summaryrefslogtreecommitdiff
path: root/macros/latex/contrib/ifnextok/ifnextok.sty
blob: 552b8b73ec965f92bce60cef0338adc8a5de1ff2 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
\NeedsTeXFormat{LaTeX2e}[1994/12/01]
\ProvidesPackage{ifnextok}[2011/06/27 v0.3 test next token (UL)]

%% Copyright (C) 2011 Uwe Lueck, 
%% http://www.contact-ednotes.sty.de.vu 
%% -- author-maintained in the sense of LPPL below -- 
%%
%% This file can be redistributed and/or modified under 
%% the terms of the LaTeX Project Public License; either 
%% version 1.3c of the License, or any later version.
%% The latest version of this license is in
%%     http://www.latex-project.org/lppl.txt
%% We did our best to help you, but there is NO WARRANTY. 
%%
%% Please report bugs, problems, and suggestions via 
%% 
%%   http://www.contact-ednotes.sty.de.vu 
%%
%% == Outline ==
%% The 'ifnextok' package deals with the behavior of 
%% \LaTeX's internal `\@ifnextchar' to skip blank spaces. 
%% This sometimes has surprising or for some users really 
%% \emph{unwanted} effects, especially with brackets following 
%% `\\' when                    %% v0.1a: was `where'
%% the user does \emph{not} intend to specify an 
%% optional argument, rather wants that brackets are \emph{printed.} 
%% The package offers commands and options for modifying this 
%% behavior, maybe limited to certain parts of the 
%% document source. They are described in the sections below 
%% together with the presentation of the implementation.
%% 
%% As after multiletter commands blank spaces are skipped anyway 
%% (\TeX book p.~46f.), the package makes a \emph{difference only 
%% for one-symbol commands} such as |\\|, or for %% \emph 2011/05/27
%% \emph{active characters} such as the double quote with 
%% \ctanpkgref{german}`.sty' and \ctanpkgref{babel}.
%% (v0.21: Or also optional arguments 
%%  \emph{following mandatory ones}---``trailing" optional arguments 
%%  mentioned by Lars Hellstr\"om and Bruno Le Floch on `LATEX-L'. 
%%  v0.21a: Moreover, with ``starred" command versions having a 
%%  first optional argument!)   %% 2011/06/14
%% 
%% Similar things happen in \ctanpkgref{amsmath} and 
%% \ctanpkgref{mathtools}, and as of v0.21,     %% v0.21a 2011/06/14
%% we discuss relations to these packages.
%% 
%% A little \strong{overview} of the package's commands and options:
%%                               %% structured 2011/05/27
%% === For Macro Writers ===
%% \begin{enumerate}
%%  \item |\IfNextToken| is an alternative to `\@ifnextchar', 
%%      not skipping spaces (\secref{main}). 
%%      This macro is the \strong{low-level} 
%%      backbone of all other modifications of \LaTeX\ commands.
%%  \item |\IfStarNextToken| is an alternative to `\@ifstar', 
%%      not skipping spaces, using `\IfNextToken' in lieu of 
%%      `\@ifnextchar' (\secref{star}).
%%  \item Some ``\strong{patching}" commands aim at modifying 
%%      existing (\LaTeX) macros without specifying the resulting 
%%      new definition explicitly 
%%      (Sections~\ref{sec:patch} and \ref{sec:star}). 
%%      As a package writer, 
%%      you just must know which macros need to be modified
%%      and specify their names as arguments for the patching macros. 
%%  \item There are low-level commands 
%%      |\INTstore| and |\INTrestore|
%%      for undoing modifications of existing macros 
%%      (\secref{stored}).
%% \end{enumerate}
%%
%% === For End-Users ===
%%  % \item 
%%      There are \strong{high-level} commands for modifying 
%%      |\\| and selecting \LaTeX\ \strong{environments} 
%%      to be affected (\secref{break}). 
%%      Package \strong{options} execute some of them 
%%      (\secref{options}), e.g., `[stdbreaks]'.
%%
%% === Intermediate ===
%% |\MakeNotSkipping{<target>}{<on-space>}| 
%% described in \secref{genpatch} is somewhat ``intermediate." 
%% It acts on a document-level command <target> without 
%% any assumptions about its internals. 
%% On the other hand, choosing <on-space> for the new behavior 
%% of <target> in front of a space token may need some knowledge ...
