summaryrefslogtreecommitdiff
path: root/macros/latex/base/ltdefns.dtx
blob: bd820e7cf2193fc20597d834ae657fdb493320e1 (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
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
% \iffalse meta-comment
%
% Copyright (C) 1993-2022
% The LaTeX Project and any individual authors listed elsewhere
% in this file.
%
% This file is part of the LaTeX base system.
% -------------------------------------------
%
% It may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either version 1.3c
% of this license or (at your option) any later version.
% The latest version of this license is in
%    https://www.latex-project.org/lppl.txt
% and version 1.3c or later is part of all distributions of LaTeX
% version 2008 or later.
%
% This file has the LPPL maintenance status "maintained".
%
% The list of all files belonging to the LaTeX base distribution is
% given in the file `manifest.txt'. See also `legal.txt' for additional
% information.
%
% The list of derived (unpacked) files belonging to the distribution
% and covered by LPPL is defined by the unpacking scripts (with
% extension .ins) which are part of the distribution.
%
% \fi
%
% \iffalse
%%% From File: ltdefns.dtx
%<*driver>
% \fi
\ProvidesFile{ltdefns.dtx}
             [2022/05/22 v1.5r LaTeX Kernel (definition commands)]
% \iffalse
\documentclass{ltxdoc}
\GetFileInfo{ltdefns.dtx}
\title{\filename}
\date{\filedate}
 \author{%
  Johannes Braams\and
  David Carlisle\and
  Alan Jeffrey\and
  Leslie Lamport\and
  Frank Mittelbach\and
  Chris Rowley\and
  Rainer Sch\"opf}
\begin{document}
 \MaintainedByLaTeXTeam{latex}
 \maketitle
 \DocInput{\filename}
\end{document}
%</driver>
% \fi
%
%
% \changes{v1.0n}{1994/05/10}{(ASAJ) Added
%    \cs{DeclareProtectedCommand}.}
% \changes{v1.0p}{1994/05/12}{(ASAJ) Fixed a bug with \cs{relax}
%    which was
%    using \cs{@gobble} before defining it.}
% \changes{v1.0q}{1994/05/13}{(ASAJ) Renamed
%    \cs{DeclareProtectedCommand} to
%    \cs{DeclareRobustCommand}.  Removed \cs{@if@short@command}.}
% \changes{v1.0q}{1994/05/13}{(ASAJ) Replaces \cs{space} by `~' in
%    \cs{csname}.}
% \changes{v1.0r}{1994/05/13}{(ASAJ) Added logging message to
%    \cs{DeclareProtectedCommand}.}
% \changes{v1.0s}{1994/05/13}{(ASAJ) Added \cs{@backslashchar}.}
% \changes{v1.0s}{1994/05/13}{(ASAJ) Coded \cs{@ifdefinable} more
%    efficiently.}
% \changes{v1.1a}{1994/05/16}{(ASAJ) Split from ltinit.dtx.}
% \changes{v1.1b}{1994/05/17}{(ASAJ) Removed warnings and logging to
%    lterror.dtx.}
% \changes{v1.1b}{1994/05/17}{(ASAJ) Added definitions for protect.}
% \changes{v1.1c}{1994/05/17}{(ASAJ) Redid definitions for protect.}
% \changes{v1.1d}{1994/05/19}{(RmS) Added definitions for
%           \cs{@namedef} and \cs{@nameuse} again.}
% \changes{v1.1e}{1994/05/20}{Changed command name from
%    \cs{@checkcommand} to \cs{CheckCommand}.}
% \changes{v1.1f}{1994/05/22}{Use new warning and error cmds}
% \changes{v1.2a}{1994/10/18}{Add star-forms for all commands}
% \changes{v1.2a}{1994/10/18}{Add extra test for \cs{endgraf}}
% \changes{v1.2b}{1994/10/25}{Documentation improvements}
% \changes{v1.2c}{1994/10/30}{(CAR)\cs{@onelevel@sanitize} added}
% \changes{v1.2f}{1994/10/30}{(DPC)\cs{newwrite}'s moved to ltfiles}
% \changes{v1.0g}{1994/11/17}
%         {\cs{@tempa} to \cs{reserved@a}}
% \changes{v1.0p}{1995/07/13}{Updates to documentation}
% \changes{v1.4b}{2015/02/21}
%         {Removed autoload support}
% \changes{v1.5l}{2020/08/21}{Integration of new hook management interface}
%
% \section{Definitions}
%
% This section contains commands used in defining other macros.
%
% \MaybeStop{}
%
%    \begin{macrocode}
%<*2ekernel>
%    \end{macrocode}
%
%
% \subsection{Initex initializations}
%
% \task{???}{This section needs extension}
%
% \begin{macro}{\two@digits}
% \changes{LaTeX2e}{1993/11/23}{Macro added}
%    Prefix a number less than 10 with `0'.
%    \begin{macrocode}
\def\two@digits#1{\ifnum#1<10 0\fi\number#1}
%    \end{macrocode}
% \end{macro}
%
% \changes{v1.2e}{1994/11/04}{Added \cs{set@display@protect} to
%    \cs{typeout}.  ASAJ.}
%
%  \begin{macro}{\typeout}
%    Display something on the terminal.
% \changes{v1.5g}{2020/05/15}{Allow \cs{par} in the argument (gh/335)}
%    \begin{macrocode}
%</2ekernel>
%<*2ekernel|latexrelease>
%<latexrelease>\IncludeInRelease{2020/10/01}%
%<latexrelease>                 {\typeout}{Allow "par" in \typeout}%
\protected\long\def\typeout#1{\begingroup
  \set@display@protect
  \def\par{^^J^^J}%
  \immediate\write\@unused{#1}\endgroup}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000/00/00}%
%<latexrelease>                 {\typeout}{Allow "par" in \typeout}%
%<latexrelease>
%<latexrelease>\def\typeout#1{\begingroup\set@display@protect
%<latexrelease>    \immediate\write\@unused{#1}\endgroup}
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
%  \end{macro}
%
%  \begin{macro}{\newlinechar}
%    A char to be used as new-line in output to files.
%    \begin{macrocode}
\newlinechar`\^^J
%    \end{macrocode}
%  \end{macro}
%
% \subsection{Saved versions of \TeX{} primitives}
%
%  The TeX primitive |\foo| is saved as |\@@foo|.
%  The following primitives are handled in this way:
% \begin{macro}{\@@par}
%    \begin{macrocode}
\let\@@par=\par
%\let\@@input=\input    %%% moved earlier
%\let\@@end=\end        %%%
%    \end{macrocode}
%  \end{macro}
%
% \begin{macro}{\@@hyph}
% Save original primitive definition.
%    \begin{macrocode}
\let\@@hyph=\-
%    \end{macrocode}
% \end{macro}
%
%
%  \begin{macro}{\@@italiccorr}
% Save the original italic correction.
% \changes{v1.0a}{1994/03/07}{Macro added}
%    \begin{macrocode}
\let\@@italiccorr=\/
%    \end{macrocode}
%  \end{macro}
%
%  \begin{macro}{\@height}
%  \begin{macro}{\@depth}
%  \begin{macro}{\@width}
%  \begin{macro}{\@minus}
% \changes{LaTeX2e}{1993/11/22}{Macro added}
%  \begin{macro}{\@plus}
% \changes{LaTeX2e}{1993/11/22}{Macro added}
%
%    The following definitions save token space.  E.g., using
%    |\@height| instead of height saves 5 tokens at the cost in time
%    of one macro expansion.
%    \begin{macrocode}
\def\@height{height} \def\@depth{depth} \def\@width{width}
\def\@minus{minus}
\def\@plus{plus}
%    \end{macrocode}
%  \begin{macro}{\hb@xt@}
% \changes{v1.2k}{1995/05/07}{Macro added}
%    The next one is another 100 tokens worth.
%    \begin{macrocode}
\def\hb@xt@{\hbox to}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%  \end{macro}
%  \end{macro}
%  \end{macro}
%  \end{macro}
%
%    \begin{macrocode}
\message{hacks,}
%    \end{macrocode}
%
% \subsection{Command definitions}
%
% This section defines the following commands:
%
% \DescribeMacro
%  {\@namedef}\marg{NAME}\\ Expands to |\def\|\marg{NAME},
%   except name can contain any characters.
%
% \DescribeMacro
%  {\@nameuse}\marg{NAME}\\
%   Expands to |\|\marg{NAME}.
%
% \DescribeMacro
%  {\@ifnextchar} X\marg{YES}\marg{NO}\\
%    Expands to \meta{YES} if next character is an `X',
%          and to \meta{NO} otherwise.
%          (Uses |\reserved@a|--|\reserved@c|.)
%          NOTE: GOBBLES ANY SPACE FOLLOWING IT.
%
% \DescribeMacro
%  {\@ifstar}\marg{YES}\marg{NO}\\
%          Gobbles following spaces and then tests if next the
%          character is a '*'.  If it is, then it gobbles the
%   `*' and expands to \meta{YES}, otherwise it expands to \meta{NO}.
%
% \DescribeMacro
%  {\@dblarg}\marg{CMD}\marg{ARG}\\
%     Expands to |\|\marg{CMD}\oarg{ARG}\marg{ARG}.  Use
%          |\@dblarg\CS| when |\CS| takes arguments |[ARG1]{ARG2}|,
%          where default is| ARG1| = |ARG2|.
%
% \DescribeMacro
%  {\@ifundefined}\marg{NAME}\marg{YES}\marg{NO}\\
%          : If \cs{NAME} is undefined then it executes \meta{YES},
%            otherwise it executes \meta{NO}.  More precisely,
%            true if \cs{NAME} either undefined or = |\relax|.
%
% \DescribeMacro
%  {\@ifdefinable}|\NAME|\marg{YES}
%       Executes \meta{YES} if the user is allowed to define |\NAME|,
%            otherwise it gives an error.  The user can define |\NAME|
%            if |\@ifundefined{NAME}| is true, '|NAME|' $\neq$ '|relax|'
%            and the first three letters of '|NAME|' are not
%           '|end|', and if |\endNAME| is not defined.
%
% \DescribeMacro
%  \newcommand|*|\marg{\cs{FOO}}\oarg{i}\marg{TEXT}\\
%         User command to define |\FOO| to be a macro with
%            i arguments (i = 0 if missing) having the definition
%            \meta{TEXT}.  Produces an error if |\FOO| already
%            defined.
%
%            Normally the command is defined to be |\long| (ie it may
%            take multiple paragraphs in its argument). In the
%            star-form, the command is not defined as |\long| and a
%            blank line in any argument to the command would generate
%            an error.
%
% \DescribeMacro
%  \renewcommand|*|\marg{\cs{FOO}}\oarg{i}\marg{TEXT}\\
%  Same as |\newcommand|, except it checks if |\FOO| already defined.
%
% \DescribeMacro
%  \newenvironment|*|\marg{FOO}\oarg{i}\marg{DEF1}\marg{DEF2}\\
%         equivalent to:\\
%         |\newcommand{\FOO}[i]{DEF1}| |\def{\endFOO}{DEF2}|\\
% (or the appropriate star forms).
%
% \DescribeMacro
%  \renewenvironment\\ Obvious companion to |\newenvironment|.
%
% \DescribeMacro
%  \@cons : See description of |\output| routine.
%
% \DescribeMacro{\@car}
%  |\@car T1 T2 ... Tn\@nil| == |T1|  (unexpanded)
%
% \DescribeMacro{\@cdr}
%  |\@cdr T1 T2 ... Tn\@ni|l == |T2 ... Tn|     (unexpanded)
%
% \DescribeMacro
%  \typeout\marg{message}\\ Produces a warning message on the terminal.
%
% \DescribeMacro
%  \typein\marg{message}\\
%        Types message, asks the user to type in a command, then
%            executes it
%
% \DescribeMacro
%  \typein\oarg{\cs{CS}}\marg{MSG}\\
%  Same as above, except defines |\CS| to be the input
%                      instead of executing it.
