summaryrefslogtreecommitdiff
path: root/Master/texmf-dist/source/latex/l3kernel/l3token.dtx
blob: 408f65c239760dd57f1ad6fabb57c45381bdb22a (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
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
% \iffalse meta-comment
%
%% File: l3token.dtx
%
% Copyright (C) 2005-2019 The LaTeX3 Project
%
% It may be distributed and/or modified under the conditions of the
% LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version.  The latest version
% of this license is in the file
%
%    https://www.latex-project.org/lppl.txt
%
% This file is part of the "l3kernel bundle" (The Work in LPPL)
% and all files in that bundle must be distributed together.
%
% -----------------------------------------------------------------------
%
% The development version of the bundle can be found at
%
%    https://github.com/latex3/latex3
%
% for those people who are interested.
%
%<*driver>
\documentclass[full,kernel]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \pkg{l3token} package\\ Token manipulation^^A
% }
%
% \author{^^A
%  The \LaTeX3 Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released 2019-10-27}
%
% \maketitle
%
% \begin{documentation}
%
% This module deals with tokens. Now this is perhaps not the most
% precise description so let's try with a better description: When
% programming in \TeX{}, it is often desirable to know just what a
% certain token is: is it a control sequence or something
% else. Similarly one often needs to know if a control sequence is
% expandable or not, a macro or a primitive, how many arguments it
% takes etc. Another thing of great importance (especially when it
% comes to document commands) is looking ahead in the token stream to
% see if a certain character is present and maybe even remove it or
% disregard other tokens while scanning. This module provides
% functions for both and as such has two primary function
% categories: |\token_| for anything that deals with tokens and
% |\peek_| for looking ahead in the token stream.
%
% Most functions we describe here can be used on control sequences,
% as those are tokens as well.
%
% It is important to distinguish two aspects of a token: its
% \enquote{shape} (for lack of a better word), which affects the
% matching of delimited arguments and the comparison of token lists
% containing this token, and its \enquote{meaning}, which affects
% whether the token expands or what operation it performs.  One can have
% tokens of different shapes with the same meaning, but not the
% converse.
%
% For instance, \cs{if:w}, \cs{if_charcode:w}, and \cs{tex_if:D} are
% three names for the same internal operation of \TeX{}, namely the
% primitive testing the next two characters for equality of their
% character code.  They have the same meaning hence behave identically
% in many situations.  However, \TeX{} distinguishes them when searching
% for a delimited argument.  Namely, the example function
% |\show_until_if:w| defined below takes everything until \cs{if:w}
% as an argument, despite the presence of other copies of \cs{if:w}
% under different names.
% \begin{verbatim}
% \cs_new:Npn \show_until_if:w #1 \if:w { \tl_show:n {#1} }
% \show_until_if:w \tex_if:D \if_charcode:w \if:w
% \end{verbatim}
% A list of all possible shapes and a list of all possible meanings are
% given in section~\ref{sec:l3token:all-tokens}.
%
% \section{Creating character tokens}
%
% \begin{function}[updated = 2015-11-12]
%   {
%     \char_set_active_eq:NN,  \char_set_active_eq:Nc,
%     \char_gset_active_eq:NN, \char_gset_active_eq:Nc
%   }
%   \begin{syntax}
%      \cs{char_set_active_eq:NN} \meta{char} \meta{function}
%   \end{syntax}
%   Sets the behaviour of the \meta{char} in situations where it is
%   active (category code $13$) to be equivalent to that of the
%   \meta{function}. The category code of the \meta{char} is
%   \emph{unchanged} by this process. The \meta{function} may itself
%   be an active character.
% \end{function}
%
% \begin{function}[added = 2015-11-12]
%   {
%     \char_set_active_eq:nN,  \char_set_active_eq:nc,
%     \char_gset_active_eq:nN, \char_gset_active_eq:nc
%   }
%   \begin{syntax}
%      \cs{char_set_active_eq:nN} \Arg{integer expression} \meta{function}
%   \end{syntax}
%   Sets the behaviour of the \meta{char} which has character
%   code as given by the \meta{integer expression} in situations
%   where it is active (category code $13$) to be equivalent to that of the
%   \meta{function}. The category code of the \meta{char} is
%   \emph{unchanged} by this process. The \meta{function} may itself
%   be an active character.
% \end{function}
%
% \begin{function}[EXP, added = 2015-09-09, updated = 2019-01-16]
%   {\char_generate:nn}
%   \begin{syntax}
%      \cs{char_generate:nn} \Arg{charcode} \Arg{catcode}
%   \end{syntax}
%   Generates a character token of the given \meta{charcode} and \meta{catcode}
%   (both of which may be integer expressions). The \meta{catcode} may be
%   one of
%   \begin{itemize}
%     \item $1$ (begin group)
%     \item $2$ (end group)
%     \item $3$ (math toggle)
%     \item $4$ (alignment)
%     \item $6$ (parameter)
%     \item $7$ (math superscript)
%     \item $8$ (math subscript)
%     \item $11$ (letter)
%     \item $12$ (other)
%     \item $13$ (active)
%   \end{itemize}
%   and other values raise an error. The \meta{charcode} may be any one valid
%   for the engine in use.
%   Active characters cannot be generated in older versions of \XeTeX{}.
%   \begin{texnote}
%     Exactly two expansions are needed to produce the character.
%   \end{texnote}
% \end{function}
%
% \begin{variable}[added = 2011-09-05]{\c_catcode_other_space_tl}
%   Token list containing one character with category code $12$,
%   (\enquote{other}), and character code $32$ (space).
