extracted @-command name, and tries to
410 % execute the correct control sequence derived from it.
411 %
412 % \begin{macrocode}
413 \def\at@decode#1{%
414 \at@test\ifx#1\relax\then{%
415 \PackageError{at}{Unknown @-command `@\at@cmdname#1'}{%
416 The @-command you typed wasn't recognised, so I've ignored it.
417 }%
418 }{%
419 #1%
420 }%
421 }
422 % \end{macrocode}
423 %
424 % \end{macro}
425 %
426 % \begin{macro}{\@at}
427 %
428 % We'd like a measure of compatibility with @p{amsmath}. The @-commands
429 % provided by @p{amsmath} work only in maths mode, so this gives us a way of
430 % distinguishing. If the control sequence |\Iat| is defined, and we're in
431 % maths mode, we'll call that instead of doing our own thing.
432 %
433 % \begin{macrocode}
434 \def\@at{%
435 \def\@tempa{\at@read@name\at@decode}%
436 \ifmmode\ifx\Iat\not@@defined\else%
437 \let\@tempa\Iat%
438 \fi\fi%
439 \@tempa%
440 }
441 % \end{macrocode}
442 %
443 % \end{macro}
444 %
445 %
446 % \subsection{Defining new commands}
447 %
448 % \begin{macro}{\at@buildcmd}
449 %
450 % First, we define a command to build these other commands:
451 %
452 % \begin{macrocode}
453 \def\at@buildcmd#1#2{%
454 \expandafter\def\csname\expandafter
455 \@gobble\string#1@decode\endcsname##1{#2##1}%
456 \edef#1{%
457 \noexpand\at@read@name%
458 \expandafter\noexpand%
459 \csname\expandafter\@gobble\string#1@decode\endcsname%
460 }%
461 }
462 % \end{macrocode}
463 %
464 % \end{macro}
465 %
466 % \begin{macro}{\newatcommand}
467 % \begin{macro}{\renewatcommand}
468 % \begin{macro}{\provideatcommand}
469 % \begin{macro}{\atdef}
470 % \begin{macro}{\atshow}
471 %
472 % Now we define the various operations on @-commands.
473 %
474 % \begin{macrocode}
475 \at@buildcmd\newatcommand\newcommand
476 \at@buildcmd\renewatcommand\renewcommand
477 \at@buildcmd\provideatcommand\providecommand
478 \at@buildcmd\atdef\def
479 \at@buildcmd\atshow\show
480 % \end{macrocode}
481 %
482 % \end{macro}
483 % \end{macro}
484 % \end{macro}
485 % \end{macro}
486 % \end{macro}
487 %
488 % \begin{macro}{\atlet}
489 %
490 % |\atlet| is rather harder than the others, because we want to allow people
491 % to say things like @.{"\\atlet"<name>"=@"<name>}. The following hacking
492 % does the trick. I'm trying very hard to duplicate |\let|'s behaviour with
493 % respect to space tokens here, to avoid any surprises, although there
494 % probably will be some differences. In particular, |\afterassignment|
495 % won't work in any sensible way.
496 %
497 % First, we read the name of the @-command we're defining. We also open
498 % a group, to stop messing other people up, and make `@@' into an `other'
499 % token, so that it doesn't irritatingly look like its meaning as a control
500 % sequence.
501 %
502 % \begin{macrocode}
503 \def\atlet{%
504 \begingroup%
505 \@makeother\@%
506 \at@read@name\atlet@i%
507 }
508 % \end{macrocode}
509 %
510 % Put the name into a scratch macro for later use. Now see if there's an
511 % equals sign up ahead. If not, this will gobble any spaces in between the
512 % @-command name and the argument.
513 %
514 % \begin{macrocode}
515 \def\atlet@i#1{%
516 \def\at@temp{#1}%
517 \@ifnextchar=\atlet@ii{\atlet@ii=}%
518 }
519 % \end{macrocode}
520 %
521 % Now we gobble the equals sign (whatever catcode it is), and peek at the
522 % next token up ahead using |\let| with no following space.