%%
%% (TODO: how command names are composed)   %% 2011/05/23, v0.1
%% 
%%   \pagebreak                                 %% 2011/06/26
%% == Caveats ==
%% \begin{enumerate}
%% \item Testing has not been very comprehensive so far. 
%% % \item 
%%       Usage together with \textcolor{blue}{\ctanpkgref{amsmath}}
%%       may require special care or fail altogether.
%% \item Switching into ``don't-skip-spaces" mode 
%%       \emph{two times} without switching back into normal mode 
%%       in between won't work with this version (v0.1--v0.3 TODO)
%%       of the package (TODO: permanent aliases).
%%       You will get the 
%%       \[\texttt{Argument of <patching> has an extra }`}.'\]
%%       error. This also applies to commands that have been issued 
%%       by package options.
%% \item Implementation may change much. 
%%       % , and certain auxiliary 
%%       % commands may move to separate packages. 
%%       (TODO 0.3)
%% \end{enumerate}
%%
%% == For Making Macros ==                      %% 2011/05/27
%% \label{sec:mac}
%% === The Main Command &\IfNextToken ===       %% & 2011/05/27
%% \label{sec:main}
%% |\IfNextToken<match>{<if>}{<else>}| %% add. miss. `\' 2011/05/31
%% is the obvious variant 
%% of \LaTeX's internal `\@ifnextchar' executing <if> 
%% if actually the \emph{``very next"} token is 
%% <match>                      %% v0.1a: was `mathc'
%% and executing <else> otherwise. If <match> is \emph{not} 
%% a \emph{space token} (\LaTeX's `\@sptoken') but 
%% the next token \emph{is}, <else> is executed; 
%% while `\@ifnextchar' tries matching the next token 
%% after ensuing space tokens. 
\newcommand{\IfNextToken}[3]{%
    \let\nextok@match= #1% 
%% ... v0.21 adds \qtd{`= '} after Heiko Oberdiek's explanation 
%% on \texhaxpref{11-05-017590}{texhax},        %% 2011/06/14
%% this allows `\@sptoken' as a possible `#1'. 
    \def\nextok@if{#2}\def\nextok@else{#3}%
    \futurelet\@let@token\nextok@decide}
%% ... apart from using different names, this is the same as 
%% `\new@ifnextchar' in `amsgen.sty' of the \ctanpkgref{amsmath}
%% bundle:
%% \begin{verbatim}
%% \long\def\new@ifnextchar#1#2#3{%
%%   \let\reserved@d= #1%
%%   \def\reserved@a{#2}\def\reserved@b{#3}%
%%   \futurelet\@let@token\new@ifnch
%% }
%% \end{verbatim}
%% ... and the behavior is essentially the same ...
\def\nextok@decide{%
    \ifx\@let@token\nextok@match \expandafter\nextok@if
    \else                        \expandafter\nextok@else
    \fi}
%% The analogue to our `\nextok@decide' in 'amsmath'/`amsgen.sty' 
%% is `\new@ifnch': 
%% \begin{verbatim}
%% \def\new@ifnch{%
%%   \ifx\@let@token\reserved@d \let\reserved@b\reserved@a \fi
%%   \reserved@b
%% }
%% \end{verbatim}
%%  ... and these two macros ('ifnextok''s and 'amsmath''s) 
%% actually make the difference to Standard \LaTeX. 
%% The latter's `\@ifnch' tests for `\@sptoken' before 
%% looking for the actually wanted `char', 'ifnextok'  and 
%% 'amsmath' don't. As to `\new@ifnch' vs.\ `\nextok@decide', 
%% the first has one token less than the latter, but one 
%% assignment more. What does this mean? TODO
%% 
%% When I decided to create the 'ifnextok' package, I was not aware 
%% of the similarity to 'amsmath', and I am not sure what I would 
%% have done had I ...
%%
%% |\NoNextSkipping| now switches into ``don't-skip-spaces" 
%% mode ``altogether" (however ...):
\newcommand*{\NoNextSkipping}{\let\@ifnextchar\IfNextToken}
%% This appears so dangerous to me that I don't want to support it 
%% much right now. |\RestoreNextSkipping| just switches back 
%% to \LaTeX's original version, so some support for 
%% \ctanpkgref{amsmath} may be missing here. 