% \end{variable}
%
% \section{Manipulating and interrogating character tokens}
%
% \begin{function}[updated = 2015-11-11]
%   {
%     \char_set_catcode_escape:N           ,
%     \char_set_catcode_group_begin:N      ,
%     \char_set_catcode_group_end:N        ,
%     \char_set_catcode_math_toggle:N      ,
%     \char_set_catcode_alignment:N        ,
%     \char_set_catcode_end_line:N         ,
%     \char_set_catcode_parameter:N        ,
%     \char_set_catcode_math_superscript:N ,
%     \char_set_catcode_math_subscript:N   ,
%     \char_set_catcode_ignore:N           ,
%     \char_set_catcode_space:N            ,
%     \char_set_catcode_letter:N           ,
%     \char_set_catcode_other:N            ,
%     \char_set_catcode_active:N           ,
%     \char_set_catcode_comment:N          ,
%     \char_set_catcode_invalid:N
%   }
%   \begin{syntax}
%     \cs{char_set_catcode_letter:N} \meta{character}
%   \end{syntax}
%   Sets the category code of the \meta{character} to that indicated in
%   the function name. Depending on the current category code of the
%   \meta{token} the escape token may also be needed:
%   \begin{verbatim}
%     \char_set_catcode_other:N \%
%   \end{verbatim}
%   The assignment is local.
% \end{function}
%
% \begin{function}[updated = 2015-11-11]
%   {
%     \char_set_catcode_escape:n           ,
%     \char_set_catcode_group_begin:n      ,
%     \char_set_catcode_group_end:n        ,
%     \char_set_catcode_math_toggle:n       ,
%     \char_set_catcode_alignment:n        ,
%     \char_set_catcode_end_line:n         ,
%     \char_set_catcode_parameter:n        ,
%     \char_set_catcode_math_superscript:n ,
%     \char_set_catcode_math_subscript:n   ,
%     \char_set_catcode_ignore:n           ,
%     \char_set_catcode_space:n            ,
%     \char_set_catcode_letter:n           ,
%     \char_set_catcode_other:n            ,
%     \char_set_catcode_active:n           ,
%     \char_set_catcode_comment:n          ,
%     \char_set_catcode_invalid:n
%   }
%   \begin{syntax}
%     \cs{char_set_catcode_letter:n} \Arg{integer expression}
%   \end{syntax}
%   Sets the category code of the \meta{character} which has character
%   code as given by the \meta{integer expression}. This version can be
%   used to set up characters which cannot otherwise be given
%   (\emph{cf.}~the \texttt{N}-type variants). The assignment is local.
% \end{function}
%
% \begin{function}[updated = 2015-11-11]{\char_set_catcode:nn}
%   \begin{syntax}
%     \cs{char_set_catcode:nn} \Arg{intexpr_1} \Arg{intexpr_2}
%   \end{syntax}
%   These functions set the category code of the \meta{character} which
%   has character code as given by the \meta{integer expression}.
%   The first \meta{integer expression}
%   is the character code and the second is the category code to apply.
%   The setting applies within the current \TeX{} group. In general, the
%   symbolic functions \cs[no-index]{char_set_catcode_\meta{type}} should be preferred,
%   but there are cases where these lower-level functions may be useful.
% \end{function}
%
% \begin{function}[EXP]{\char_value_catcode:n}
%   \begin{syntax}
%     \cs{char_value_catcode:n} \Arg{integer expression}
%   \end{syntax}
%   Expands to the current category code of the \meta{character} with
%   character code given by the
%   \meta{integer expression}.
% \end{function}
%
% \begin{function}{\char_show_value_catcode:n}
%   \begin{syntax}
%     \cs{char_show_value_catcode:n} \Arg{integer expression}
%   \end{syntax}
%   Displays the current category code of the \meta{character} with
%   character code given by the  \meta{integer expression} on the
%   terminal.
% \end{function}
%
% \begin{function}[updated = 2015-08-06]{\char_set_lccode:nn}
%   \begin{syntax}
%     \cs{char_set_lccode:nn} \Arg{intexpr_1} \Arg{intexpr_2}
%   \end{syntax}
%   Sets up the behaviour of the \meta{character} when
%   found inside \cs{tl_lower_case:n}, such that \meta{character_1}
%   will be converted into \meta{character_2}. The two \meta{characters}
%   may be specified using an \meta{integer expression} for the character code
%   concerned. This may include the \TeX{} |`|\meta{character}
%   method for converting a single character into its character
%   code:
%   \begin{verbatim}
%     \char_set_lccode:nn { `\A } { `\a } % Standard behaviour
%     \char_set_lccode:nn { `\A } { `\A + 32 }
%     \char_set_lccode:nn { 50 } { 60 }
%   \end{verbatim}
%   The setting applies within the current \TeX{} group.
% \end{function}
%
% \begin{function}[EXP]{\char_value_lccode:n}
%   \begin{syntax}
%     \cs{char_value_lccode:n} \Arg{integer expression}
%   \end{syntax}
%   Expands to the current lower case code of the \meta{character} with
%   character code given by the
%   \meta{integer expression}.
% \end{function}
%
% \begin{function}{\char_show_value_lccode:n}
%   \begin{syntax}
%     \cs{char_show_value_lccode:n} \Arg{integer expression}
%   \end{syntax}
%   Displays the current lower case code of the \meta{character} with
%   character code given by the  \meta{integer expression} on the
%   terminal.