523 %
524 % \begin{macrocode}
525 \def\atlet@ii#1{\afterassignment\atlet@iii\global\let\at@gnext=}
526 % \end{macrocode}
527 %
528 % The control sequence |\at@gnext| is now |\let| to be whatever we want the
529 % @-command to be, unless it's picked up an `@@' sign. If it has, we've
530 % eaten the |@| token, so just read the name and pass it on. Otherwise,
531 % we can |\let| the @-command directly to |\at@gnext|. There's some
532 % nastiness here to make |\the\toks@| expand before we close the group and
533 % restore its previous definition.
534 %
535 % \begin{macrocode}
536 \def\atlet@iii{%
537 \if @\noexpand\at@gnext%
538 \expandafter\at@read@name\expandafter\atlet@iv%
539 \else%
540 \expandafter\endgroup%
541 \expandafter\let\at@temp= \at@gnext%
542 \fi%
543 }
544 % \end{macrocode}
545 %
546 % We've read the source @-command name, so just copy the definitions over.
547 %
548 % \begin{macrocode}
549 \def\atlet@iv#1{%
550 \expandafter\endgroup%
551 \expandafter\let\at@temp=#1%
552 }
553 % \end{macrocode}
554 %
555 % \end{macro}
556 %
557 %
558 % \subsection{Robustness of @-commands}
559 %
560 % We want all @-commands to be robust. We could leave them all being
561 % fragile, although making robust @-commands would then be almost impossible.
562 % There are two problems which we must face:
563 %
564 % \begin{itemize}
565 %
566 % \item The `|\@at|' command which scans the @-command name is (very)
567 % fragile. I could have used |\DeclareRobustCommand| for it (and in
568 % fact I did in an earlier version), but that doesn't help the other
569 % problem at all.
570 %
571 % \item The `name' of the @-command may contain active characters or control
572 % sequences, which will be expanded at the wrong time unless we do
573 % something about it now.
574 %
575 % \end{itemize}
576 %
577 % We must also be careful not to introduce extra space characters into any
578 % files written, because spaces are significant in @-commands. Finally,
579 % we have a minor problem in that most auxiliary files are read in with
580 % the `@@' character set to be a letter.
581 %
582 % \begin{macro}{\at}
583 %
584 % Following the example of \LaTeX's `short' command handling, we'll define
585 % |\at| to decide what to do depending on what |\protect| looks like. If
586 % we're typesetting, we just call |\@at| (above) and expect it to cope.
587 % Otherwise we call |\at@protect|, which scoops up the |\fi| and the |\@at|,
588 % and inserts other magic.
589 %
590 % \begin{macrocode}
591 \def\at{\ifx\protect\@typeset@protect\else\at@protect\fi\@at}
592 % \end{macrocode}
593 %
594 % \end{macro}
595 %
596 % \begin{macro}{\at@protect}
597 %
598 % Since we gobbled the |\fi| from the above, we must put that back. We then
599 % need to do things which are more complicated. If |\protect| is behaving
600 % like |\string|, then we do one sort of protection. Otherwise, we assume
601 % that |\protect| is being like |\noexpand|.
602 %
603 % \begin{macrocode}
604 \def\at@protect\fi#1{%
605 \fi%
606 \ifx\protect\string%
607 \expandafter\at@protect@string%
608 \else%
609 \expandafter\at@protect@noexpand%
610 \fi%
611 }
612 % \end{macrocode}
613 %
614 % \end{macro}
615 %
616 % \begin{macro}{\at@protect@string}
617 %
618 % When |\protect| is |\string|, we don't need to be able to recover the
619 % original text particularly accurately -- it's for the user to look at.
620 % Therefore, we just output a $|@|_{11}$ and use |\string| on the next
621 % token. This must be sufficient, since we only allow multi-token command
622 % names if the first token is a letter (code~11).
623 %
624 % \begin{macrocode}
625 \def\at@protect@string{@\string}
626 % \end{macrocode}
627 %
628 % \end{macro}
629 %
630 % \begin{macro}{\at@protect@noexpand}
631 %
632 % This is a little more complex, since we're still expecting to be executed
633 % properly at some stage. However, there's a cheeky dodge we can employ
634 % since the |\at| command is thoroughly robustified (or at least it will be
635 % by the time we've finished this). All |\@unexpandable@protect| does
636 % is confer repeated robustness on a fragile command. Since our command
637 % is robust, we don't need this and we can get away with just using a
638 % single |\noexpand|, both for the |\@at@| command and the following token
639 % (which we must robustify, because no-one else can do it for us -- if
640 % anyone tries, they end up using the |@\protect| command which is rather
641 % embarassing).