\newcommand*{\RestoreNextSkipping}{%
    \let\@ifnextchar\kernel@ifnextchar}
%% Actually, because `\NoNextSkipping' does not affect 
%% `\kernel@ifnextchar', those of \LaTeX's commands using the latter 
%% still will skip spaces (with package version v0.1). 
%% 
%% As opposed to 'amsmath', 'ifnextok' aims at more choices 
%% as to what document-level commands are affected by the 
%% modified `next' checking. Of course, 'amsmath' deals with 
%% breaks between math display lines, while the present package 
%% rather was motivated by experiences in the humanities. 
%%
%% `\@sptoken' was discussed under \qtd{Some puzzling TeX} on 
%% texhax in 2011 (February/May/June), and the matter is discussed in 
%% \meta{The \TeX book} in Exercise~24.6 and on pp.~376f.
%%
%% === ``Bold" Patching Commands ===
%% \label{sec:patch}
%% % The little auxilary |\@xp@xp@xp| (cf.~`\@xp' in `amsgen.sty'):
%% % \providecommand*{\@xpxpxp}{\expandafter\expandafter\expandafter}
%% |\INTpatch<replacer><macro>| replaces 
%% something in the definition of <macro> according to 
%% the replacement macro <replacer>. 
%% This seems to work with the macros I thought of. 
%% It does \emph{not} work when 
%% (for replacing `\@ifnextchar')   %% mod. 2011/06/24:
%% (a)~there are \emph{more} `\@ifnextchar's in the 
%% macro to patch (outside braces), 
%% or when (b)~the only `\@ifnextchar' 
%% is inside a pair of braces.
\newcommand*{\INTpatch}[2]{%
  \expandafter\expandafter\expandafter \def
    \expandafter\expandafter\expandafter #2%
      \expandafter\expandafter\expandafter {%
        \expandafter #1#2}}                     %% red. 2011/06/24
%% |\NextTestPatch<macro>| replaces 
%% `\@ifnextchar' in the definition of <macro> by 
%% `\IfNextToken'. 
\newcommand*{\NextTestPatch}{\INTpatch\nextok@patch}
\def\nextok@patch#1\@ifnextchar{#1\IfNextToken} %% red. 2011/06/24
%% Another application of `\INTpatch' is `\StarTestPatch' in \secref{star}.
%% 
%% === Storing and Restoring ===
%% \label{sec:stored}
%% % |\WCS@namemod@apply@cs{<mod>}<apply>{<cs>}| 
%% % applies the token <apply> to (the token) <cs> modified by <mod>:
%% %\providecommand*\WCS@namemod@apply@cs[3]{%
%% %    \expandafter#2\csname#1#3\endcsname}
%% % Note the order of the arguments in the replacement text. 
%% % It allows shorthands like the following 
%% % |\INT@save@nmod<apply>{<cs>}|
%% % when one name modifyer is used for various purposes:
%% % \newcommand*{\INT@save@nmod}{\WCS@namemod@apply@cs\INT@save}
%% |\INTstore<macro>| stores the meaning of the macro <macro> 
%% in a special name space. 
%% %\newcommand*{\INTstore}[1]{\INT@save@nmod\let#1#1} 
\newcommand*{\INTstore}[1]{%
    \expandafter\let\csname\INT@save#1\endcsname#1}
%% In order to apply `\MakeNotSkipping' even to 
%% active characters below (v0.2), nothing must be gobbled 
%% from `\string<token>':
%% % `\INT@save<token>' is a \emph{``name modifier":}
% \newcommand*{\INT@save}{INT.save\expandafter\@gobble\string}
\newcommand*{\INT@save}{INT.save\string}
%% % For restoring, we use another general auxiliary 
%% % \[|\WCS@namemod@copy@to@from{<mod>}<new>{<old>}|:\]
%% %\providecommand*{\WCS@namemod@copy@to@from}[3]{%
%% %    \expandafter\let\expandafter#2\csname#1#3\endcsname}
%% % \providecommand*{\WCS@namemod@copy@to@from}[2]{%
%% %     \def\WCS@apply{\let#2}%
%% %     \WCS@namemod@apply@cs#1\WCS@apply}
%% |\INTrestore<macro>| \emph{re}stores the meaning of <macro> 
%% that is expected to have been stored with `\INTstore':
%% %\newcommand*{\INTrestore}[1]{\WCS@namemod@copy@to@from\INT@save#1#1}
\newcommand*{\INTrestore}[1]{%
    \expandafter\let\expandafter#1\csname\INT@save#1\endcsname}
%%
%% === The Star Test ===
%% \label{sec:star}
%% Before a \LaTeX\ line-break command tests for an optional argument, 
%% it tests for a star using `\@ifstar', which in turn invokes 
%% `\@ifnextchar'. So already `\@ifstar' needs to be modified. 