%
% \changes{LaTeX209}{1992/03/18}
%  {(RMS) changed input channel from 0 to \cs{@inputcheck} to avoid
%     conflicts with other channels allocated by \cs{newread}}
%
%  \begin{macro}{\typein}
%
% \changes{v1.2k}{1995/05/08}{Use \cs{@firstofone}}
% \changes{v1.2l}{1995/05/08}{Remove unnecessary braces}
% \changes{v1.2l}{1995/05/08}{Replace \cs{def} by \cs{let}}
% \changes{v1.2m}{1995/05/24}{(DPC) New implementation}
% \changes{v1.2u}{1995/10/16}{(DPC) Use \cs{@testopt} /1911}
%    \begin{macrocode}
\def\typein{%
  \let\@typein\relax
  \@testopt\@xtypein\@typein}
%    \end{macrocode}
%
% \changes{v1.2r}{1995/10/03}
%     {Add missing \cs{@typein} for /1710 (from patch file)}
% \changes{v1.4a}{2015/01/03}{use modified definition in luatex}
%    \begin{macrocode}
\ifx\directlua\@undefined
%    \end{macrocode}
%
%    \begin{macrocode}
\def\@xtypein[#1]#2{%
 \typeout{#2}%
 \advance\endlinechar\@M
 \read\@inputcheck to#1%
 \advance\endlinechar-\@M
 \@typein}%
%    \end{macrocode}
%
%    \begin{macrocode}
\else
%    \end{macrocode}
%
%    \begin{macrocode}
\def\@xtypein[#1]#2{%
  \typeout{#2}%
  \begingroup \endlinechar\m@ne
  \read\@inputcheck to#1%
  \expandafter\endgroup
  \expandafter\def\expandafter#1\expandafter{#1}%
  \@typein}%
%    \end{macrocode}
%
%    \begin{macrocode}
\fi
%    \end{macrocode}
%  \end{macro}
%
%  \begin{macro}{\@namedef}
%    \begin{macrocode}
\def\@namedef#1{\expandafter\def\csname #1\endcsname}
%    \end{macrocode}
%  \end{macro}
%
%  \begin{macro}{\@nameuse}
%    \begin{macrocode}
\def\@nameuse#1{\csname #1\endcsname}
%    \end{macrocode}
%  \end{macro}
%
%  \begin{macro}{\@cons}
%    \begin{macrocode}
\def\@cons#1#2{\begingroup\let\@elt\relax\xdef#1{#1\@elt #2}\endgroup}
%    \end{macrocode}
%  \end{macro}
%
%  \begin{macro}{\@car}
%  \begin{macro}{\@cdr}
%    \begin{macrocode}
\def\@car#1#2\@nil{#1}
\def\@cdr#1#2\@nil{#2}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
%  \begin{macro}{\@carcube}
% |\@carcube T1 ... Tn\@nil| = |T1| |T2| |T3| , $n > 3$
% \changes{v1.5k}{2020/08/19}{Made \cs{long} for \cs{NewCommandCopy}}
% \changes{v1.5o}{2020/11/25}{Added missing latexrelease entry}
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020/10/01}{\@carcube}{Make \@carcube long}%
%<*2ekernel|latexrelease>
\long\def\@carcube#1#2#3#4\@nil{#1#2#3}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{0000/00/00}{\@carcube}{Undo: Make \@carcube long}%
%<latexrelease>\def\@carcube#1#2#3#4\@nil{#1#2#3}
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
%  \end{macro}
%
%
%
%  \begin{macro}{\@onlypreamble}
%  \begin{macro}{\@preamblecmds}
%    This macro adds its argument to the list of commands stored in
%    |\@preamblecmds| to be
%    disabled after |\begin{document}|. These commands are redefined
%    to generate |\@notprerr| at this point.
%    \begin{macrocode}
\def\@preamblecmds{}
\def\@onlypreamble#1{%
  \expandafter\gdef\expandafter\@preamblecmds\expandafter{%
       \@preamblecmds\do#1}}
\@onlypreamble\@onlypreamble
\@onlypreamble\@preamblecmds
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
%
% \begin{macro}{\@star@or@long}
% \changes{v1.2a}{1994/10/18}{macro added}
% Look ahead for a |*|. If present reset |\l@ngrel@x| so that
% the next definition, |#1|,  will be non-long.
%    \begin{macrocode}
\def\@star@or@long#1{%
  \@ifstar
   {\let\l@ngrel@x\relax#1}%
   {\let\l@ngrel@x\long#1}}
%    \end{macrocode}
% \end{macro}
%

%  \begin{macro}{\l@ngrel@x}
% This is either |\relax| or |\long| depending on whether the |*|-form
% of a definition command is being executed.
%    \begin{macrocode}
\let\l@ngrel@x\relax
%    \end{macrocode}
%  \end{macro}
%
% \begin{macro}{\newcommand}
% \changes{LaTeX2e}{1993/11/23}{Macro reimplemented and extended}
%    User level |\newcommand|.%
%    \begin{macrocode}
\def\newcommand{\@star@or@long\new@command}
%    \end{macrocode}
%
% \begin{macro}{\new@command}
% \changes{v1.2u}{1995/10/16}{(DPC) Use \cs{@testopt} /1911}
%    \begin{macrocode}
\def\new@command#1{%
  \@testopt{\@newcommand#1}0}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%
%  \begin{macro}{\@newcommand}
% \changes{LaTeX2e}{1993/11/23}{Macro added}
%  \begin{macro}{\@argdef}
% \changes{LaTeX2e}{1993/11/23}{Macro added}
%  \begin{macro}{\@xargdef}
% \changes{LaTeX2e}{1993/11/23}{Macro interface changed}
% \changes{v1.1g}{2004/01/23}{Use kernel version of
%                             \cs{@ifnextchar} (pr/3501)}
%    Handling arguments for |\newcommand|.
%    \begin{macrocode}
\def\@newcommand#1[#2]{%
  \kernel@ifnextchar [{\@xargdef#1[#2]}%
                {\@argdef#1[#2]}}
%    \end{macrocode}
%    Define |#1| if it is definable.
%
%    Both here and in |\@xargdef| the replacement text is absorbed as
%    an argument because if we are not allowed to make the definition
%    we have to get rid of it completely.
%    \begin{macrocode}
\long\def\@argdef#1[#2]#3{%
   \@ifdefinable #1{\@yargdef#1\@ne{#2}{#3}}}
%    \end{macrocode}
%
% \changes{v1.2q}{1995/10/02}
%     {New implementation, using \cs{@test@opt}}
%    Handle the second optional argument.
%    \begin{macrocode}
\long\def\@xargdef#1[#2][#3]#4{%
  \@ifdefinable#1{%
%    \end{macrocode}
%    Define the actual command to be:\\
%    |\def\foo{\@protected@testopt\foo\\foo{default}}|\\
%    where |\\foo| is a csname generated from applying |\csname| and
%    |\string| to |\foo|, ie the actual name contains a backslash and
%    therefore can't clash easily with existing command names.
%    ``Default'' is the contents of the second optional argument of
%    |(re)newcommand|.
%
% \changes{v1.2z2}{1998/03/04}
%     {Unnecessary \cs{expandafter} removed: pr/2758}
%    \begin{macrocode}
     \expandafter\def\expandafter#1\expandafter{%
          \expandafter
          \@protected@testopt
          \expandafter
          #1%
          \csname\string#1\endcsname
          {#3}}%
%    \end{macrocode}
%    Now we define the internal macro ie |\\foo| which is supposed to
%    pick up all arguments (optional and mandatory).
%    \begin{macrocode}
       \expandafter\@yargdef
          \csname\string#1\endcsname
           \tw@
           {#2}%
           {#4}}}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%  \end{macro}
%
% \begin{macro}{\@testopt}
% \changes{v1.2q}{1995/10/02}
%     {Macro added}
% \changes{v1.3a}{1999/01/07}
%     {made long and brace optional arg. latex/2896}
% This macro encapsulates the most common call to |\@ifnextchar|, saving
% several tokens each time it is used in the definition of a command
% with an optional argument.
% |#1| The code to execute in the case that there is a |[| need not be
% a single token but can be any sequence of commands that `expects' to
% be followed by |[|. If this command were only used in |\newcommand|
% definitions then |#1| would be a single token and the braces could
% be omitted from |{#1}| in the definition below, saving a bit of
% memory.
% \changes{v1.1g}{2004/01/23}{Use kernel version of
%                             \cs{@ifnextchar} (pr/3501)}
%    \begin{macrocode}
\long\def\@testopt#1#2{%
  \kernel@ifnextchar[{#1}{#1[{#2}]}}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@protected@testopt}
% \changes{v1.2q}{1995/10/02}
%     {Macro added}
% Robust version of |\@testopt|. The extra argument (|#1|) must be a
% single token. If protection is needed the call expands to |\protect|
% applied to this token, and the 2nd and 3rd arguments are
% discarded (by |\@x@protect|). Otherwise |\@testopt| is called on
% the 2nd and 3rd arguments.
%
% This method of making commands robust avoids the need for using up
% two csnames per command, the price is the extra expansion time
% for the |\ifx| test.
%    \begin{macrocode}
\def\@protected@testopt#1{%
  \ifx\protect\@typeset@protect
    \expandafter\@testopt
  \else
    \@x@protect#1%
  \fi}
%    \end{macrocode}
% \end{macro}
%
%  \begin{macro}{\@yargdef}
%  \begin{macro}{\@yargd@f}
% \changes{v1.3f}{1999/04/29}{New macro added}
%
% \changes{LaTeX2e}{1993/11/23}{Macro interface changed}
% \changes{LaTeX2e}{1993/11/23}{Avoid \cs{@?@?} token}
% \changes{v1.0b}{1994/03/12}{Name changed from \cs{XXX@argdef}}
% \changes{v1.3c}{1999/01/18}{New implementation DPC /2942}
% \changes{v1.3d}{1999/02/09}{catch bad argument forms by re-inserting \#3}
% \changes{v1.3f}{1999/04/29}{Full expansion and conversion needed
%    for digit in new version, see pr/3013}
%
%    These generate a primitive argument specification, from a
%    \LaTeX\ |[|\meta{digit}|]| form; in fact \meta{digit} can be
%    anything such that |\number|~\meta{digit} is single digit.
%
%    Reorganised slightly so that |\renewcommand{\reserved@a}[1]{foo}|
%    works.  I am not sure this is worth it, as a following
%    |\newcommand| would over-write the definition of |\reserved@a|.
%
%    Recall that \LaTeX2.09 goes into an infinite loop with\\
%    |\renewcommand[1]{\@tempa}{foo}|\\
%    (DPC 6 October 93).
%
%    Reorganised again (DPC 1999). Rather than make a loop to
%    construct the argument spec by counting, just extract the
%    required argument spec by using a delimited argument (delimited
%    by the digit).  This is faster and uses less tokens. The coding
%    is slightly odd to preserve the old interface (using |#2| =
%    |\tw@| as the flag to surround the first argument with |[]|.  But
%    the new method did not allow for the number of arguments |#3| not
%    being given as an explicit digit; hence (further expansion of
%    this argument and use of) |\number| was added later in 1999.
%
%    It is not clear why these are still |\long|.
%
%    \begin{macrocode}
\long \def \@yargdef #1#2#3{%
  \ifx#2\tw@
    \def\reserved@b##11{[####1]}%
  \else
    \let\reserved@b\@gobble
  \fi
  \expandafter
    \@yargd@f \expandafter{\number #3}#1%
}
%    \end{macrocode}
%
%    \begin{macrocode}
\long \def \@yargd@f#1#2{%
  \def \reserved@a ##1#1##2##{%
    \expandafter\def\expandafter#2\reserved@b ##1#1%
    }%
  \l@ngrel@x \reserved@a 0##1##2##3##4##5##6##7##8##9###1%
}
%    \end{macrocode}
%
%  \end{macro}
%  \end{macro}
%
% \begin{macro}{\@reargdef}
%  \changes{LaTeX2e}{1993/12/20}
%                {Kept old version of \cs{@reargdef}, for array.sty}
% \changes{v1.0b}{1994/03/12}{New defn, in terms of \cs{@yargdef}}
%  \changes{v1.2y}{1996/07/26}{third arg picked up by \cs{@yargdef}}
%    \begin{macrocode}
\long\def\@reargdef#1[#2]{%
  \@yargdef#1\@ne{#2}}
%    \end{macrocode}
% \end{macro}
%
%  \begin{macro}{\renewcommand}
%    Check the command name is already used.  If not give an error
%    message. Then temporarily
%    disable |\@ifdefinable| then call |\newcommand|. (Previous
%    version |\let#1=\relax| but this does not work too well if |#1|
%    is |\@temp|\emph{a--e}.)