% \end{function}
%
% \begin{function}[updated = 2015-08-06]{\char_set_uccode:nn}
%   \begin{syntax}
%     \cs{char_set_uccode:nn} \Arg{intexpr_1} \Arg{intexpr_2}
%   \end{syntax}
%   Sets up the behaviour of the \meta{character} when
%   found inside \cs{tl_upper_case:n}, such that \meta{character_1}
%   will be converted into \meta{character_2}. The two \meta{characters}
%   may be specified using an \meta{integer expression} for the character code
%   concerned. This may include the \TeX{} |`|\meta{character}
%   method for converting a single character into its character
%   code:
%   \begin{verbatim}
%     \char_set_uccode:nn { `\a } { `\A } % Standard behaviour
%     \char_set_uccode:nn { `\A } { `\A - 32 }
%     \char_set_uccode:nn { 60 } { 50 }
%   \end{verbatim}
%   The setting applies within the current \TeX{} group.
% \end{function}
%
% \begin{function}[EXP]{\char_value_uccode:n}
%   \begin{syntax}
%     \cs{char_value_uccode:n} \Arg{integer expression}
%   \end{syntax}
%   Expands to the current upper case code of the \meta{character} with
%   character code given by the
%   \meta{integer expression}.
% \end{function}
%
% \begin{function}{\char_show_value_uccode:n}
%   \begin{syntax}
%     \cs{char_show_value_uccode:n} \Arg{integer expression}
%   \end{syntax}
%   Displays the current upper case code of the \meta{character} with
%   character code given by the  \meta{integer expression} on the
%   terminal.
% \end{function}
%
% \begin{function}[updated = 2015-08-06]{\char_set_mathcode:nn}
%   \begin{syntax}
%     \cs{char_set_mathcode:nn} \Arg{intexpr_1} \Arg{intexpr_2}
%   \end{syntax}
%   This function sets up the math code of \meta{character}.
%   The \meta{character} is specified as
%   an \meta{integer expression} which will be used as the character
%   code of the relevant character. The setting applies within the
%   current \TeX{} group.
% \end{function}
%
% \begin{function}[EXP]{\char_value_mathcode:n}
%   \begin{syntax}
%     \cs{char_value_mathcode:n} \Arg{integer expression}
%   \end{syntax}
%   Expands to the current math code of the \meta{character} with
%   character code given by the
%   \meta{integer expression}.
% \end{function}
%
% \begin{function}{\char_show_value_mathcode:n}
%   \begin{syntax}
%     \cs{char_show_value_mathcode:n} \Arg{integer expression}
%   \end{syntax}
%   Displays the current math code of the \meta{character} with
%   character code given by the  \meta{integer expression} on the
%   terminal.
% \end{function}
%
% \begin{function}[updated = 2015-08-06]{\char_set_sfcode:nn}
%   \begin{syntax}
%     \cs{char_set_sfcode:nn} \Arg{intexpr_1} \Arg{intexpr_2}
%   \end{syntax}
%   This function sets up the space factor for the \meta{character}.
%   The \meta{character} is specified as
%   an \meta{integer expression} which will be used as the character
%   code of the relevant character. The setting applies within the
%   current \TeX{} group.
% \end{function}
%
% \begin{function}[EXP]{\char_value_sfcode:n}
%   \begin{syntax}
%     \cs{char_value_sfcode:n} \Arg{integer expression}
%   \end{syntax}
%   Expands to the current space factor for the \meta{character} with
%   character code given by the
%   \meta{integer expression}.
% \end{function}
%
% \begin{function}{\char_show_value_sfcode:n}
%   \begin{syntax}
%     \cs{char_show_value_sfcode:n} \Arg{integer expression}
%   \end{syntax}
%   Displays the current space factor for the \meta{character} with
%   character code given by the  \meta{integer expression} on the
%   terminal.
% \end{function}
%
% \begin{variable}[added = 2012-01-23, updated = 2015-11-11]{\l_char_active_seq}
%   Used to track which tokens may require special handling at the document
%   level as they are (or have been at some point)
%   of category \meta{active} (catcode~$13$). Each entry in
%   the sequence consists of a single escaped token, for example |\~|.
%   Active tokens should be added to the sequence when they are defined for
%   general document use.
% \end{variable}
%
% \begin{variable}[added = 2012-01-23, updated = 2015-11-11]{\l_char_special_seq}
%   Used to track which tokens will require special handling when working with
%   verbatim-like material at the document level as they are not of categories
%   \meta{letter} (catcode~$11$) or \meta{other} (catcode~$12$). Each entry in
%   the sequence consists of a single escaped token, for example |\\| for the
%   backslash or |\{| for an opening brace.^^A \}
%   Escaped tokens should be added to the sequence when they are defined for
%   general document use.
% \end{variable}
%
% \section{Generic tokens}
%
% \begin{variable}
%   {
%     \c_group_begin_token,
%     \c_group_end_token,
%     \c_math_toggle_token,
%     \c_alignment_token,
%     \c_parameter_token,
%     \c_math_superscript_token,
%     \c_math_subscript_token,
%     \c_space_token
%   }
%   These are implicit tokens which have the category code described
%   by their name. They are used internally for test purposes but
%   are also available to the programmer for other uses.
% \end{variable}
%
% \begin{variable}
%   {
%     \c_catcode_letter_token,
%     \c_catcode_other_token
%   }
%   These are implicit tokens which have the category code described
%   by their name. They are used internally for test purposes and should
%   not be used other than for category code tests.