%% We do not so much want to change `\@ifstar' altogether, 
%% rather we will replace it at some places by a non-skipping 
%% variant |\IfStarNextToken|,\footnote{TODO or &\IfNextStar, 
%%                                      cf.~&\IfNextSpace.} 
%% using the patching command |\StarTestPatch<macro>|.
%% (`\@ifstar' has an argument and therefore cannot be patched as 
%%  nicely as the line-break commands.)
\newcommand*{\IfStarNextToken}[1]{\IfNextToken*{\@firstoftwo{#1}}}
\newcommand*{\StarTestPatch}{\INTpatch\nextok@starpatch}
%% The macro to be patched may contain a \cs{par} 
%% (`\@centercr' is an example), so we need `\long':
%%  %% red. 2011/06/24
\long\def\nextok@starpatch#1\@ifstar{#1\IfStarNextToken}
%% |\StoreStarSkipping| stores the current meaning of 
%% `\@ifstar' ...
%% % is executed before changing `\@ifstar' temporarily:
\newcommand*{\StoreStarSkipping}{\INTstore\@ifstar}
%% ... so that it can be restored by |\RestoreStarSkipping|:
\newcommand*{\RestoreStarSkipping}{\INTrestore\@ifstar}
%% |\NoStarSkipping| renders `\@ifstar' non-skipping altogether:
\newcommand*{\NoStarSkipping}{\let\@ifstar\IfStarNextToken}
%% This again seems to be so dangerous that it will not be 
%% supported much with package version v0.1 (by a package option). 
%% 
%% On the other hand, 'amsmath' (`amsgen.sty') is not as scrupulous 
%% as we are and indeed redefines `\@ifstar' \emph{altogether},
%% equivalent to our `\NoStarSkipping', except that the latter 
%% provides a method to restore. I.e., as soon as you have loaded
%% `amsgen.sty' (invoked by any 'amsmath' package), 
%% you have decided that a star appearing after 
%% whitespace is printed as a star, rather than choosing the 
%% ``starred" version of the respective command.
%% %% v0.21a 2011/06/14:
%% What we actually find in `amsgen.sty' is
%% \begingroup \hfuzz=33pt
%% \begin{verbatim}
%% \def\@ifstar#1#2{\new@ifnextchar *{\def\reserved@a*{#1}\reserved@a}{#2}}
%% \end{verbatim}
%% \endgroup \noindent
%% The `\reserved@a' trick seems to be due to 'amsmath''s idea 
%% of implementing the conditional 
%% (see the code we are quoting in \secref{main}).
%%
%% === More General Patching with \cs{@sptoken} ===
%% \label{sec:genpatch}
%% This section deals with modifying macros by a more general 
%% technique than employed in \secref{patch}. We do not use any 
%% knowledge of internals of the target user command 
%% (a ``control symbol" like `\\' or an ``active character"), 
%% and the command may take arguments, as the active double quote 
%% does with 'german.sty' or 'babel'.
%%
%% ==== &\IfNextSpace ====
%% |\IfNextSpace{<if>}{<else>}| is an auxiliary macro that 
%% executes <if> if the next token is a space token 
%% (\LaTeX's `\@sptoken'), otherwise it executes <else>:
\newcommand*{\IfNextSpace}{\IfNextToken\@sptoken}
%% This did not work with the v0.2 version of `\IfNextToken' 
%% that didn't have \qtd{`= '}, due to `\@sptoken' being an 
%% ``implicit space token," as Heiko Oberdiek pointed out 
%% on texhax. He also provided the remedy that actually was 
%% 'amsmath''s way ...