% \changes{LaTeX2e}{1993/11/23}{Macro reimplemented and extended}
% \changes{v1.1f}{1994/05/02}{Removed surplus \cs{space} in error}
%    \begin{macrocode}
\def\renewcommand{\@star@or@long\renew@command}
%    \end{macrocode}
%
%  \begin{macro}{\renew@command}
%  \changes{v1.2y}{1996/07/26}{use \cs{relax} in place of empty arg}
%  \changes{v1.2y}{1996/07/26}{use \cs{noexpand} instead of \cs{string}}
% \changes{v1.2z1}{1997/10/21}{Use \cs{begingroup}/\cs{endgroup} rather
%    than braces for grouping, to avoid generating empty math atom.}
% \changes{v1.5e}{2018/09/26}{Always explicitly generate a space after the csname and
%    not rely on \cs{noexpand} to save tokens (git/41)}
%    \begin{macrocode}
\def\renew@command#1{%
  \begingroup \escapechar\m@ne\xdef\@gtempa{{\string#1}}\endgroup
  \expandafter\@ifundefined\@gtempa
     {\@latex@error{Command \string#1 undefined}\@ehc}%
     \relax
  \let\@ifdefinable\@rc@ifdefinable
  \new@command#1}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
% \changes{v1.0n}{1994/05/10}{Removed braces around \cs{@ifundefined}
%    argument.  ASAJ.}
% \changes{v1.0s}{1994/05/13}{Coded more efficiently, thanks to FMi.}
%
%  \begin{macro}{\@ifdefinable}
%  \begin{macro}{\@@ifdefinable}
%  \begin{macro}{\@rc@ifdefinable}
%    Test if user is allowed to define a command.
%    \begin{macrocode}
\long\def\@ifdefinable #1#2{%
      \edef\reserved@a{\expandafter\@gobble\string #1}%
     \@ifundefined\reserved@a
         {\edef\reserved@b{\expandafter\@carcube \reserved@a xxx\@nil}%
          \ifx \reserved@b\@qend \@notdefinable\else
            \ifx \reserved@a\@qrelax \@notdefinable\else
              #2%
            \fi
          \fi}%
         \@notdefinable}
%    \end{macrocode}
%    Saved definition of |\@ifdefinable|.
%    \begin{macrocode}
\let\@@ifdefinable\@ifdefinable
%    \end{macrocode}
%    Version of |\@ifdefinable| for use with |\renewcommand|.  Does
%    not do the check this time, but restores the normal definition.
%    \begin{macrocode}
\long\def\@rc@ifdefinable#1#2{%
  \let\@ifdefinable\@@ifdefinable
  #2}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%  \end{macro}
%
% \begin{macro}{\newenvironment}
%    Define a new user environment.
%    |#1| is the environment name. |#2#| Grabs all the tokens up to
%    the first |{|. These will be any optional arguments. They are not
%    parsed at this point, but are just passed to |\@newenv| which
%    will eventually call |\newcommand|. Any optional arguments will
%    then be parsed by |\newcommand| as it defines the command that
%    executes the `begin code' of the environment.
%
% This |#2#| trick removed with version 1.2i as it fails if a |{|
% occurs in the optional argument. Now use |\@ifnextchar| directly.
%    \begin{macrocode}
\def\newenvironment{\@star@or@long\new@environment}
%    \end{macrocode}
%
% \begin{macro}{\new@environment}
% \changes{v1.2i}{1995/04/25}{Parse arguments slowly but safely /1507}
% \changes{v1.2u}{1995/10/16}{(DPC) Use \cs{@testopt} /1911}
%    \begin{macrocode}
\def\new@environment#1{%
  \@testopt{\@newenva#1}0}
%    \end{macrocode}
%
% \begin{macro}{\@newenva}
% \changes{v1.1g}{2004/01/23}{Use kernel version of
%                             \cs{@ifnextchar} (pr/3501)}
%    \begin{macrocode}
\def\@newenva#1[#2]{%
   \kernel@ifnextchar [{\@newenvb#1[#2]}{\@newenv{#1}{[#2]}}}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@newenvb}
% \changes{v1.3a}{1999/01/07}
%     {made long and brace optional arg. latex/2896}
%    \begin{macrocode}
\def\@newenvb#1[#2][#3]{\@newenv{#1}{[#2][{#3}]}}
%    \end{macrocode}
% \end{macro}
%
% \end{macro}
% \end{macro}
%
%
%  \begin{macro}{\renewenvironment}
%    Redefine an environment.
%    For |\renewenvironment| disable |\@ifdefinable| and then call
%    |\newenvironment|.  It is OK to |\let| the argument to |\relax|
%    here as there should not be a |@temp|\ldots\ environment.
% \changes{LaTeX2e}{1993/11/23}{Macro reimplemented and extended}
% \changes{v1.1f}{1994/05/02}{Removed surplus \cs{space} in error}
%    \begin{macrocode}
\def\renewenvironment{\@star@or@long\renew@environment}
%    \end{macrocode}
%
%  \begin{macro}{\renew@environment}
% \changes{v1.2a}{1994/10/18}{reset end command}
% \changes{v1.2y}{1996/07/26}{use \cs{relax} in place of empty arg}
%    \begin{macrocode}
\def\renew@environment#1{%
  \@ifundefined{#1}%
     {\@latex@error{Environment #1 undefined}\@ehc
     }\relax
  \expandafter\let\csname#1\endcsname\relax
  \expandafter\let\csname end#1\endcsname\relax
  \new@environment{#1}}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
%
% \begin{macro}{\@newenv}
% \changes{LaTeX2e}{1993/11/23}{Macro interface changed}
% \changes{v1.2h}{1994/11/24}{Added test for \cs{endgraf}}
%    The internal version of |\newenvironment|.
%
%    Call |\newcommand| to define the \meta{begin-code} for the
%    environment.  |\def| is used for the \meta{end-code} as it does
%    not take arguments. (but may contain |\par|s)
%
% Make sure that an attempt to define a `graf' or `group' environment
% fails.
%    \begin{macrocode}
\long\def\@newenv#1#2#3#4{%
  \@ifundefined{#1}%
    {\expandafter\let\csname#1\expandafter\endcsname
                         \csname end#1\endcsname}%
    \relax
  \expandafter\new@command
     \csname #1\endcsname#2{#3}%
     \l@ngrel@x\expandafter\def\csname end#1\endcsname{#4}}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\newif}
% \changes{v1.1l}{1995/05/24}{(DPC) New implementation}
% And here's a different sort of allocation:
% For example,
% |\newif\iffoo| creates |\footrue|, |\foofalse| to go with |\iffoo|.
%    \begin{macrocode}
\def\newif#1{%
  \count@\escapechar \escapechar\m@ne
    \let#1\iffalse
    \@if#1\iftrue
    \@if#1\iffalse
  \escapechar\count@}
%    \end{macrocode}
%
% \begin{macro}{\@if}
%    \begin{macrocode}
\def\@if#1#2{%
  \expandafter\def\csname\expandafter\@gobbletwo\string#1%
                    \expandafter\@gobbletwo\string#2\endcsname
                       {\let#1#2}}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%
% \begin{macro}{\providecommand}
% |\providecommand| takes the same arguments as |\newcommand|, but
% discards them if |#1| is already defined, Otherwise it just acts like
% |\newcommand|. This implementation currently leaves any discarded
% definition in |\reserved@a| (and possibly |\\reserved@a|) this
% wastes a bit of space, but it will be reclaimed as soon as these
% scratch macros are redefined.
%
% \changes{LaTeX2e}{1993/11/22}{Macro added}
%    \begin{macrocode}
\def\providecommand{\@star@or@long\provide@command}
%    \end{macrocode}
%
% \begin{macro}{\provide@command}
% \changes{v1.2z}{1997/09/09}{Use \cs{begingroup} to avoid generating
%    math ords if used in math mode. pr/2573}
%    \begin{macrocode}
\def\provide@command#1{%
  \begingroup
    \escapechar\m@ne\xdef\@gtempa{{\string#1}}%
  \endgroup
  \expandafter\@ifundefined\@gtempa
    {\def\reserved@a{\new@command#1}}%
    {\def\reserved@a{\renew@command\reserved@a}}%
   \reserved@a}%
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\CheckCommand}
% \changes{LaTeX2e}{1993/11/22}{Macro added}
% \changes{v1.1e}{1994/05/20}{Changed name from \cs{@checkcommand} to
%    \cs{CheckCommand}.}
%    |\CheckCommand| takes the same arguments as |\newcommand|. If
%    the command already exists, with the same definition, then
%    nothing happens, otherwise a warning is issued. Useful for
%    checking the current state before a macro package starts
%    redefining things.  Currently two macros are considered to have
%    the same definition if they are the same except for different
%    default arguments.  That is, if the old definition was:
%    |\newcommand\xxx[2][a]{(#1)(#2)}| then
%    |\CheckCommand\xxx[2][b]{(#1)(#2)}| would \emph{not} generate a
%    warning, but, for instance |\CheckCommand\xxx[2]{(#1)(#2)}|
%    would.
%    \begin{macrocode}
\def\CheckCommand{\@star@or@long\check@command}
%    \end{macrocode}
%    |\CheckCommand| is only available in the preamble part of the
%    document.
%    \begin{macrocode}
\@onlypreamble\CheckCommand
%    \end{macrocode}
%
% \begin{macro}{\check@command}
%    \begin{macrocode}
\def\check@command#1#2#{\@check@c#1{#2}}
\@onlypreamble\check@command
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@check@c}
% \changes{v1.2i}{1995/04/25}{Make \cs{long} for latex/1346}
%    |\CheckCommand| itself just grabs all the arguments we need,
%    without actually looking for |[| optional argument forms.  Now
%    define |\reserved@a|. If |\\reserved@a| is then defined, compare it
%    with the ``|\#1|' otherwise compare |\reserved@a| with |#1|.
%    \begin{macrocode}
\long\def\@check@c#1#2#3{%
  \expandafter\let\csname\string\reserved@a\endcsname\relax
  \renew@command\reserved@a#2{#3}%
  \@ifundefined{\string\reserved@a}%
   {\@check@eq#1\reserved@a}%
   {\expandafter\@check@eq
           \csname\string#1\expandafter\endcsname
           \csname\string\reserved@a\endcsname}}
\@onlypreamble\@check@c
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@check@eq}
%     Complain if |#1| and |#2| are not |\ifx| equal.
%    \begin{macrocode}
\def\@check@eq#1#2{%
  \ifx#1#2\else
     \@latex@warning@no@line
               {Command \noexpand#1 has
                changed.\MessageBreak
                Check if current package is valid}%
  \fi}
\@onlypreamble\@check@eq
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@gobble}
% \begin{macro}{\@gobbletwo}
% \begin{macro}{\@gobblethree}
% \changes{v1.5g}{2020/02/27}{Macro added}
% \begin{macro}{\@gobblefour}
% \changes{v1.2n}{1995/05/26}{(CAR) Added \cs{long}s}
%    The |\@gobble| macro is used to get rid of its argument.
%    \begin{macrocode}
\long\def \@gobble #1{}
\long\def \@gobbletwo #1#2{}
\long\def \@gobblethree #1#2#3{}
\long\def \@gobblefour #1#2#3#4{}
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@firstofone}
% \begin{macro}{\@firstoftwo}
% \begin{macro}{\@secondoftwo}
%    Some argument-grabbers.