% \end{variable}
%
% \begin{variable}{\c_catcode_active_tl}
%   A token list containing an active token. This is used internally
%   for test purposes and should not be used other than in
%   appropriately-constructed category code tests.
% \end{variable}
%
% \section{Converting tokens}
%
% \begin{function}[EXP]{\token_to_meaning:N, \token_to_meaning:c}
%   \begin{syntax}
%     \cs{token_to_meaning:N} \meta{token}
%   \end{syntax}
%   Inserts the current meaning of the \meta{token} into the input
%   stream as a series of characters of category code $12$ (other).
%   This is the primitive \TeX{} description of the \meta{token},
%   thus for example both functions defined by \cs{cs_set_nopar:Npn}
%   and token list variables defined using \cs{tl_new:N} are described
%   as |macro|s.
%   \begin{texnote}
%     This is the \TeX{} primitive \tn{meaning}.
%     The \meta{token} can thus be an explicit space tokens or an
%     explicit begin-group or end-group character token (|{|~or~|}| when
%     normal \TeX{} category codes apply) even though these are not
%     valid \texttt{N}-type arguments.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP]{\token_to_str:N, \token_to_str:c}
%   \begin{syntax}
%     \cs{token_to_str:N} \meta{token}
%   \end{syntax}
%   Converts the given \meta{token} into a series of characters with
%   category code $12$ (other). If the \meta{token} is a control
%   sequence, this will start with the current escape character with
%   category code $12$ (the escape character is part of the
%   \meta{token}). This function requires only a single expansion.
%   \begin{texnote}
%     \cs{token_to_str:N} is the \TeX{} primitive \tn{string} renamed.
%     The \meta{token} can thus be an explicit space tokens or an
%     explicit begin-group or end-group character token (|{|~or~|}| when
%     normal \TeX{} category codes apply) even though these are not
%     valid \texttt{N}-type arguments.
%   \end{texnote}
% \end{function}
%
% \section{Token conditionals}
%
% \begin{function}[EXP,pTF]{\token_if_group_begin:N}
%   \begin{syntax}
%     \cs{token_if_group_begin_p:N} \meta{token} \\
%     \cs{token_if_group_begin:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a begin group token
%   (|{| when normal \TeX{} category codes are in ^^A }
%   force).
%   Note that an explicit begin group token cannot be tested in this way,
%   as it is not a valid \texttt{N}-type argument.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_group_end:N}
%   \begin{syntax}
%     \cs{token_if_group_end_p:N} \meta{token} \\
%     \cs{token_if_group_end:NTF} \meta{token} \Arg{true code} \Arg{false code}
%    \end{syntax}
%   Tests if \meta{token} has the category code of an end group token
%   (^^A {
%   |}| when normal \TeX{} category codes are in force).
%   Note that an explicit end group token cannot be tested in this way,
%   as it is not a valid \texttt{N}-type argument.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_math_toggle:N}
%   \begin{syntax}
%     \cs{token_if_math_toggle_p:N} \meta{token} \\
%     \cs{token_if_math_toggle:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a math shift token
%   (|$| when normal \TeX{} category codes are in force).
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_alignment:N}
%   \begin{syntax}
%     \cs{token_if_alignment_p:N} \meta{token} \\
%     \cs{token_if_alignment:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of an alignment token
%   (|&| when normal \TeX{} category codes are in force).
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_parameter:N}
%   \begin{syntax}
%     \cs{token_if_parameter_p:N} \meta{token} \\
%     \cs{token_if_alignment:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a macro parameter token
%   (|#| when normal \TeX{} category codes are in force).
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_math_superscript:N}
%   \begin{syntax}
%     \cs{token_if_math_superscript_p:N} \meta{token} \\
%     \cs{token_if_math_superscript:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a superscript token
%   (|^| when normal \TeX{} category codes are in force).
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_math_subscript:N}
%   \begin{syntax}
%     \cs{token_if_math_subscript_p:N} \meta{token} \\
%     \cs{token_if_math_subscript:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a subscript token
%   (|_| when normal \TeX{} category codes are in force).
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_space:N}
%   \begin{syntax}
%     \cs{token_if_space_p:N} \meta{token} \\
%     \cs{token_if_space:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a space token.
%   Note that an explicit space token with character code $32$ cannot
%   be tested in this way, as it is not a valid \texttt{N}-type argument.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_letter:N}
%   \begin{syntax}
%     \cs{token_if_letter_p:N} \meta{token} \\
%     \cs{token_if_letter:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of a letter token.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_other:N}
%   \begin{syntax}
%     \cs{token_if_other_p:N} \meta{token} \\
%     \cs{token_if_other:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of an \enquote{other}
%   token.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_active:N}
%   \begin{syntax}
%     \cs{token_if_active_p:N} \meta{token} \\
%     \cs{token_if_active:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if \meta{token} has the category code of an active character.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_eq_catcode:NN}
%   \begin{syntax}
%     \cs{token_if_eq_catcode_p:NN} \meta{token_1} \meta{token_2} \\
%     \cs{token_if_eq_catcode:NNTF} \meta{token_1} \meta{token_2} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the two \meta{tokens} have the same category code.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_eq_charcode:NN}
%   \begin{syntax}
%     \cs{token_if_eq_charcode_p:NN} \meta{token_1} \meta{token_2} \\
%     \cs{token_if_eq_charcode:NNTF} \meta{token_1} \meta{token_2} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the two \meta{tokens} have the same character code.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_eq_meaning:NN}
%   \begin{syntax}
%     \cs{token_if_eq_meaning_p:NN} \meta{token_1} \meta{token_2} \\
%     \cs{token_if_eq_meaning:NNTF} \meta{token_1} \meta{token_2} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the two \meta{tokens} have the same meaning when expanded.