%%
%% v0.2 was:
%% \begin{verbatim}
%% \newcommand*{\IfNextSpace}[2]{%
%%     \def\nextok@if{#1}\def\nextok@else{#2}%
%%     \futurelet\@let@token\nextok@ifspace}
%% \newcommand*{\nextok@ifspace}{%
%%     \ifx\@let@token\@sptoken \expandafter \nextok@if
%%     \else                    \expandafter \nextok@else
%%     \fi}
%% \end{verbatim}
%% ... not so bad from an efficiency point of view, but ...---% 
%% % Is `\IfNextSpace' useful? I don't know of a shorthand 
%% % like `\@ifnextcharleftbracket' for   %% 2011/06/24 \qtd:
%% % \qtd{&\@ifnextchar[} ...             %% rm. 2011/06/26
%%
%% ==== &\MakeNotSkipping ====
%% |\MakeNotSkipping{<target>}{<on-space>}| should modify `<target>'
%% so that it acts in its usual way when no space token is ahead 
%% while executing <on-space> otherwise. E.g., <target> may be 
%% the active double quote |"| from 'babel', and on the left of a space 
%% token you want that the double quote just prints an ordinary 
%% double quote from the ligature 
%% \lq\verb+''+\rq\                 %% \ 2011/06/03
%% (the first pair of argument braces may be omitted):
%% \[\verb+\MakeNotSkipping{"}{''}+\] 
%% ... while I don't really recommend this right now (v0.2f.). 
%% %% rm. 2011/06/27 useless:
%% % For some unknown |\\|, 
%% % you may want to block searching for an optional argument by 
%% % \[`\MakeNotSkipping{\\}{\\\relax}'\]
%% 
%% <target>, being on document level and probably appearing 
%% in moving arguments, must be robust, while `\IfNextSpace' is not. 
%% When <target> has been defined using `\DeclareRobustCommand'
%% (`\\' from the `document' environment is an example), 
%% we would loose the original behavior of <target> if we used 
%% `\DeclareRobustCommand' ourselves. 
%%
%% v0.21 was horribly flawed at this point; I had not tested 
%% all cases, I had not studied how \LaTeX's `\DeclareRobustCommand' 
%% handles control symbols (such as `\_', see `source.pdf'), 
%% and my implementation did not obey the warning in my 
%% ancompanying documentation ...
%%
%% % |\RobustDeferName<cs>| may move to a general package 
%% % about robust commands TODO:
%% %\providecommand*{\RobustDeferName}[1]{\expandafter\@gobble\string#1 }
%% When <target> is a single (active) character, 
%% it may have been robustified by making it expand 
%% to a robust control sequence token, such as `~' 
%% via `\nobreakspace{}'---we don't know, 
%% or don't try to find out now. We make it robust, 
%% accepting that this may just introduce an unnecessary extra 
%% macro. %% rest of paragraph was nonsense 2011/06/27
\newcommand*{\INT@modified}[1]{%
    \ifx\protect\@typeset@protect 
        \expandafter\expandafter\expandafter \IfNextSpace
            \csname\INT@mod#1\expandafter\endcsname
    \else 
        \protect#1%
    \fi}
%% `\INT@mod<cs>' is another \emph{``name modifier":}
%% % `\WCS@namemod@apply@cs' from \secref{stored} 
%% % provides both arguments for `\IfNextSpace' associated with 
%% % #1 (TODO!?):
%% %     \edef\@tempa{\WCS@namemod@apply@cs\INT@mod\noexpand#1}%
%% %     \expandafter\expandafter\expandafter \IfNextSpace \@tempa}
\newcommand*{\INT@mod}{INT@mod.\string}
%% Here is the main command of the section. 
%% `\@tempa' will store the meaning of the command that <target> 
%% would call in typesetting mode after 
%% `\DeclareRobustCommand<target>'.
%% `\@tempb' will store what <target> does in in typesetting 
%% mode, maybe that is just the meaning of <target> 
%% (kind of flag, like `\if@tempswa'):
\newcommand*{\MakeNotSkipping}[2]{%
    \expandafter \let \expandafter \@tempa
        \csname\expandafter\@gobble\string#1 \endcsname
    \let\@tempb#1%
%% When #1 is a control word, and its name, extended by a space, 
%% is the name of a defined token, we \emph{lazily} 
%% (like \ctanpkgref{makerobust}, TODO) assume that 
%% its current meaning was assigned by `\DeclareRobustCommand'.
%% One exception: if that token is the control space~`\ ', 
%% #1 is a single character (hopefully active, TODO check!?).