%    \begin{macrocode}
\long\def\@firstofone#1{#1}
\long\def\@firstoftwo#1#2{#1}
\long\def\@secondoftwo#1#2{#2}
%    \end{macrocode}
% \begin{macro}{\@iden}
%    |\@iden| is another name for |\@firstofone| for
%    compatibility reasons.
%    \begin{macrocode}
\let\@iden\@firstofone
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@thirdofthree}
%    Another grabber now used in the encoding specific
%    section.
% \changes{v1.2z3}{1998/03/20}{Macro added}
%    \begin{macrocode}
\long\def\@thirdofthree#1#2#3{#3}
%    \end{macrocode}
% \end{macro}
%
%
% \begin{macro}{\@expandtwoargs}
%    A macro to totally expand two arguments to another macro
% \changes{v1.5r}{2022/10/22}{Use \cs{protected@edef}.}
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2022/11/01}%
%<latexrelease>        {\@expandtwoargs}{protected edef}%
%<*2ekernel|latexrelease>
\def\@expandtwoargs#1#2#3{%
\protected@edef\reserved@a{\noexpand#1{#2}{#3}}\reserved@a}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{00/00/00}%
%<latexrelease>        {\@expandtwoargs}{protected edef}%
%<latexrelease>\def\@expandtwoargs#1#2#3{%
%<latexrelease>\edef\reserved@a{\noexpand#1{#2}{#3}}\reserved@a}
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@backslashchar}
%    A category code 12 backslash.
%    \begin{macrocode}
\edef\@backslashchar{\expandafter\@gobble\string\\}
%    \end{macrocode}
% \end{macro}
%
% \changes{v1.0n}{1994/05/10}{Added \cs{DeclareProtectedCommand}}
% \changes{v1.0p}{1994/05/12}{Fixed a bug with \cs{relax } which was
%    using \cs{@gobble} before defining it.}
% \changes{v1.0q}{1994/05/13}{Renamed \cs{DeclareProtectedCommand} to
%    \cs{DeclareRobustCommand}.  Removed \cs{@if@short@command}.
%    Moved to after the definition of \cs{@gobble}.}
% \changes{v1.0r}{1994/05/13}{Added logging message to
%    \cs{DeclareProtectedCommand}.}
%
% \subsection{Robust commands and protect}
%
% \changes{v1.1b}{1994/05/17}{Added the discussion of protected
%    commands, defined the values that \cs{protect} should have.}
% \changes{v1.1c}{1994/05/18}{Redid the discussion and definitions, in
%    line with the proposed new setting of \cs{protect} in the output
%    routine.}
%
% Fragile and robust commands are one of the thornier issues in
% \LaTeX's commands.  Whilst typesetting documents, \LaTeX{} makes use
% of many of \TeX's features, such as arithmetic, defining macros, and
% setting variables.  However, there are (at least) three different
% occasions when these commands are not safe.  These are called
% `moving arguments' by \LaTeX, and consist of:
% \begin{itemize}
% \item writing information to a file, such as indexes or tables of
%    contents.
% \item writing information to the screen.
% \item inside an |\edef|, |\message|, |\mark|, or other command which
%    evaluates its argument fully.
% \end{itemize}
% The method \LaTeX{} uses for making fragile commands robust is to
% precede them with |\protect|.  This can have one of four possible
% values:
% \begin{itemize}
% \item |\relax|, for normal typesetting.  So |\protect\foo| will
%    execute |\foo|.
% \item |\string|, for writing to the screen.  So |\protect\foo| will
%    write |\foo|.
% \item |\noexpand|, for writing to a file.  So |\protect\foo| will
%    write |\foo| followed by a space.
% \item |\@unexpandable@protect|, for writing a moving argument to a
%    file.  So |\protect\foo| will write |\protect\foo| followed by a
%    space.  This value is also used inside |\edef|s, |\mark|s and
%    other commands which evaluate their arguments fully.
%    More precisely, whenever the content of an |\edef| or |\xdef| etc.\
%    can contain arbitrary user input not under the direct control of the
%    programmer, one should use |\proetected@edef| instead of |\edef|, etc.,
%    so that |\protect| has a suitable definition and the user input will
%    not break if it contains fragile commands.
% \end{itemize}
%
% \changes{1.1b}{1994/05/17}
%     {(ASAJ) Added the \cs{@protect@...} commands.}
% \changes{1.1c}{1994/05/18}
%     {(ASAJ) Renamed the commands, and removed
%       one which is no longer needed.}
%
% \begin{macro}{\@unexpandable@protect}
% \changes{1.2w}{1995/12/05}{Removed \cs{unexpandable@noexpand} as never used. internal/1733}
%    \begin{macrocode}
\def\@unexpandable@protect{\noexpand\protect\noexpand}
%    \end{macrocode}
% \end{macro}
%
% \changes{v1.2e}{1994/11/04}{Rewrote protected short commands
%    using \cs{x@protect}. ASAJ.}
%
% \begin{macro}{\DeclareRobustCommand}
% \begin{macro}{\declare@robustcommand}
%    This is a package-writers command, which has the same syntax as
%    |\newcommand|, but which declares a protected command.  It does
%    this by having\\
%    |\DeclareRobustCommand\foo|\\
%    define |\foo| to be
%    |\protect\foo<space>|,\\
%    and then use |\newcommand\foo<space>|.\\
%    Since the internal command is |\foo<space>|, when it is written
%    to an auxiliary file, it will appear as |\foo|.
%
%    We have to be a
%    bit cleverer if we're defining a short command, such as |\_|, in
%    order to make sure that the auxiliary file does not include a
%    space after the command, since |\_ a| and |\_a| aren't the same.
%    In this case we define |\_| to be:
%\begin{verbatim}
%    \x@protect\_\protect\_<space>
%\end{verbatim}
%    which expands to:
%\begin{verbatim}
%    \ifx\protect\@typeset@protect\else
%       \@x@protect@\_
%    \fi
%    \protect\_<space>
%\end{verbatim}
%    Then if |\protect| is |\@typeset@protect| (normally |\relax|)
%    then we just perform |\_<space>|, and otherwise
%    |\@x@protect@| gobbles everything up and expands to
%    |\protect\_|.
%
%    \emph{Note}: setting |\protect| to any value other than |\relax|
%    whilst in `typesetting' mode will cause commands to go into an
%    infinite loop!  In particular, setting |\protect| to |\@empty| will
%    cause |\_| to loop forever.  It will also break lots of other
%    things, such as protected |\ifmmode|s inside |\halign|s.  If you
%    really have to do such a thing, then please set
%    |\@typeset@protect| to be |\@empty| as well.  (This is what the
%    code for |\patterns| does, for example.)
%
%    More fun with |\expandafter| and |\csname|.
%    \begin{macrocode}
\def\DeclareRobustCommand{\@star@or@long\declare@robustcommand}
%    \end{macrocode}
%
%    \begin{macrocode}
\def\declare@robustcommand#1{%
   \ifx#1\@undefined\else\ifx#1\relax\else
      \@latex@info{Redefining \string#1}%
   \fi\fi
   \edef\reserved@a{\string#1}%
   \def\reserved@b{#1}%
   \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
   \edef#1{%
      \ifx\reserved@a\reserved@b
         \noexpand\x@protect
         \noexpand#1%
      \fi
      \noexpand\protect
      \expandafter\noexpand\csname
         \expandafter\@gobble\string#1 \endcsname
   }%
   \let\@ifdefinable\@rc@ifdefinable
   \expandafter\new@command\csname
      \expandafter\@gobble\string#1 \endcsname
}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%
% \begin{macro}{\@x@protect}
% \begin{macro}{\x@protect}
%
%    \begin{macrocode}
\def\x@protect#1{%
   \ifx\protect\@typeset@protect\else
      \@x@protect#1%
   \fi
}
\def\@x@protect#1\fi#2#3{%
   \fi\protect#1%
}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@typeset@protect}
%     We set \cs{@typeset@protect} to \cs{relax} rather than \cs{@empty}
%     to make sure that the protection mechanism stops the look-ahead
%     and expansion performed at the start of \cs{halign} cells.
%    \begin{macrocode}
\let\@typeset@protect\relax
%    \end{macrocode}
% \end{macro}
%
% \changes{v1.2e}{1994/11/04}{Added commands for setting and restoring
%    \cs{protect}.  ASAJ.}
%
% \begin{macro}{\set@display@protect}
% \begin{macro}{\set@typeset@protect}
%    These macros set |\protect| appropriately for typesetting or
%    displaying.
% \changes{v1.2o}{1995/07/03}{Use \cs{@typeset@protect} for init}
%    \begin{macrocode}
\def\set@display@protect{\let\protect\string}
\def\set@typeset@protect{\let\protect\@typeset@protect}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\protected@edef}
% \begin{macro}{\protected@xdef}
% \begin{macro}{\unrestored@protected@xdef}
% \begin{macro}{\restore@protect}
%    The commands |\protected@edef| and |\protected@xdef| perform
%    `safe' |\edef|s and |\xdef|s, saving and restoring |\protect|
%    appropriately.  For cases where restoring |\protect| doesn't
%    matter, there's an `unsafe' |\unrestored@protected@xdef|, useful
%    if you know what you're doing!
%    \begin{macrocode}
\def\protected@edef{%
   \let\@@protect\protect
   \let\protect\@unexpandable@protect
   \afterassignment\restore@protect
   \edef
}
\def\protected@xdef{%
   \let\@@protect\protect
   \let\protect\@unexpandable@protect
   \afterassignment\restore@protect
   \xdef
}
\def\unrestored@protected@xdef{%
   \let\protect\@unexpandable@protect
   \xdef
}
\def\restore@protect{\let\protect\@@protect}
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
%
%  \begin{macro}{\protect}
%    The normal meaning of |\protect|
% \changes{v1.2j}{1995/04/29}{Init \cs{protect} here}
%    \begin{macrocode}
\set@typeset@protect
%    \end{macrocode}
%  \end{macro}
%
% \begin{macro}{\MakeRobust}
% \changes{v1.4a}{2015/01/08}{Added macro}
% \changes{v1.5f}{2019/08/27}{Make the assignments global as we may
%   need to apply them inside a group}
% \changes{v1.5m}{2020/08/21}{Make \cs{MakeRobust} produce the same
%   command structure as \cs{DeclareRobustCommand}}
%
%    This macro makes an existing fragile macro robust, but only if it
%    hasn't been robust in the past, i.e., it checks for the existence
%    of the macro
%    \verb*=\<name> = and if that exists it assumes that
%    \verb=\<name>= is already robust. In that case either undefine
%    the inner macro first or use \cs{DeclareRobustCommand} to
%    define it in a robust way directly.  We could probably test the
%    top-level definition to have the right kind of structure, but
%    this is somewhat problematical as we then have to distinguish
%    between \cs{long} macros and others and also take into account
%    that sometimes the top-level is deliberately done manually (like
%    with \cs{begin}).
%
%    The macro firstly checks if the control sequence in question exists
%    at all.
% \changes{v1.5p}{2021/05/26}{Normalize error message in \cs{MakeRobust}}
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020/10/01}{\MakeRobust}{\MakeRobust}%
%<*2ekernel|latexrelease>
\def\MakeRobust#1{%
  \count@=\escapechar
  \escapechar=`\\
  \@ifundefined{\expandafter\@gobble\string#1}{%
    \@latex@error{Command `\string#1' undefined.%
      \MessageBreak There is nothing here to make robust}%
    \@eha
  }%
%    \end{macrocode}
% Then we check if the macro is already robust. We do this by testing
% if the internal name for a robust macro is defined, namely
% \verb*=\foo =. If it is already defined do nothing, otherwise set
% \verb*=\foo = equal to \verb*=\foo= and redefine \verb*=\foo= so
% that it acts like a macro defined with \verb=\DeclareRobustCommand=.
% We use \cs{@kernel@rename@newcommand} to copy \verb*=\foo= over to
% \verb*=\foo =, including a possible default optional argument.