% \end{function}
%
% \begin{function}[updated = 2011-05-23, EXP,pTF]{\token_if_macro:N}
%   \begin{syntax}
%     \cs{token_if_macro_p:N} \meta{token} \\
%     \cs{token_if_macro:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is a \TeX{} macro.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_cs:N}
%   \begin{syntax}
%     \cs{token_if_cs_p:N} \meta{token} \\
%     \cs{token_if_cs:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is a control sequence.
% \end{function}
%
% \begin{function}[EXP,pTF]{\token_if_expandable:N}
%   \begin{syntax}
%     \cs{token_if_expandable_p:N} \meta{token} \\
%     \cs{token_if_expandable:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is expandable. This test returns \meta{false}
%   for an undefined token.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_long_macro:N}
%   \begin{syntax}
%     \cs{token_if_long_macro_p:N} \meta{token} \\
%     \cs{token_if_long_macro:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is a long macro.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_protected_macro:N}
%   \begin{syntax}
%     \cs{token_if_protected_macro_p:N} \meta{token} \\
%     \cs{token_if_protected_macro:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is a protected macro: for a macro which
%   is both protected and long this returns \texttt{false}.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_protected_long_macro:N}
%   \begin{syntax}
%     \cs{token_if_protected_long_macro_p:N} \meta{token} \\
%     \cs{token_if_protected_long_macro:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is a protected long macro.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_chardef:N}
%   \begin{syntax}
%     \cs{token_if_chardef_p:N} \meta{token} \\
%     \cs{token_if_chardef:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a chardef.
%   \begin{texnote}
%     Booleans, boxes and small integer constants are implemented as
%     \tn{chardef}s.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_mathchardef:N}
%   \begin{syntax}
%     \cs{token_if_mathchardef_p:N} \meta{token} \\
%     \cs{token_if_mathchardef:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a mathchardef.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_dim_register:N}
%   \begin{syntax}
%     \cs{token_if_dim_register_p:N} \meta{token} \\
%     \cs{token_if_dim_register:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a dimension register.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_int_register:N}
%   \begin{syntax}
%     \cs{token_if_int_register_p:N} \meta{token} \\
%     \cs{token_if_int_register:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a integer register.
%   \begin{texnote}
%     Constant integers may be implemented as integer registers,
%     \tn{chardef}s, or \tn{mathchardef}s depending on their value.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP,pTF, added=2012-02-15]{\token_if_muskip_register:N}
%   \begin{syntax}
%     \cs{token_if_muskip_register_p:N} \meta{token} \\
%     \cs{token_if_muskip_register:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a muskip register.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_skip_register:N}
%   \begin{syntax}
%     \cs{token_if_skip_register_p:N} \meta{token} \\
%     \cs{token_if_skip_register:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a skip register.
% \end{function}
%
% \begin{function}[EXP,pTF, updated=2012-01-20]{\token_if_toks_register:N}
%   \begin{syntax}
%     \cs{token_if_toks_register_p:N} \meta{token} \\
%     \cs{token_if_toks_register:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is defined to be a toks register
%   (not used by \LaTeX3).
% \end{function}
%
% \begin{function}[updated = 2011-05-23, EXP,pTF]{\token_if_primitive:N}
%   \begin{syntax}
%     \cs{token_if_primitive_p:N} \meta{token} \\
%     \cs{token_if_primitive:NTF} \meta{token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{token} is an engine primitive.
% \end{function}
%
% \section{Peeking ahead at the next token}
%
% There is often a need to look ahead at the next token in the input
% stream while leaving it in place. This is handled using the
% \enquote{peek} functions. The generic \cs{peek_after:Nw} is
% provided along with a family of predefined tests for common cases.
% As peeking ahead does \emph{not} skip spaces the predefined tests
% include both a space-respecting and space-skipping version.
%
% \begin{function}{\peek_after:Nw}
%   \begin{syntax}
%     \cs{peek_after:Nw} \meta{function} \meta{token}
%   \end{syntax}
%   Locally sets the test variable \cs{l_peek_token} equal to \meta{token}
%   (as an implicit token, \emph{not} as a token list), and then
%   expands the \meta{function}. The \meta{token} remains in
%   the input stream as the next item after the \meta{function}.
%   The \meta{token} here may be \verb*| |, |{| or |}| (assuming
%   normal \TeX{} category codes), \emph{i.e.}~it is not necessarily the
%   next argument which would be grabbed by a normal function.
% \end{function}
%
% \begin{function}{\peek_gafter:Nw}
%   \begin{syntax}
%     \cs{peek_gafter:Nw} \meta{function} \meta{token}
%   \end{syntax}
%   Globally sets the test variable \cs{g_peek_token} equal to \meta{token}
%   (as an implicit token, \emph{not} as a token list), and then
%   expands the \meta{function}. The \meta{token} remains in
%   the input stream as the next item after the \meta{function}.
%   The \meta{token} here may be \verb*| |, |{| or |}| (assuming
%   normal \TeX{} category codes), \emph{i.e.}~it is not necessarily the
%   next argument which would be grabbed by a normal function.