    \ifx\@tempa\relax \else \ifx\@tempa\ \else
        \let\@tempb\@tempa \fi \fi
    \expandafter\let\csname\INT@save#1\endcsname\@tempb
%% We have analyzed #1 and now may modify it:
    \def#1{\INT@modified#1}%
%% We do not know beforehand what <on-space> will contain, 
%% in any case it should \emph{not} be expanded right here, 
%% that's why we use the token register `\@toks':
    \toks@{#2}%
    \expandafter\edef\csname\INT@mod#1\endcsname{%
        {\the\toks@}%
        \expandafter\noexpand\csname\INT@save#1\endcsname}%
}
%% This still is experimental, and you must care not to apply 
%% the patch two times when it has not been undone in between. 
%% The main application may be a macro like |\\| that some 
%% (non-standard) environment defines; then you could redefine 
%% the environmont so that its start finally modifies that 
%% macro according to your wishes. In the latter situation, 
%% the end of the environment will undo your `\MakeNotSkipping'; 
%% 
%% TODO: %% v0.2
%% In the case of |"|, this might be a starting point 
%% for handling conventions about moving an ensuing 
%% punctuation mark to the left of the quotation mark. 
%% Moreover, getting something really useful would require 
%% dealing with `"' at the left of a bracket too.
%%
%% %  \pagebreak                                 %% 2011/06/26
%% == ``Manual" Line Breaks ==
%% \label{sec:break}
%% === Outline of Implementation ===
%% In the first instance, the present package aims at rendering 
%% |\\| a command that interpretes a left-hand square bracket 
%% as a start of an optional argument only if the bracket is not 
%% preceded by any other token (apart from the star in `\\*'), 
%% especially not by a space token. 
%% 
%% Indeed, an author may expect that when a bracket opens in a 
%% \emph{different} line than the `\\', then it will be 
%% \emph{printed} rather than interpreted as an 
%% \emph{optional-argument delimiter}
%% (the package author has been such an author some times). 
%% Now, when the bracket only is in a line \emph{following} 
%% the line carrying the `\\', the end-line character 
%% normally produces a space token (\TeX book p.~47), 
%% so the present idea of implementation will cover 
%% the case of a bracket in the next line.
%% 
%% In `latex.ltx', the names of the commands implementing the 
%% line break have some ``pivot" part <pivot> that we can use 
%% to patch them in a uniform way. They are two in each case: 
%% The first starts with `\@<pivot>' and invokes `\@ifstar', 
%% the second starts with `\@x<pivot>' and invokes the 
%% left-hand-bracket test. Both of them need to be patched. 
%% 
%% === ``Normal" Manual Line Breaks ===
%% \label{sec:newline}
%% If I had been aware of the difficulties of this part, 
%% I probably would not have started writing this package, 
%% hoping it would be the work of about an hour. 
%% 
%% `\@xnewline' must be patched in order to get a non-skipping
%% version of the bracket test, and this patch suffices for the 
%% optional-argument goal. 
%% 
%% %% more accurately 2011/05/31:
%% The `\@ifstar' call is in `\@normalcr' and its alias `\\ '; 
%% the latter is invoked by `\\' according to `\DeclareRobustCommand\\'.
%% 
%% Things seem to be easier when `\\ ' \emph{expands} to `\@normalcr' 
%% instead of being an \emph{alias} of it 
%% (\strong{\textcolor{blue}{CAUTION!}}).
%% Then we just need to control `\@normalcr':
\@namedef{\@backslashchar\space}{\@normalcr}
%% |\StoreNewlineSkipping| \emph{stores} the skipping behavior of |\\|
%% outside special environments:
\newcommand*{\StoreNewlineSkipping}{%
    \INTstore\@normalcr \INTstore\@xnewline}
%% |\RestoreNewlineSkipping| \emph{re}stores the skipping behavior of |\\|
%% outside special environments:
\newcommand*{\RestoreNewlineSkipping}{%
    \INTrestore\@normalcr \INTrestore\@xnewline}
%% |\NoNewlineSkipping| \emph{suppresses} skipping blank spaces with 
%% |\\| outside special environments:
\newcommand*{\NoNewlineSkipping}{%
    \StarTestPatch\@normalcr \NextTestPatch\@xnewline}
%%
%% === Manual Line Breaks in \LaTeX\ Environments ===
%% \label{sec:envs}
%% The macros in the present section should modify \LaTeX's |\\| 
%% in environments (<env> being one of:) 
%% |center|, |tab|, |array|, and |tabular|. 