%    \begin{macrocode}
  {%
    \@ifundefined{\expandafter\@gobble\string#1\space}%
    {%
      \expandafter\@kernel@rename@newcommand
        \csname\expandafter\@gobble\string#1\space\endcsname
        #1%
      \edef\reserved@a{\string#1}%
      \def\reserved@b{#1}%
      \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
      \xdef#1{%
        \ifx\reserved@a\reserved@b
          \noexpand\x@protect\noexpand#1%
        \fi
        \noexpand\protect\expandafter\noexpand
        \csname\expandafter\@gobble\string#1\space\endcsname}%
    }%
    {\@latex@info{Command `\string#1' is already robust}}%
  }%
  \escapechar=\count@
}%
%    \end{macrocode}
%
%  \begin{macro}{\@kernel@rename@newcommand}
%    This macro renames a command, possibly with an optional argument (defined
%    with \cs{newcommand}) from |#2| to |#1|, by renaming the internal macro
%    \verb=\\#2= to \verb=\\#1= and defining \verb=\#1= appropriately, then
%    undefining \verb=\#2= and \verb=\\#2=.  The \cs{afterassignment} trick is
%    to make both definitions in \cs{@copy@newcommand} global (which are local
%    by default).
%
%    In case the macro was defined with \cs{newcommand} and an optional
%    argument, to replicate exactly the behaviour of \cs{DeclareRobustCommand}
%    we have to move also the internal \verb*=\\foo= to \verb*=\\foo =.  In that
%    case, \verb=#1= will be a parameterless macro (\cs{robust@command@chk@safe}
%    checks that), and \cs{@if@newcommand} will return true (both defined below
%    in this file).  If so, we can use \cs{@copy@newcommand} rather than plain
%    \cs{let} to copy the command over.  \cs{@kernel@rename@newcommand} does
%    this test and carries out the renaming.
%    \begin{macrocode}
\def\@kernel@rename@newcommand#1#2{%
  \robust@command@chk@safe#2%
    {\@if@newcommand#2%
      {\afterassignment\global
       \global\@copy@newcommand#1#2%
       \global\let#2\@undefined
       \global\expandafter\let\csname\string#2\endcsname\@undefined}%
      {\global\let#1=#2}}%
    {\global\let#1=#2}}
%    \end{macrocode}
%  \end{macro}
%
%    \begin{macrocode}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{2019/10/01}{\MakeRobust}{\MakeRobust}%
%<latexrelease>\def\MakeRobust#1{%
%<latexrelease>  \@ifundefined{\expandafter\@gobble\string#1}{%
%<latexrelease>    \@latex@error{The control sequence `\string#1' is undefined!%
%<latexrelease>      \MessageBreak There is nothing here to make robust}%
%<latexrelease>    \@eha
%<latexrelease>  }%
%<latexrelease>  {%
%<latexrelease>    \@ifundefined{\expandafter\@gobble\string#1\space}%
%<latexrelease>    {%
%<latexrelease>      \global\expandafter\let\csname
%<latexrelease>      \expandafter\@gobble\string#1\space\endcsname=#1%
%<latexrelease>      \edef\reserved@a{\string#1}%
%<latexrelease>      \def\reserved@b{#1}%
%<latexrelease>      \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
%<latexrelease>      \xdef#1{%
%<latexrelease>        \ifx\reserved@a\reserved@b
%<latexrelease>          \noexpand\x@protect\noexpand#1%
%<latexrelease>        \fi
%<latexrelease>        \noexpand\protect\expandafter\noexpand
%<latexrelease>        \csname\expandafter\@gobble\string#1\space\endcsname}%
%<latexrelease>    }%
%<latexrelease>    {\@latex@info{The control sequence `\string#1' is already robust}}%
%<latexrelease>   }%
%<latexrelease>}%
%<latexrelease>\let\@kernel@rename@newcommand\@undefined
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{2015/01/01}{\MakeRobust}{\MakeRobust}%
%<latexrelease>\def\MakeRobust#1{%
%<latexrelease>  \@ifundefined{\expandafter\@gobble\string#1}{%
%<latexrelease>    \@latex@error{The control sequence `\string#1' is undefined!%
%<latexrelease>      \MessageBreak There is nothing here to make robust}%
%<latexrelease>    \@eha
%<latexrelease>  }%
%<latexrelease>  {%
%<latexrelease>    \@ifundefined{\expandafter\@gobble\string#1\space}%
%<latexrelease>    {%
%<latexrelease>      \expandafter\let\csname
%<latexrelease>      \expandafter\@gobble\string#1\space\endcsname=#1%
%<latexrelease>      \edef\reserved@a{\string#1}%
%<latexrelease>      \def\reserved@b{#1}%
%<latexrelease>      \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
%<latexrelease>      \edef#1{%
%<latexrelease>        \ifx\reserved@a\reserved@b
%<latexrelease>          \noexpand\x@protect\noexpand#1%
%<latexrelease>        \fi
%<latexrelease>        \noexpand\protect\expandafter\noexpand
%<latexrelease>        \csname\expandafter\@gobble\string#1\space\endcsname}%
%<latexrelease>    }%
%<latexrelease>    {\@latex@info{The control sequence `\string#1' is already robust}}%
%<latexrelease>   }%
%<latexrelease>}%
%<latexrelease>\let\@kernel@rename@newcommand\@undefined
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{0000/00/00}{\MakeRobust}{\MakeRobust}%
%<latexrelease>\let\MakeRobust\@undefined
%<latexrelease>\let\@kernel@rename@newcommand\@undefined
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
%
%  \begin{macro}{\kernel@make@fragile}
%    The opposite of |\MakeRobust| except that it doesn't do many
%    checks as it is internal to the kernel. Why does one want such a
%    thing?
%    Only for compatibility reasons if \texttt{latexrelease} requests
%    a rollback of the kernel. For this reason we pretend that this
%    command existed in all earlier versions of \LaTeX{} i.e., we are
%    not rolling it back since we need it precisely then. But we have
%    to get it into the \texttt{latexrelease} file so that a roll
%    forward is possible too.
%    \begin{macrocode}
%</2ekernel>
%<*2ekernel|latexrelease>
%<latexrelease>\IncludeInRelease{2020/10/01}%
%<latexrelease>                 {\kernel@make@fragile}{Undo robustness}%
\def\kernel@make@fragile#1{%
  \@ifundefined{\expandafter\@gobble\string#1\space}%
%    \end{macrocode}
%    If not robust do nothing.
%    \begin{macrocode}
     {}%
%    \end{macrocode}
%    Otherwise copy \verb*=\foo = back to \verb=\foo=.
%    Then use \cs{@kernel@rename@newcommand} to check and copy
%    \verb*=\\foo = back to \verb*=\\foo= in case the command has an optional
%    argument.  If so, also undefine \verb*=\\foo =, and at the end undefine
%    \verb*=\foo =.
%    \begin{macrocode}
     {%
      \global\expandafter\let\expandafter #1\csname
        \expandafter\@gobble\string#1\space\endcsname
      \expandafter\@kernel@rename@newcommand
        \csname\expandafter\@gobble\string#1\expandafter\endcsname
        \csname\expandafter\@gobble\string#1\space\endcsname
      \global\expandafter\let\csname
        \expandafter\@gobble\string#1\space\endcsname\@undefined
     }%
}
%    \end{macrocode}
%
%    \begin{macrocode}
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{0000/00/00}%
%<latexrelease>                 {\kernel@make@fragile}{Undo robustness}%
%<latexrelease>\def\kernel@make@fragile#1{%
%<latexrelease>  \@ifundefined{\expandafter\@gobble\string#1\space}%
%<latexrelease>     {}%
%<latexrelease>     {%
%<latexrelease>      \global\expandafter\let\expandafter #1\csname
%<latexrelease>      \expandafter\@gobble\string#1\space\endcsname
%<latexrelease>      \global\expandafter\let\csname
%<latexrelease>      \expandafter\@gobble\string#1\space\endcsname\@undefined
%<latexrelease>     }%
%<latexrelease>}
%<latexrelease>\EndIncludeInRelease
%</2ekernel|latexrelease>
%<*2ekernel>
%    \end{macrocode}
%  \end{macro}
%
%
% \subsection{Acting on robust commands}
%
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020-10-01}{\robust@command@act}
%<latexrelease>  {Add \robust@command@act}%
%<*2ekernel|latexrelease>
%    \end{macrocode}
%
%   With most document level commands being robust now there is more of a
%   requirement to have a standard way of aliasing (or copying) a command to a
%   new name, for example to save an original definition before changing a
%   command.  \cs{DeclareCommandCopy} is analogous to \TeX's \cs{let}, except
%   that it copes with the different types of robust commands defined by
%   \LaTeX's mechanisms.
%
%   A couple of ``types of robustness'' are defined by the \LaTeXe{} kernel,
%   namely robust commands defined with \cs{DeclareRobustCommand} and commands
%   with optional arguments defined with \cs{newcommand}.  However there are
%   other types of robust commands that are frequently used, which are not
%   defined in the \LaTeXe{} kernel, like commands defined with
%   \textsf{xparse}'s \cs{NewDocumentCommand} and \textsf{etoolbox}'s
%   \cs{newrobustcmd}.
%
%   In this section we will define a generic extensible machinery to act on
%   robust commands.  This code will then be used to test if a command is
%   robust, considered the different types of robustness, and then either copy
%   that definition, if \cs{DeclareCommandCopy} (or similar) is used, or show
%   the definition of the command, if \cs{ShowCommand} is used.
%
% \begin{macro}{\robust@command@act}
% \changes{v1.5k}{2020/08/19}{Made \cs{robust@command@act}
%     (was \cs{declare@command@copy}) more generic}
%
%   The looping machinery is generic and knows nothing about what is to be done
%   for each case.  The syntax of the main macro \cs{robust@command@act} is:
%   \begin{quote}
%     |\robust@command@act|\meta{action-list}\meta{robust-cmd}\\
%       \meta{fallback-action}\meta{act-arg}
%   \end{quote}
%   \meta{action-list} is a token list of the form:
%   \begin{quote}
%     |{|\meta{if-type-1} \meta{act-type-1}|}|\\
%     |{|\meta{if-type-2} \meta{act-type-2}|}|\\
%     \ldots
%   \end{quote}
%   \cs{robust@command@act} will iterate over the \meta{action-list}, evaluating
%   each \meta{if-type-$n$} \meta{robust-cmd} |{|\meta{true}|}{|\meta{false}|}|.
%   If the \meta{if-type-$n$} conditional returns \meta{true}, then
%   \meta{act-type-$n$}\meta{act-arg} is executed, and the loop ends.  If the
%   conditional returns \meta{false}, then \meta{if-type-$n+1$} is executed in
%   the same way, until either one of the conditionals return \meta{true}, or
%   the end of the \meta{action-list} is reached.  If the end is reached, then
%   \meta{fallback-action}\meta{act-arg} is executed before
%   \cs{robust@command@act} exits.
%
%   \cs{robust@command@act} will start by using \cs{robust@command@act@chk@args}
%   to check if the \meta{robust-cmd} (|#2|) is a parameterless (possibly
%   \cs{protected}) macro.  If it is not, the command is not a robust command:
%   these always start with a parameterless user-level macro;  in that case,
%   \cs{robust@command@act@end} is used to short-circuit the process and do the
%   \meta{fallback-action} (|#3|).  This first test is necessary because later
%   on we need to be able to expand the \meta{robust-cmd} without the risk of it
%   Breaking Badly, and as a bonus, this speeds up the process in case we used
%   \cs{NewCommandCopy} in a ``normal'' macro.
%    \begin{macrocode}
\long\def\robust@command@act#1#2#3#4{%
  \robust@command@chk@safe#2%
    {\expandafter\robust@command@act@loop
       \expandafter#2%
         #1{\@nnil\@nnil}%
     \robust@command@act@end}%
    {\robust@command@act@end}%
      {#3}{#4}}%
%    \end{macrocode}
%
% \begin{macro}{\robust@command@act@loop}
% \begin{macro}{\robust@command@act@loop@aux}
% \begin{macro}{\robust@command@act@do}
%   If \cs{robust@command@act@chk@args} branched to false, then
%   \cs{robust@command@act@loop} will loop over the list of items in the
%   \meta{action-list} (|#1|), and process each item as described earlier.