% \end{function}
%
% \begin{variable}{\l_peek_token}
%  Token set by \cs{peek_after:Nw} and available for testing
%  as described above.
% \end{variable}
%
% \begin{variable}{\g_peek_token}
%  Token set by \cs{peek_gafter:Nw} and available for testing
%  as described above.
% \end{variable}
%
% \begin{function}[updated = 2012-12-20, TF]{\peek_catcode:N}
%   \begin{syntax}
%     \cs{peek_catcode:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream has the same
%   category code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_catcode:NNTF}). Spaces are respected by the test
%   and the \meta{token} is left in the input stream after
%   the \meta{true code} or \meta{false code} (as appropriate to the
%   result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]{\peek_catcode_ignore_spaces:N}
%   \begin{syntax}
%     \cs{peek_catcode_ignore_spaces:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next non-space \meta{token} in the input stream has the
%   same category code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_catcode:NNTF}).  Explicit and implicit space tokens
%   (with character code 32 and category code 10) are ignored and
%   removed by the test and the \meta{token} is left in the input
%   stream after the \meta{true code} or \meta{false code} (as
%   appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]{\peek_catcode_remove:N}
%   \begin{syntax}
%     \cs{peek_catcode_remove:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream has the same
%   category code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_catcode:NNTF}). Spaces are respected by the test
%   and the \meta{token} is removed from the input stream if the
%   test is true. The function then places either the
%   \meta{true code} or \meta{false code} in the input stream (as
%   appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]
%   {\peek_catcode_remove_ignore_spaces:N}
%   \begin{syntax}
%     \cs{peek_catcode_remove_ignore_spaces:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next non-space \meta{token} in the input stream has the
%   same category code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_catcode:NNTF}).  Explicit and implicit space tokens
%   (with character code 32 and category code 10) are ignored and
%   removed by the test and the \meta{token} is removed from the
%   input stream if the test is true. The function then places
%   either the \meta{true code} or \meta{false code} in the input stream
%   (as appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]{\peek_charcode:N}
%   \begin{syntax}
%     \cs{peek_charcode:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream has the same
%   character code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_charcode:NNTF}). Spaces are respected by the test
%   and the \meta{token} is left in the input stream after
%   the \meta{true code} or \meta{false code} (as appropriate to the
%   result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]{\peek_charcode_ignore_spaces:N}
%   \begin{syntax}
%     \cs{peek_charcode_ignore_spaces:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next non-space \meta{token} in the input stream has the
%   same character code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_charcode:NNTF}).  Explicit and implicit space tokens
%   (with character code 32 and category code 10) are ignored and removed by
%   the test and the \meta{token} is left in the input stream after
%   the \meta{true code} or \meta{false code} (as appropriate to the
%   result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]{\peek_charcode_remove:N}
%   \begin{syntax}
%     \cs{peek_charcode_remove:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream has the same
%   character code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_charcode:NNTF}). Spaces are respected by the test
%   and the \meta{token} is removed from the input stream if the
%   test is true. The function then places either the
%   \meta{true code} or \meta{false code} in the input stream (as
%   appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-20, TF]
%   {\peek_charcode_remove_ignore_spaces:N}
%   \begin{syntax}
%     \cs{peek_charcode_remove_ignore_spaces:NTF} \meta{test token}
%     ~~\Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next non-space \meta{token} in the input stream has the
%   same character code as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_charcode:NNTF}).  Explicit and implicit space tokens
%   (with character code 32 and category code 10) are ignored and
%   removed by the test and the \meta{token} is removed from the
%   input stream if the test is true. The function then places
%   either the \meta{true code} or \meta{false code} in the input stream
%   (as appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2011-07-02, TF]{\peek_meaning:N}
%   \begin{syntax}
%     \cs{peek_meaning:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream has the same
%   meaning as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_meaning:NNTF}). Spaces are respected by the test
%   and the \meta{token} is left in the input stream after
%   the \meta{true code} or \meta{false code} (as appropriate to the
%   result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-05, TF]{\peek_meaning_ignore_spaces:N}
%   \begin{syntax}
%     \cs{peek_meaning_ignore_spaces:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next non-space \meta{token} in the input stream has the
%   same meaning as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_meaning:NNTF}).  Explicit and implicit space tokens
%   (with character code 32 and category code 10) are ignored and
%   removed by the test and the \meta{token} is left in the input
%   stream after the \meta{true code} or \meta{false code} (as
%   appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2011-07-02, TF]{\peek_meaning_remove:N}
%   \begin{syntax}
%     \cs{peek_meaning_remove:NTF} \meta{test token} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream has the same
%   meaning as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_meaning:NNTF}). Spaces are respected by the test
%   and the \meta{token} is removed from the input stream if the
%   test is true. The function then places either the
%   \meta{true code} or \meta{false code} in the input stream (as
%   appropriate to the result of the test).
% \end{function}
%
% \begin{function}[updated = 2012-12-05, TF]
%   {\peek_meaning_remove_ignore_spaces:N}
%   \begin{syntax}
%     \cs{peek_meaning_remove_ignore_spaces:NTF} \meta{test token}
%     ~~\Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next non-space \meta{token} in the input stream has the
%   same meaning as the \meta{test token} (as defined by the test
%   \cs{token_if_eq_meaning:NNTF}).  Explicit and implicit space tokens
%   (with character code 32 and category code 10) are ignored and
%   removed by the test and the \meta{token} is removed from the
%   input stream if the test is true. The function then places
%   either the \meta{true code} or \meta{false code} in the input stream
%   (as appropriate to the result of the test).