642 %
643 % I'll give the definition, and then examine how this expands in various
644 % cases.
645 %
646 % \begin{macrocode}
647 \def\at@protect@noexpand{\noexpand\@at@ @\noexpand}
648 \def\@at@#1{\at}
649 % \end{macrocode}
650 %
651 % A few points, before we go into the main examination of the protection.
652 % I've inserted a $|@|_{11}$ token, which is gobbled by |\@at@| when the
653 % thing is finally expanded fully. This prevents following space tokens
654 % in an |\input| file from being swallowed because they follow a control
655 % sequence. (I can't use the normal $|@|_{13}$ token, because when files
656 % like the |.aux| file are read in, |@| is given code~11 by
657 % |\makeatletter|.)
658 %
659 % \setbox0\hbox{|@at@|}
660 % Now for a description of why this works. When |\at| is expanded, it works
661 % out that |\protect| is either |\noexpand| or |\@unexpandable@protect|, and
662 % becomes |\at@protect@noexpand|. Because of the |\noexpand| tokens, this
663 % stops being expanded once it reaches $\fbox{\box0}\,|@|_{11}\,x$ (where
664 % $x$ is the token immediately following the $|@|_{13}$ character). If this
665 % is expanded again, for example in another |\edef|, or in a |\write| or a
666 % |\mark|, the |\@at@| wakes up, gobbles the following |@| (whatever catcode
667 % it is -- there may be intervening |\write| and |\input| commands) and
668 % becomes |\at|, and the whole thing can start over again.
669 %
670 % \end{macro}
671 %
672 %
673 % \subsection{Enabling and disabling @-commands}
674 %
675 % \begin{macro}{\aton}
676 %
677 % We define the |\aton| command to enable all of our magic. We store
678 % the old catcode in the |\atoff| command, make `@@' active, and make it
679 % do the stuff.
680 %
681 % \begin{macrocode}
682 \def\aton{%
683 \ifnum\catcode`\@=\active\else%
684 \edef\atoff{\catcode`\noexpand\@\the\catcode`\@}%
685 \catcode`\@\active%
686 \lccode`\~`\@%
687 \lowercase{\let~\at}%
688 \fi%
689 }
690 % \end{macrocode}
691 %
692 % \end{macro}
693 %
694 % \begin{macro}{\atoff}
695 %
696 % The |\atoff| command makes `@@' do the stuff it's meant to. We remember
697 % the old catcode and revert to it. This is largely unnecessary.
698 %
699 % \begin{macrocode}
700 \def\atoff{\catcode`\@12}
701 % \end{macrocode}
702 %
703 % \end{macro}
704 %
705 % \begin{macro}{\makeatother}
706 %
707 % Now we make our active `@@' the default outside of package files.
708 %
709 % \begin{macrocode}
710 \let\makeatother\aton
711 % \end{macrocode}
712 %
713 % \end{macro}
714 %
715 % And we must make sure that the user can use all of our nice commands.
716 % Once the document starts, we allow @-commands.
717 %
718 % \begin{macrocode}
719 \AtBeginDocument{\aton}
720 % \end{macrocode}
721 %
722 % \begin{macro}{\dospecials}
723 % \begin{macro}{\@sanitize}
724 %
725 % We must add the `@@' character to the various specials lists.
726 %
727 % \begin{macrocode}
728 \expandafter\def\expandafter\dospecials\expandafter{\dospecials\do\@}
729 \expandafter\def\expandafter\@sanitize\expandafter{%
730 \@sanitize\@makeother\@}
731 % \end{macrocode}
732 %
733 % \end{macro}
734 % \end{macro}
735 %
736 % \subsection{Default @-commands}
737 %
738 % We define some trivial examples to get the user going.
739 %
740 % \begin{macrocode}
741 \expandafter\chardef\csname at.@\endcsname=`\@
742 \atdef*#1*{\ifmmode\mathbf{#1}\else\textbf{#1}\fi}
743 \atdef/#1/{\ifmmode\mathit{#1}\else\emph{#1}\fi}
744 \atlet i=\index
745 \atdef I#1{#1\index{#1}}
746 %</package>
747 % \end{macrocode}
748 %
749 % \hfill Mark Wooding, \today
750 %
751 % \Finale
752 %
753 \endinput