%   If the \meta{if-type-$n$} command expands to \meta{true} then
%   \cs{robust@command@act@do} is used to execute \meta{act-type-$n$} on the
%   \meta{act-arg}, otherwise the loop resumes with the next item.
%    \begin{macrocode}
\long\def\robust@command@act@loop#1#2{\robust@command@act@loop@aux#1#2}
\long\def\robust@command@act@loop@aux#1#2#3{%
  \ifx\@nnil#2%
  \else
    #2{#1}%
      {\robust@command@act@do{#3}}%
      {\expandafter\robust@command@act@loop\expandafter#1}%
  \fi}
\long\def\robust@command@act@do#1%
  \fi#2%
  \robust@command@act@end#3#4{%
  \fi
  #1#4}
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\robust@command@act@end}
%   If the end is reached and no action was taken, then do
%   \meta{fallback-action}\meta{act-arg}.
%    \begin{macrocode}
\long\def\robust@command@act@end#1#2{#1#2}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\robust@command@chk@safe}
% \begin{macro}{\robust@command@act@chk@args}
%    \begin{macrocode}
\long\def\robust@command@chk@safe#1{%
  \begingroup
    \escapechar=`\\
  \expandafter\endgroup\expandafter
  \robust@command@act@chk@args\meaning#1:->\@nil}
\def\robust@command@act@chk@args#1:->#2\@nil{%
  \@expl@str@if@eq@@nnTF{#1}{macro}%
    {\@firstoftwo}%
    {\@expl@str@if@eq@@nnTF{#1}{\protected macro}%
      {\@firstoftwo}%
      {\@secondoftwo}}}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%    \begin{macrocode}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000-00-00}{\robust@command@act}
%<latexrelease>  {Add \robust@command@act}%
%<latexrelease>\let\robust@command@act\@undefined
%<latexrelease>\let\robust@command@act@loop\@undefined
%<latexrelease>\let\robust@command@act@loop@aux\@undefined
%<latexrelease>\let\robust@command@act@do\@undefined
%<latexrelease>\let\robust@command@act@end\@undefined
%<latexrelease>\let\robust@command@chk@safe\@undefined
%<latexrelease>\let\robust@command@act@chk@args\@undefined
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
%
% \end{macro}
%
%
% \subsubsection{Copying robust commands}
%
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020-10-01}{\DeclareCommandCopy}
%<latexrelease>  {Add \NewCommandCopy, \RenewCommandCopy, and \DeclareCommandCopy}%
%<*2ekernel|latexrelease>
%    \end{macrocode}
%
% \begin{macro}{\NewCommandCopy}
% \begin{macro}{\RenewCommandCopy}
% \begin{macro}{\DeclareCommandCopy}
% \changes{v1.5j}{2020/05/09}{Added \cs{DeclareCommandCopy} (gh/239)}
%
%   \cs{NewCommandCopy} starts by checking if \verb=#1= is already defined, and
%   raises an error if so, otherwise the definition is carried out.
%   \cs{RenewCommandCopy} does (almost) the opposite.  If the command is
%   \emph{not} defined, then an error is raised.  But the definition is carried
%   out anyhow, so the behaviour is consistent with \cs{renewcommand}.
%
%   A \cs{ProvideCommandCopy} isn't defined because it's not reasonably useful.
%   \verb=\provide...= commands mean ``define this if there's no other
%   definition'', but copying a command (usually) implies that the command being
%   copied is defined, so \cs{ProvideCommandCopy} doesn't make a lot of sense.
%   But more importantly, the most common use case of copying a command is to
%   redefine it later, while preserving the old definition, as in:
% \begin{verbatim}
%   \ProvideCommandCopy \A \B
%   \renewcommand \B { ... \A ... }
% \end{verbatim}
%   then, if \verb=\A= is already defined the first line is skipped, an in this
%   case \verb=\B= won't work as expected.
%
%   The three versions call the internal \cs{declare@commandcopy} with the
%   proper action.  \cs{@firstofone} will carry out the copy.  The only case
%   when the copy is not made is the \meta{false} case for \cs{NewCommandCopy},
%   in which the command already exists and the definition is aborted.
%    \begin{macrocode}
\def\NewCommandCopy{%
  \declare@commandcopy
    {\@firstofone}%
    {\@firstoftwo\@notdefinable}}
\def\RenewCommandCopy{%
  \declare@commandcopy
    {\@latex@error{Command \@backslashchar\reserved@a\space undefined}\@ehc
     \@firstofone}%
    {\@firstofone}}
\def\DeclareCommandCopy{%
  \declare@commandcopy
    {\@firstofone}%
    {\@firstofone}}
%    \end{macrocode}
%
% \begin{macro}{\declare@commandcopy}
%   Start by checking if the command is already defined.  The proper action is
%   taken by each specific command above.  If all's good, then
%   \cs{robust@command@act} is called with the proper arguments as described
%   earlier, with \cs{@declarecommandcopylisthook} as the \meta{action-list} and
%   \cs{declare@commandcopy@let} as the \meta{fallback-action}.
%    \begin{macrocode}
\long\def\declare@commandcopy#1#2#3#4{%
  \edef\reserved@a{\@expl@cs@to@str@@N#3}%
  \@ifundefined\reserved@a{#1}{#2}%
    {\robust@command@act
       \@declarecommandcopylisthook#4%
       \declare@commandcopy@let{#3#4}}}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@declarecommandcopylisthook}
%   The initial definition of \cs{@declarecommandcopylisthook} contains the
%   tests for the two types of robust command in the kernel.
%    \begin{macrocode}
\def\@declarecommandcopylisthook{%
  {\@if@DeclareRobustCommand \@copy@DeclareRobustCommand}%
  {\@if@newcommand \@copy@newcommand}}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\declare@commandcopy@let}
%   The initial definition of \cs{@declarecommandcopylisthook} contains the
%   tests for the two types of robust command in the kernel.
%    \begin{macrocode}
\long\def\declare@commandcopy@let#1#2{\let#1=#2\relax}
%    \end{macrocode}
% \end{macro}
%
%   Now the rollback code.
%    \begin{macrocode}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000-00-00}{\DeclareCommandCopy}
%<latexrelease>  {Undefine \NewCommandCopy, \RenewCommandCopy, and \DeclareCommandCopy}%
%<latexrelease>\let\NewCommandCopy\@undefined
%<latexrelease>\let\RenewCommandCopy\@undefined
%<latexrelease>\let\DeclareCommandCopy\@undefined
%<latexrelease>\let\declare@commandcopy\@undefined
%<latexrelease>\let\@declarecommandcopylisthook\@undefined
%<latexrelease>\let\declare@commandcopy@let\@undefined
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
%
% \subsubsection{Showing robust commands}
%
% \begin{macro}{\ShowCommand}
% \changes{v1.5k}{2020/08/19}{Added \cs{ShowCommand} (gh/373)}
%
%   Most of the machinery defined for \cs{NewCommandCopy} can be used to show
%   the definition of a robust command, in a similar fashion to \texttt{texdef}.
%   The difference is that after the command's is detected to has a given type
%   of robustness, rather than making a copy, we use a separate routine to show
%   its definition.
%
%   With all the machinery in place, \cs{ShowCommand} itself is quite simple:
%   use \cs{robust@command@act} to iterate through the \cs{@showcommandlisthook}
%   list, and if nothing is found, fallback to \cs{show}.
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020-10-01}{\ShowCommand}%
%<latexrelease>  {Add \ShowCommand}%
%<*2ekernel|latexrelease>
%    \end{macrocode}
%
%    \begin{macrocode}
\long\def\ShowCommand#1{%
  \robust@command@act
    \@showcommandlisthook#1%
    \show#1}
%    \end{macrocode}
%
% \begin{macro}{\@showcommandlisthook}
%   The initial definition of \cs{@showcommandlisthook} contains the same tests
%   as used for copying, but \cs{@show@...} commands instead of \cs{@copy@...}.
%   Same as before, it is initialized to cope with \cs{DeclareRobustCommand} and
%   \cs{newcommand} with optional arguments.
%    \begin{macrocode}
\def\@showcommandlisthook{%
  {\@if@DeclareRobustCommand \@show@DeclareRobustCommand}%
  {\@if@newcommand \@show@newcommand}}
%    \end{macrocode}
% \end{macro}
%
%   Now the rollback code.
%    \begin{macrocode}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000-00-00}{\ShowCommand}
%<latexrelease>  {Undefine \ShowCommand}%
%<latexrelease>\let\ShowCommand\@undefined
%<latexrelease>\let\@showcommandlisthook\@undefined
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
%
%
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020-10-01}{\@if@DeclareRobustCommand}
%<latexrelease>  {Add \@if@DeclareRobustCommand, \@if@newcommand,
%<latexrelease>       \@copy@DeclareRobustCommand, \@copy@newcommand,
%<latexrelease>       \@show@DeclareRobustCommand, \@show@newcommand}%
%<*2ekernel|latexrelease>
%    \end{macrocode}
%
% \subsubsection{Commands defined with \cs{DeclareRobustCommand}}
%
% \begin{macro}{\@if@DeclareRobustCommand}
% \changes{v1.5j}{2020/05/09}{Added \cs{DeclareCommandCopy} (gh/239)}
%
%   Now that we provided a generic way to copy one macro to another, we need to
%   define a way to check if a command is one of \LaTeXe's robust types.  These
%   tests are heavily based on Heiko's \cs{LetLtxMacro}, but chopped into
%   separate macros.
%
%   \cs{@if@DeclareRobustCommand} checks if a command \verb=\cmd= was defined by
%   \cs{DeclareRobustCommand}.  The test returns true if the expansion of
%   \verb=\cmd= is exactly \verb*=\protect\cmd =.
%
%    \begin{macrocode}
\long\def\@if@DeclareRobustCommand#1{%
  \begingroup
    \escapechar=`\\
    \edef\reserved@a{\string#1}%
    \edef\reserved@b{\detokenize{#1}}%
    \xdef\@gtempa{%
      \ifx\reserved@a\reserved@b
         \noexpand\x@protect
         \noexpand#1%
      \fi
      \noexpand\protect
      \expandafter\noexpand\csname\@expl@cs@to@str@@N#1 \endcsname}%
  \endgroup
  \ifx\@gtempa#1\relax
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}
%    \end{macrocode}
%
% \begin{macro}{\@copy@DeclareRobustCommand}
% \begin{macro}{\copy@kernel@robust@command}
%   If a command was defined by \cs{DeclareRobustCommand} (that is,
%   \cs{@if@DeclareRobustCommand} returns true), then to make a copy of \verb=\cmd=
%   into \verb=\foo= we define the latter such that it expands to
%   \verb*=\protect\foo =, then make \verb*=\foo = equal to \verb*=\cmd =.
%
%   There is one detail we need to take care of:  if a command was defined with
%   \cs{DeclareRobustCommand} it may still have an optional argument, in which
%   case there is one more macro layer before the actual definition of the
%   command.  We use \cs{@if@newcommand} to check that and
%   \cs{@copy@newcommand} to do the copying.
%    \begin{macrocode}
\long\def\@copy@DeclareRobustCommand#1#2{%
  \begingroup
    \escapechar=`\\
    \edef\reserved@a{\string#1}%
    \edef\reserved@b{\detokenize{#1}}%
    \edef\reserved@a{%
  \endgroup
  \def\noexpand#1{%
    \ifx\reserved@a\reserved@b
       \noexpand\x@protect
       \noexpand#1%
    \fi
    \noexpand\protect
    \expandafter\noexpand\csname\@expl@cs@to@str@@N#1 \endcsname}%
  \noexpand\copy@kernel@robust@command
    \expandafter\noexpand\csname\@expl@cs@to@str@@N#1 \endcsname
    \expandafter\noexpand\csname\@expl@cs@to@str@@N#2 \endcsname}%
  \reserved@a}
\long\def\copy@kernel@robust@command#1#2{%
  \robust@command@chk@safe#2%
    {\@if@newcommand#2%
       {\@copy@newcommand}%
       {\declare@commandcopy@let}}
    {\declare@commandcopy@let}%
  #1#2}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@show@DeclareRobustCommand}
% \begin{macro}{\show@kernel@robust@command}
%   Showing the command is pretty simple.  This command prints the top-level
%   expansion as \TeX's \cs{show} would, but with |robust macro:| rather than
%   just |macro:|, then a blank line and then \cs{show} the inner command.