% \end{function}
%
% \begin{function}[TF, updated = 2012-12-20]{\peek_N_type:}
%   \begin{syntax}
%     \cs{peek_N_type:TF} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the next \meta{token} in the input stream can be safely
%   grabbed as an \texttt{N}-type argument. The test is \meta{false}
%   if the next \meta{token} is either an explicit or implicit
%   begin-group or end-group token (with any character code), or
%   an explicit or implicit space character (with character code $32$
%   and category code $10$), or an outer token (never used in \LaTeX3)
%   and \meta{true} in all other cases.
%   Note that a \meta{true} result ensures that the next \meta{token} is
%   a valid \texttt{N}-type argument. However, if the next \meta{token}
%   is for instance \cs{c_space_token}, the test takes the
%   \meta{false} branch, even though the next \meta{token} is in fact
%   a valid \texttt{N}-type argument. The \meta{token} is left
%   in the input stream after the \meta{true code} or \meta{false code}
%   (as appropriate to the result of the test).
% \end{function}
%
% \section{Description of all possible tokens}
% \label{sec:l3token:all-tokens}
%
% Let us end by reviewing every case that a given token can fall into.
% This section is quite technical and some details are only meant for
% completeness.  We distinguish the meaning of the token, which controls
% the expansion of the token and its effect on \TeX{}'s state, and its
% shape, which is used when comparing token lists such as for delimited
% arguments.  Two tokens of the same shape must have the same meaning,
% but the converse does not hold.
%
% A token has one of the following shapes.
% \begin{itemize}
%   \item A control sequence, characterized by the sequence of
%     characters that constitute its name: for instance, \cs{use:n} is a
%     five-letter control sequence.
%   \item An active character token, characterized by its character code
%     (between $0$ and $1114111$ for \LuaTeX{} and \XeTeX{} and less for
%     other engines) and category code~$13$.
%   \item A character token, characterized by its character code and
%     category code (one of $1$, $2$, $3$, $4$, $6$, $7$, $8$, $10$,
%     $11$ or~$12$ whose meaning is described below).\footnote{In
%     \LuaTeX{}, there is also the case of \enquote{bytes}, which behave as
%     character tokens of category code $12$~(other) and character code
%     between $1114112$ and~$1114366$.  They are used to output
%     individual bytes to files, rather than UTF-8.}
% \end{itemize}
% There are also a few internal tokens.  The following list may be
% incomplete in some engines.
% \begin{itemize}
%   \item Expanding \tn{the}\tn{font} results in a token that looks
%     identical to the command that was used to select the current font
%     (such as \tn{tenrm}) but it differs from it in shape.
%   \item A \enquote{frozen} |\relax|, which differs from the primitive in
%     shape (but has the same meaning), is inserted when the closing \tn{fi} of a
%     conditional is encountered before the conditional is evaluated.
%   \item Expanding \tn{noexpand} \meta{token} (when the \meta{token} is
%     expandable) results in an internal token, displayed (temporarily)
%     as \cs{notexpanded: \meta{token}}, whose shape coincides with the
%     \meta{token} and whose meaning differs from \tn{relax}.
%   \item An |\outer endtemplate:| can be encountered when peeking ahead
%     at the next token; this expands to another internal token,
%     |end of alignment template|.
%   \item Tricky programming might access a frozen |\endwrite|.
%   \item Some frozen tokens can only be accessed in interactive
%     sessions: |\cr|, |\right|, |\endgroup|, |\fi|, |\inaccessible|.
% \end{itemize}
%
% The meaning of a (non-active) character token is fixed by its category
% code (and character code) and cannot be changed.  We call these
% tokens \emph{explicit} character tokens.  Category codes that a
% character token can have are listed below by giving a sample output of
% the \TeX{} primitive \tn{meaning}, together with their \LaTeX3 names
% and most common example:
% \begin{itemize}
%   \item[1] begin-group character (|group_begin|, often |{|),
%   \item[2] end-group character (|group_end|, often |}|),
%   \item[3] math shift character (|math_toggle|, often |$|),
%   \item[4] alignment tab character (|alignment|, often |&|),
%   \item[6] macro parameter character (|parameter|, often |#|),
%   \item[7] superscript character (|math_superscript|, often |^|),
%   \item[8] subscript character (|math_subscript|, often |_|),
%   \item[10] blank space (|space|, often character code~$32$),
%   \item[11] the letter (|letter|, such as |A|),
%   \item[12] the character (|other|, such as |0|).
% \end{itemize}
% Category code~$13$ (|active|) is discussed below.  Input characters
% can also have several other category codes which do not lead to
% character tokens for later processing: $0$~(|escape|),
% $5$~(|end_line|), $9$~(|ignore|), $14$~(|comment|), and
% $15$~(|invalid|).
%
% The meaning of a control sequence or active character can be identical
% to that of any character token listed above (with any character code),
% and we call such tokens \emph{implicit} character tokens.  The
% meaning is otherwise in the following list:
% \begin{itemize}
%   \item a macro, used in \LaTeX3 for most functions and some variables
%     (|tl|, |fp|, |seq|, \ldots{}),
%   \item a primitive such as \tn{def} or \tn{topmark}, used in \LaTeX3
%     for some functions,
%   \item a register such as \tn{count}|123|, used in \LaTeX3{} for the
%     implementation of some variables (|int|, |dim|, \ldots{}),
%   \item a constant integer such as \tn{char}|"56| or \tn{mathchar}|"121|,
%   \item a font selection command,
%   \item undefined.