%% These \emph{environment names} are the expected 
%% \emph{arguments} of those macros. 
%% However, argument |center| also affects the 
%% %% 2011/05/27 v0.11a verse:
%% |flushleft|, |flushright|, and 
%% |verse| environments,\footnote{`verse' 
%%      is provided by \LaTeX's standard \ctanpkgref{classes} only, 
%%      while `flushleft' and `flushright' belong to the \LaTeX\ 
%%      kernel.} 
%% and |array| and |tabular| should also affect 
%% their enhanced variants from other \LaTeX\ packages. 
%% When this internal structure of \LaTeX\ changes, 
%% the present section may become obsolete ...
%%
%% |\INTactOnEnv{<action1>}{<action2>}{<env>}| 
%% is the backbone of these macros. 
%% <action1> and <action2> are one of 
%% \[`\INTstore', `\INTrestore', `\StarTestPatch', 
%% `\NextTestPatch'.\] 
%% <action1> deals with `\@ifstar', 
%% <action2> deals with `\@ifnextchar':
\newcommand*{\INTactOnEnv}[3]{%
    \expandafter#1\csname  @#3cr\endcsname
    \expandafter#2\csname @x#3cr\endcsname}
%% |\StoreSkippingCRs{<env>}| \emph{stores} the skipping behavior 
%% of |\\| in environments <env>:
\newcommand*{\StoreSkippingCRs}{%
    \INTactOnEnv\INTstore\INTstore}
%% |\RestoreSkippingCRs{<env>}| \emph{re}stores the skipping behavior 
%% of |\\| in environments <env>:
\newcommand*{\RestoreSkippingCRs}{%
    \INTactOnEnv\INTrestore\INTrestore}
%% |\NotSkippingCRs{<env>}| \emph{suppresses} space skipping
%% of |\\| in environments <env>: 
\newcommand*{\NotSkippingCRs}{%
    \INTactOnEnv\StarTestPatch\NextTestPatch}
%% % \NotSkippingCRs{center}
%% \begin{center}
%%  % \MakeNotSkipping\\{\\\relax}
%%  %  \show\\
%%    Do these commands work?\\ [\,Or do they not?\,]
%% \end{center}
%% 
%% %% 2011/05/27 v0.11a:
%% By contrast, the environments |quotation| and |quote| 
%% from \LaTeX's standard \ctanpkgref{classes} use the 
%% ``normal" newline command essentially provided by 
%% `\@normalcr'.
%% === 'amsmath' and 'mathtools' ===
%% Just discussing related functionality in 
%% the \ctanpkgref{amsmath} and \ctanpkgref{mathtools}
%% packages, without any own code: 
%% %% modifications v0.21a 2011/06/14:
%% 
%% 'amsmath' modifies the star (|*|) test all over the document 
%% (see our \secref{star}), while providing own 
%% (not skipping) versions of |\\| rather in math displays 
%% and math environments only. This applies quite obviously 
%% to the `{cases}' and `{matrix}' environments. 
%% I am not sure about 'amsmath''s use of `\displaybreak',
%% (cf.~`amstex.sty' and the `\intertext' command) and `\math@cr'. 
%%
%% 'mathtools' modifies 'amsmath''s line breaking behavior 
%% in turn on its options `[allowspaces]' and `[disallowspaces]', 
%% referring to some strange behavior of 'amsmath'. 
%% Still I don't understand what is going on entirely, 
%% and my impression is that nobody else has understood these 
%% things entirely so far. 'mathtools' is not the first 
%% package suppressing space skipping with `\\', 'amsmath' has done 
%% this already; the question is where, where not, and why ... 
%% 'mathtools''s `[disallowspaces]' just seems to provide 
%% a more straightforward policy ... TODO
%% % Again, please see the 'mathtools' documentation for more details. 
%% %% <- rm. 2011/06/14 
%%
%% == Package Options ==
%% \label{sec:options} 
%% === Behavior \emph{without} Options ===
%% If the package is called without any option, 
%% it only defines `\IfNextToken', `\IfStarNextToken' 
%% and the other package-writer or user commands, 
%% without actually changing behavior of any \LaTeX\ command.