%   For a macro defined with, say, |\DeclareRobustCommand\foo[1]{bar}|, it will
%   print:
% \begin{verbatim}
%   > \foo=robust macro:
%   ->\protect \foo  .
%   
%   > \foo =\long macro:
%   #1->bar.
% \end{verbatim}
%   If the inner command is defined with an optional argument, then
%   \cs{@show@newcommand} is also used.
%
%   The value of \cs{escapechar} is deliberately not enforced, so
%   \cs{ShowCommand} behaves more like \cs{show}.
%    \begin{macrocode}
\long\def\@show@DeclareRobustCommand#1{%
  \typeout{> \string#1=robust macro:}%
  \typeout{->\@expl@cs@replacement@spec@@N#1.^^J}%
  \expandafter\show@kernel@robust@command
    \csname\@expl@cs@to@str@@N#1 \endcsname}
\long\def\show@kernel@robust@command#1{%
  \robust@command@chk@safe#1%
    {\@if@newcommand#1%
       {\@show@newcommand}%
       {\show}}%
    {\show}%
  #1}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \end{macro}
%
% \subsubsection{Commands defined with \cs{newcommand} (with optional argument)}
%
% \begin{macro}{\@if@newcommand}
%   A command \verb=\cmd= (or \verb*=\cmd =, if it was defined with
%   \cs{DeclareRobustCommand}) with an optional argument will expand to
%   \verb*=\@protected@testopt\cmd\\cmd{<opt>}=.  To check that we look at the
%   first three tokens in the expansion of \verb=\cmd=, and return true or false
%   accordingly.
%
%   This test \emph{requires} that the command be a parameterless macro,
%   otherwise it will not work (and probably break).  This is ensured with
%   \cs{robust@command@chk@safe} before calling \cs{@if@newcommand}.
%    \begin{macrocode}
\long\def\@if@newcommand#1{%
  \edef\reserved@a{%
    \noexpand\@protected@testopt
    \noexpand#1%
    \expandafter\noexpand\csname\@backslashchar\@expl@cs@to@str@@N#1\endcsname}%
  \edef\reserved@b{%
    \unexpanded\expandafter\expandafter\expandafter
      {\expandafter\@carcube#1{}{}{}\@nil}}%
  \ifx\reserved@a\reserved@b
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}
%    \end{macrocode}
%
% \begin{macro}{\@copy@newcommand}
%   Then, if a command \verb=\cmd= takes an optional argument, we copy it to
%   \verb=\foo= by defining the latter to expand to
%   \verb=\@protected@testopt\foo\\foo{<opt>}=.
%    \begin{macrocode}
\long\def\@copy@newcommand#1#2{%
  \edef#1{\noexpand\@protected@testopt
    \noexpand#1%
    \expandafter\noexpand\csname\@backslashchar\@expl@cs@to@str@@N#1\endcsname
    \unexpanded\expandafter\expandafter\expandafter
      {\expandafter\@gobblethree#2}}%
  \expandafter
  \let\csname\@backslashchar\@expl@cs@to@str@@N#1\expandafter\endcsname
      \csname\@backslashchar\@expl@cs@to@str@@N#2\endcsname}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@show@newcommand}
% \begin{macro}{\@show@newcommand@aux}
%   A command being \cs{show}n here is guaranteed to have an optional argument.
%   Start by showing the top-level expansion of the command (using \cs{typeout}
%   to avoid TeX asking for interaction and extra context lines), then call
%   \cs{@show@newcommand@aux} with the internal command, which contains the
%   actual definition, and with the expansion of the command to extract the
%   default value of the optional argument.
%    \begin{macrocode}
\long\def\@show@newcommand#1{%
  \typeout{> \string#1=robust macro:}%
  \typeout{->\@expl@cs@replacement@spec@@N#1.^^J}%
  \expandafter\@show@newcommand@aux
    \csname\@backslashchar\@expl@cs@to@str@@N#1\expandafter\endcsname
    \expandafter{#1}}
%    \end{macrocode}
%
%   For a macro defined with, say, |\newcommand\foo[1][opt]{bar}|, it will
%   print:
% \begin{verbatim}
%   > \foo=robust macro:
%   ->\@protected@testopt \foo \\foo {opt}.
%   
%   > \\foo=\long macro:
%   > default #1=opt.
%   [#1]->bar.
% \end{verbatim}
%   If the command was defined with \cs{DeclareRobustCommand}, then another pair
%   of lines show the top-level expansion \verb*|\protect \foo  |.
%
%   The extra gymnastics with \cs{showtokens} ensures that \cs{showtokens}
%   itself, and the internals of this macro aren't showed in the context lines.
%    \begin{macrocode}
\long\def\@show@newcommand@aux#1#2{%
  \typeout{> \string#1=\@expl@cs@prefix@spec@@N#1macro:}%
  \edef\reserved@a{%
    default \string##1=\expandafter\detokenize\@gobblethree#2.^^J%
    \@expl@cs@argument@spec@@N#1->\@expl@cs@replacement@spec@@N#1}%
  \showtokens\expandafter\expandafter\expandafter{\expandafter\reserved@a}}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%   Now the rollback code.
%    \begin{macrocode}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000-00-00}{\@if@DeclareRobustCommand}
%<latexrelease>  {Undefine \@if@DeclareRobustCommand, \@if@newcommand,
%<latexrelease>            \@copy@DeclareRobustCommand, \@copy@newcommand,
%<latexrelease>            \@show@DeclareRobustCommand, \@show@newcommand}%
%<latexrelease>\let\@if@DeclareRobustCommand\@undefined
%<latexrelease>\let\@copy@DeclareRobustCommand\@undefined
%<latexrelease>\let\@show@DeclareRobustCommand\@undefined
%<latexrelease>\let\@if@newcommand\@undefined
%<latexrelease>\let\@copy@newcommand\@undefined
%<latexrelease>\let\@show@newcommand\@undefined
%
%<latexrelease>\let\copy@kernel@robust@command\@undefined
%<latexrelease>\let\show@kernel@robust@command\@undefined
%<latexrelease>\let\@show@newcommand@aux\@undefined
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
%
% \end{macro}
%
%
% \subsection{Internal defining commands}
%
% These commands are used internally to define other \LaTeX{}
% commands.
% \begin{macro}{\@ifundefined}
% \changes{LaTeX2e}{1993/11/23}{Redefined to remove a trailing \cs{fi}}
%    Check if first arg is undefined or \cs{relax} and execute second or
%    third arg depending,
% \changes{1.5c}{2018/01/06}{Avoid defining undefined commands to \cs{relax}}
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2018-04-01}{\@ifundefined}
%<latexrelease>{Leave commands undefined in \@ifundefined}%
%<*2ekernel|latexrelease>
%    \end{macrocode}
% Version using |\ifcsname| to avoid defining undefined tokens to |\relax|.
% Defined here to simplify using unmatched |\fi|.
%    \begin{macrocode}
\def\@ifundefined#1{%
  \ifcsname#1\endcsname\@ifundefin@d@i\else\@ifundefin@d@ii\fi{#1}}
%    \end{macrocode}
%    \begin{macrocode}
\long\def\@ifundefin@d@i#1\fi#2{\fi
  \expandafter\ifx\csname #2\endcsname\relax
    \@ifundefin@d@ii
  \fi
  \@secondoftwo}
%    \end{macrocode}
%    \begin{macrocode}
\long\def\@ifundefin@d@ii\fi#1#2#3{\fi #2}
%    \end{macrocode}
% Now test of engine.
%    \begin{macrocode}
\ifx\numexpr\@undefined
%    \end{macrocode}
% Classic version (should not be needed as etex is assumed).
%    \begin{macrocode}
\def\@ifundefined#1{%
  \expandafter\ifx\csname#1\endcsname\relax
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}
\else\ifx\directlua\@undefined
%    \end{macrocode}
% Use the |\ifcsname| defined above.
%    \begin{macrocode}
\else
%    \end{macrocode}
% Optimised version for Lua\TeX, using |\lastnamedcs|
%    \begin{macrocode}
\def\@ifundefined#1{%
  \ifcsname#1\endcsname
    \expandafter\ifx\lastnamedcs\relax\else\@ifundefin@d@i\fi
  \fi
  \@firstoftwo}
%    \end{macrocode}
%    \begin{macrocode}
\long\def\@ifundefin@d@i#1#2#3#4#5{#1#2#5}
%    \end{macrocode}
%    \begin{macrocode}
\fi
\fi
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000-00-00}{\@ifundefined}
%<latexrelease>{Leave commands undefined in \@ifundefined}%
%<latexrelease>\def\@ifundefined#1{%
%<latexrelease>  \expandafter\ifx\csname#1\endcsname\relax
%<latexrelease>    \expandafter\@firstoftwo
%<latexrelease>  \else
%<latexrelease>    \expandafter\@secondoftwo
%<latexrelease>  \fi}
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
%
%
% \begin{macro}{\@qend}
% \begin{macro}{\@qrelax}
% The following define |\@qend| and |\@qrelax| to be the strings
% `|end|' and `|relax|' with the characters |\catcode|d 12.
%    \begin{macrocode}
\edef\@qend{\expandafter\@cdr\string\end\@nil}
\edef\@qrelax{\expandafter\@cdr\string\relax\@nil}
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
%
% \begin{macro}{\@ifnextchar}
% \changes{LaTeX209}{1992/08/24}
%     {(Rms) \cs{@ifnextchar} didn't work if its
%         first argument was an equal sign.}
% \changes{v1.2q}{1995/10/02}
%     {Use \cs{@let@token} }
% \changes{v1.3a}{1999/01/07}
%     {made long}
% \changes{v1.3b}{1999/01/07}
%     {extra \cs{long}. latex/2902}
% \changes{v1.3e}{1999/03/01}
%     {remove extra \cs{long}. internal/2967}
%  |\@ifnextchar| peeks at the following character and compares it
%  with its first argument. If both are the same it executes its
%  second argument, otherwise its third.
%    \begin{macrocode}
\long\def\@ifnextchar#1#2#3{%
  \let\reserved@d=#1%
  \def\reserved@a{#2}%
  \def\reserved@b{#3}%
  \futurelet\@let@token\@ifnch}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\kernel@ifnextchar}
% \changes{v1.3g}{2004/01/23}{Added macro (pr/3501)}
%    This macro is the kernel version of |\@ifnextchar| which is used
%    in a couple of places to prevent the AMS variant from being used
%    since in some places this produced chaos (for example
%    if an \texttt{fd} file
%    is loaded in a random place then the optional argument to
%    |\ProvidesFile| could get printed there instead of being written
%    only in the log file.  This happened
%    when there was a space or a newline between the mandatory and
%    optional arguments! It should really be fixed in the
%    \texttt{amsmath} package one day, but\ldots
%
%    Note that there may be other places in the kernel where this version
%    should be used rather than the original, but variable, version.
%
%    \begin{macrocode}
\let\kernel@ifnextchar\@ifnextchar
%    \end{macrocode}
% \end{macro}
%
%
%  \begin{macro}{\@ifnch}
%    |\@ifnch| is a tricky macro to skip any space tokens that may
%    appear before the character in question. If it encounters a space
%    token, it calls \@xifnch.
% \changes{v1.2q}{1995/10/02}
%     {Use \cs{@let@token} for internal/924, save \cs{reserved@e}}
%    \begin{macrocode}
\def\@ifnch{%
  \ifx\@let@token\@sptoken
    \let\reserved@c\@xifnch
  \else
    \ifx\@let@token\reserved@d
      \let\reserved@c\reserved@a
    \else
      \let\reserved@c\reserved@b
    \fi
  \fi
  \reserved@c}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@sptoken}
%    The following code makes |\@sptoken| a space token. It is
%    important here that the control sequence |\:| consists of
%    a non-letter only, so that the following whitespace is
%    significant. Together with the fact that the equal sign
%    in a |\let| may be followed by only one optional space
%    the desired effect is achieved.
%    NOTE: the following hacking must precede the definition of |\:|
%    as math medium space.
%    \begin{macrocode}
\def\:{\let\@sptoken= } \:  % this makes \@sptoken a space token
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@xifnch}
%    In the following definition of |\@xifnch|, |\:| is again used
%    to get a space token as delimiter into the definition.
%    \begin{macrocode}
\def\:{\@xifnch} \expandafter\def\: {\futurelet\@let@token\@ifnch}
%    \end{macrocode}
% \end{macro}
%
%
% \begin{macro}{\@ifstar}
% \changes{v1.2u}{1995/10/16}{(DPC) New implementation, for /1910}
% The new implementation below avoids passing the \meta{true code}
% Through one more |\def| than the \meta{false code}, which previously
% meant that |#| had to be written as |####| in one argument, but |##|
% in the other. The |*| is gobbled by |\@firstoftwo|.
%    \begin{macrocode}
\def\@ifstar#1{\@ifnextchar *{\@firstoftwo{#1}}}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@dblarg}
% \begin{macro}{\@xdblarg}
% \changes{v1.1g}{2004/01/23}{Use kernel version of
%                             \cs{@ifnextchar} (pr/3501)}
%    \begin{macrocode}
\long\def\@dblarg#1{\kernel@ifnextchar[{#1}{\@xdblarg{#1}}}
\long\def\@xdblarg#1#2{#1[{#2}]{#2}}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@sanitize}
% The command |\@sanitize| changes the catcode of all special characters
% except for braces to `other'.  It can be used for commands like
% |\index| that want to write their arguments verbatim.  Needless to
% say, this command should only be executed within a group, or chaos
% will ensue.
%
%    \begin{macrocode}
\def\@sanitize{\@makeother\ \@makeother\\\@makeother\$\@makeother\&%
\@makeother\#\@makeother\^\@makeother\_\@makeother\%\@makeother\~}
%    \end{macrocode}
% \end{macro}
%
%  \begin{macro}{\@onelevel@sanitize}
% \changes{v1.2c}{1994/10/30}{Macro added}
%
%    This makes the whole ``meaning'' of |#1| (its one-level
%    expansion) into catcode 12 tokens: it could be used in
%    |\DeclareRobustCommand|.
%
%    If it is to be used on default float specifiers, this should be
%    done when they are defined.
%    \begin{macrocode}
\def \@onelevel@sanitize #1{%
  \edef #1{\expandafter\strip@prefix
           \meaning #1}%
}
%    \end{macrocode}
%  \end{macro}
%
%
% \begin{macro}{\string@makeletter}
% \begin{macro}{\@string@makeletter}
% \begin{macro}{\char@if@alph}
%   Iterates through a string, turning each alphabetic character into
%   a catcode-11 token (partly undoes a \cs{detokenize}).  Useful for
%   \cs{ifx}-based string comparisons where \cs{detokenize}-ing the
%   other string would break too much code.
%
%   \changes{v1.5n}{2020/09/06}
%         {Macro added}
%
%   The macro uses \textsf{expl3}'s \cs{@expl@str@map@function@@NN} to
%   iterate on the string (without losing spaces) and applies
%   \cs{@string@makeletter} on each character.  The latter checks if
%   character is between a--z or A--Z, and uses \cs{@alph} or \cs{@Alph}
%   to get the corresponding catcode-11 token.  Other tokens are passed
%   through unchanged.
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020/10/01}{\string@makeletter}
%<latexrelease>  {Add \string@makeletter}%
%<*2ekernel|latexrelease>
\def\string@makeletter#1{%
  \@expl@str@map@function@@NN#1\@string@makeletter}
\def\@string@makeletter#1{%
  \char@if@alph{#1}%
    {\@expl@char@generate@@nn{`#1}{11}}%
    {#1}}
\def\char@if@alph#1{%
  \ifnum0\ifnum`#1<`A 1\fi\ifnum`#1>`z 1\fi
      \if\ifnum`#1>`Z @\fi\ifnum`#1<`a @\fi01\fi>0
    \expandafter\@secondoftwo
  \else
    \expandafter\@firstoftwo
  \fi}
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%
%<latexrelease>\IncludeInRelease{0000/00/00}{\string@makeletter}
%<latexrelease>  {Undefine \string@makeletter}%
%<latexrelease>\let\string@makeletter\@undefined
%<latexrelease>\let\@string@makeletter\@undefined
%<latexrelease>\let\char@if@alph\@undefined
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
%
% \begin{macro}{\makeatletter}
% \begin{macro}{\makeatother}
% \changes{v1.0n}{1994/05/10}{Added \cs{makeatletter} and
%           \cs{makeatother} ASAJ.}
%    Make internal control sequences accessible or inaccessible.
%    \begin{macrocode}
\DeclareRobustCommand\makeatletter{\catcode`\@11\relax}
\DeclareRobustCommand\makeatother{\catcode`\@12\relax}
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%
% \section{Discretionary Hyphenation}
% \begin{macro}{\-}
% \changes{1.2x}{1995/12/13}{Documentation changed.}
% \changes{v1.5a}{2017/03/13}{Define \cs{-} in terms of \cs{hyphenchar}}
% \begin{macro}{\@dischyph}
% \changes{v1.0g}{1994/04/12}
%         {Define \cs{@dischyph}, was previously in ltboxes.dtx}
% \changes{v1.5b}{2017/03/27}{Define \cs{@dischyph} after \cs{-}}
% \changes{v1.5j}{2020/05/11}{Do not overwrite \cs{-} under Lua\TeX}
% \end{macro}
% Moved here to be after the definition of |\DeclareRobustCommand|.
%
% The primitive |\-| command adds a discretionary hyphen using the
% current font's |\hyphenchar|. Monospace fonts are usually declared
% with |\hyphenchar| set to $-1$ to suppress hyphenation.
%
% \LaTeX, from \LaTeX2.09 in 1986 defined |\-| by
% \begin{verbatim}
% \def\-{\discretionary{-}{}{}}
% \end{verbatim}
%  The following comment was added when these commands were first set
%  up, 19 April 1986:
% \begin{quote}
%  the |\-| command is redefined to allow it to work in the |\ttfamily|
%  type style, where automatic hyphenation is suppressed by setting
%  |\hyphenchar| to~$-1$. The original primitive \TeX{} definition is
%  saved as |\@@hyph| just in case anyone needs it.
% \end{quote}
%
% \LaTeXe, between 1993 and 2017, had a comment at this point
% saying that the definition ``would probably change'' because
% the definition always uses |-|. The definition used below
% was given in comments at this point during time.
%
% In 2017 we finally enabled this definition by default, with the
% older \LaTeX\ definition accessible via \textsf{latexrelease}
% as usual.
%
% In Lua\LaTeX\ the primitive definition of \cs{-} is used directly
% because it's use of extended hyphenation parameters means that \cs{-}
% works correctly even with \cs{hyphenchar} set to $-1$. This change
% makes \cs{-} under Lua\LaTeX\ compatible with language specific
% hyphenation characters.
%
% Temporary definition of |\@latex@info|, final definition is later.
%    \begin{macrocode}
\def\@latex@info#1{}
%    \end{macrocode}
%
%    \begin{macrocode}
%</2ekernel>
%<latexrelease>\IncludeInRelease{2020/10/01}{\-}{Use primitive \- in Lua\LaTeX}%
%<*2ekernel|latexrelease>
\ifx\directlua\@undefined
  \DeclareRobustCommand{\-}{%
    \discretionary{%
      \char \ifnum\hyphenchar\font<\z@
              \defaulthyphenchar
            \else
              \hyphenchar\font
            \fi
                 }{}{}%
  }
\else
  \let\-\@@hyph
\fi
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{2017/04/15}{\-}{Use \hyphenchar in \-}%
%<latexrelease>\DeclareRobustCommand{\-}{%
%<latexrelease>  \discretionary{%
%<latexrelease>    \char \ifnum\hyphenchar\font<\z@
%<latexrelease>            \defaulthyphenchar
%<latexrelease>          \else
%<latexrelease>            \hyphenchar\font
%<latexrelease>          \fi
%<latexrelease>               }{}{}%
%<latexrelease>}
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000/00/00}{\-}{Use \hyphenchar in \-}%
%<latexrelease>\def\-{\discretionary{-}{}{}}
%<latexrelease>\EndIncludeInRelease
%    \end{macrocode}
%    
%    \begin{macrocode}
%<*2ekernel|latexrelease>
\let\@dischyph=\-
%</2ekernel|latexrelease>
%<*2ekernel>
%    \end{macrocode}
% \end{macro}
%
% Delayed from |ltvers.dtx|
%    \begin{macrocode}
\newif\if@includeinrelease
\@includeinreleasefalse
%    \end{macrocode}
%
% \changes{v1.5f}{2019/08/27}{Make various commands robust}
%
% Delayed from |ltplain.dtx|
%    \begin{macrocode}
%</2ekernel>
%<*2ekernel|latexrelease>
%<latexrelease>\IncludeInRelease{2019/10/01}%
%<latexrelease>                 {\allowbreak}{Make various commands robust}%
\MakeRobust\allowbreak
\MakeRobust\bigbreak
\MakeRobust\break
\MakeRobust\dotfill
\MakeRobust\frenchspacing
\MakeRobust\goodbreak
\MakeRobust\hrulefill
\MakeRobust\medbreak
\MakeRobust\nobreak
\MakeRobust\nonfrenchspacing
\MakeRobust\obeylines
\MakeRobust\obeyspaces
\MakeRobust\slash
\MakeRobust\smallbreak
\MakeRobust\strut
\MakeRobust\underbar
%</2ekernel|latexrelease>
%<latexrelease>\EndIncludeInRelease
%<latexrelease>\IncludeInRelease{0000/00/00}%
%<latexrelease>                 {\allowbreak}{Make various commands robust}%
%<latexrelease>
%<latexrelease>\kernel@make@fragile\allowbreak
%<latexrelease>\kernel@make@fragile\bigbreak
%<latexrelease>\kernel@make@fragile\break
%<latexrelease>\kernel@make@fragile\dotfill
%<latexrelease>\kernel@make@fragile\frenchspacing
%<latexrelease>\kernel@make@fragile\goodbreak
%<latexrelease>\kernel@make@fragile\hrulefill
%<latexrelease>\kernel@make@fragile\medbreak
%<latexrelease>\kernel@make@fragile\nobreak
%<latexrelease>\kernel@make@fragile\nonfrenchspacing
%<latexrelease>\kernel@make@fragile\obeylines
%<latexrelease>\kernel@make@fragile\obeyspaces
%<latexrelease>\kernel@make@fragile\slash
%<latexrelease>\kernel@make@fragile\smallbreak
%<latexrelease>\kernel@make@fragile\strut
%<latexrelease>\kernel@make@fragile\underbar
%<latexrelease>
%<latexrelease>\EndIncludeInRelease
%<*2ekernel>
%    \end{macrocode}
%
%
% \begin{macro}{\g@addto@macro}
%    Globally add to the end of a macro.
%    This macro is used by the kernel to add to its internal hooks.
% \changes{v0.2a}{1993/11/14}{Made global}
% \changes{v0.2w}{1994/01/31}
%     {Use toks register to avoid `hash' problems}
% \changes{v1.0o}{1995/05/17}
%     {Make long for latex/1522}
% \changes{v1.0w}{1996/12/17}
%     {Use \cs{begingroup} to save making a mathord}
% \changes{v1.0x}{1997/02/05}
%     {missing percent /2402}
%    \begin{macrocode}
\long\def\g@addto@macro#1#2{%
  \begingroup
    \toks@\expandafter{#1#2}%
    \xdef#1{\the\toks@}%
  \endgroup}
%    \end{macrocode}
% \end{macro}
%
%
%    \begin{macrocode}
%</2ekernel>
%    \end{macrocode}
%
%
% \Finale
%