%%
%% === Option `newline' ===
%% Package option |newline| stores and disables space skipping for 
%% |\\| in ``normal" mode according to \secref{newline}:
\DeclareOption{newline}{\StoreNewlineSkipping\NoNewlineSkipping}
%% 
%% === Environments ===
%% The next package options are just the environment names 
%% according to \secref{envs} 
%% (|center|, |tab|, |array|, |tabular|).
%% Option |<env>| stores and disables the skipping behavior 
%% of |\\| in <env> environments. 
%% We abuse the our temporary macro `\nextok@match' from 
%% \secref{main}:
\def\nextok@match#1{%
    \DeclareOption{#1}{\StoreSkippingCRs{#1}\NotSkippingCRs{#1}}}
\nextok@match{center}
\nextok@match{tab}
\nextok@match{array}
\nextok@match{tabular}
%% === ``All Options" or ``Standard Options" ===
%% Package Options |all| and (v0.11:\@) |stdbreaks| have the same effect as 
%% using the `newline' option and the environment package options 
%% `center', `tab', `array', and `tabular' at once. 
\def\nextok@match#1{\csname ds@#1\endcsname}
%% (... must not be changed before `\ProcessOptions' ...)
\DeclareOption{all}{%
    \nextok@match{newline} \nextok@match{center}
    \nextok@match{tab} \nextok@match{array} \nextok@match{tabular}}
%% Behavior of option |all| may \emph{change} in the future of the package, 
%% while option |stdbreaks| should rather \emph{keep} its present behavior.
\DeclareOption{stdbreaks}{\nextok@match{all}}   %% v0.11
%%
%% 
%% == Processing Options and Leaving the Package ==
\ProcessOptions
\endinput
%%
%% == Acknowledgments ==                        %% v0.2 2011/05/27
%% While I experienced the problem myself some years ago with 
%% a critical edition, I finally decided to do this work 
%% after postings by Susan Dittmar (March 2011)     %% 017128
%% and Philipp Stephani (December 2010)             %% 106431
%% on the \texhaxref{}{'texhax'} mailing list.
%% The latter pointed to \ctanpkgref{mathtools}. 
%% 
%% Moreover, the space skipping matter was discussed on the 
%% `LATEX-L' mailing list (\qtd{xparse and space skipping}) 
%% in mid of May 2011, and the present 
%% package may be considered a contribution to that discussion 
%% (saying something like: keep the simple standard for beginners, 
%%  offer something advanced for advanced users if you think 
%%  some of them want it ... maybe just as `contrib').
%% Bruno Le Floch (May~11) and Frank Mittelbach (May~15)
%% made me aware of the similar functionality in 'amsmath'. 
%% Wordings in describing 'ifnextok' may resemble wordings 
%% in that `LATEX-L' very much. Only for v0.21, I actually 
%% read these `LATEX-L' postings, rather than only their 
%% subject lines.
%% 
%% See sections~\ref{sec:main} and~\ref{sec:genpatch} for 
%% Heiko Oberdiek's contribution.   %% 2011/06/24
%% 
%% == VERSION HISTORY ==
v0.1   2011/05/23   very first 
v0.11  2011/05/23   typo `mathc' fixed, where/when
       2011/05/27   &, more structure, option [stdbreaks]
       2011/05/27   doc. mentions `verse', `quotation', `quote'; 
                    ack.s
v0.2   2011/05/30   \IfNextSpace, \MakeNotSkipping 
       2011/05/31   using \@normalcr differently; 
                    corrected \IfNextSpace 
v0.21  2011/06/02   reworked \IfNextToken and \IfNextSpace 
                    after Heiko Oberdiek, regarding amsmath; 
                    documentation discusses amsmath and mathtools 
                    and refers to LATEX-L
       2011/06/03   ... \ , \rq\ ; corr. version string
v0.21a 2011/06/14   quoting \@ifstar from amsmath, extended comments 
                    on amsmath and mathtools, starred versions, 
                    Heiko Oberdiek for sec:main
       TO CTAN
v0.22  2011/06/24   ack's extended; reduced sec:patch; sec:genpatch: 
                    horribly fragile robustification fixed
       2011/06/25   re-implementations in sec:patch using \WCS...
       2011/06/26   was named v0.3, renamed v0.22
       JUST STORED
v0.3   2011/06/26   sec:stored as v0.11; 
                    sec:genpatch code similar to v0.2; 
                    different sectioning; \pagebreak's
       2011/06/27   \INT@modified streamlined, rm. useless \\ example 
                    and wrong description of \INT@modified