% \end{itemize}
% Macros be \tn{protected} or not, \tn{long} or not (the opposite of
% what \LaTeX3 calls |nopar|), and \tn{outer} or not (unused in
% \LaTeX3).  Their \tn{meaning} takes the form
% \begin{quote}
%   \meta{properties} |macro:|\meta{parameters}|->|\meta{replacement}
% \end{quote}
% where \meta{properties} is among \tn{protected}\tn{long}\tn{outer},
% \meta{parameters} describes parameters that the macro expects, such as
% |#1#2#3|, and \meta{replacement} describes how the parameters are
% manipulated, such as~|#2/#1/#3|.
%
% ^^A todo Bruno: discuss here some other subtleties of space tokens? when looking for numbers, when looking for equal signs in let, in expressions, etc.
%
% Now is perhaps a good time to mention some subtleties relating to
% tokens with category code $10$ (space).  Any input character with this
% category code (normally, space and tab characters) becomes a normal
% space, with character code~$32$ and category code~$10$.
%
% When a macro takes an undelimited argument, explicit space characters
% (with character code $32$ and category code $10$) are ignored.  If the
% following token is an explicit character token with category code $1$
% (begin-group) and an arbitrary character code, then \TeX{} scans ahead
% to obtain an equal number of explicit character tokens with category
% code $1$ (begin-group) and $2$ (end-group), and the resulting list of
% tokens (with outer braces removed) becomes the argument.  Otherwise, a
% single token is taken as the argument for the macro: we call such
% single tokens \enquote{N-type}, as they are suitable to be used as an
% argument for a function with the signature~\texttt{:N}.
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3token} implementation}
%
%    \begin{macrocode}
%<*initex|package>
%    \end{macrocode}
%
%    \begin{macrocode}
%<@@=char>
%    \end{macrocode}
%
% \subsection{Manipulating and interrogating character tokens}
%
% \begin{macro}{\char_set_catcode:nn}
% \begin{macro}{\char_value_catcode:n}
% \begin{macro}{\char_show_value_catcode:n}
%   Simple wrappers around the primitives.
%    \begin{macrocode}
\cs_new_protected:Npn \char_set_catcode:nn #1#2
  { \tex_catcode:D \int_eval:n {#1} = \int_eval:n {#2} \exp_stop_f: }
\cs_new:Npn \char_value_catcode:n #1
  { \tex_the:D \tex_catcode:D \int_eval:n {#1} \exp_stop_f: }
\cs_new_protected:Npn \char_show_value_catcode:n #1
  { \exp_args:Nf \tl_show:n { \char_value_catcode:n {#1} } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \char_set_catcode_escape:N           ,
%     \char_set_catcode_group_begin:N      ,
%     \char_set_catcode_group_end:N        ,
%     \char_set_catcode_math_toggle:N      ,
%     \char_set_catcode_alignment:N        ,
%     \char_set_catcode_end_line:N         ,
%     \char_set_catcode_parameter:N        ,
%     \char_set_catcode_math_superscript:N ,
%     \char_set_catcode_math_subscript:N   ,
%     \char_set_catcode_ignore:N           ,
%     \char_set_catcode_space:N            ,
%     \char_set_catcode_letter:N           ,
%     \char_set_catcode_other:N            ,
%     \char_set_catcode_active:N           ,
%     \char_set_catcode_comment:N          ,
%     \char_set_catcode_invalid:N
%   }
%    \begin{macrocode}
\cs_new_protected:Npn \char_set_catcode_escape:N #1
  { \char_set_catcode:nn { `#1 } { 0 } }
\cs_new_protected:Npn \char_set_catcode_group_begin:N #1
  { \char_set_catcode:nn { `#1 } { 1 } }
\cs_new_protected:Npn \char_set_catcode_group_end:N #1
  { \char_set_catcode:nn { `#1 } { 2 } }
\cs_new_protected:Npn \char_set_catcode_math_toggle:N #1
  { \char_set_catcode:nn { `#1 } { 3 } }
\cs_new_protected:Npn \char_set_catcode_alignment:N #1
  { \char_set_catcode:nn { `#1 } { 4 } }
\cs_new_protected:Npn \char_set_catcode_end_line:N #1
  { \char_set_catcode:nn { `#1 } { 5 } }
\cs_new_protected:Npn \char_set_catcode_parameter:N #1
  { \char_set_catcode:nn { `#1 } { 6 } }
\cs_new_protected:Npn \char_set_catcode_math_superscript:N #1
  { \char_set_catcode:nn { `#1 } { 7 } }
\cs_new_protected:Npn \char_set_catcode_math_subscript:N #1
  { \char_set_catcode:nn { `#1 } { 8 } }
\cs_new_protected:Npn \char_set_catcode_ignore:N #1
  { \char_set_catcode:nn { `#1 } { 9 } }
\cs_new_protected:Npn \char_set_catcode_space:N #1
  { \char_set_catcode:nn { `#1 } { 10 } }
\cs_new_protected:Npn \char_set_catcode_letter:N #1
  { \char_set_catcode:nn { `#1 } { 11 } }
\cs_new_protected:Npn \char_set_catcode_other:N #1
  { \char_set_catcode:nn { `#1 } { 12 } }
\cs_new_protected:Npn \char_set_catcode_active:N #1
  { \char_set_catcode:nn { `#1 } { 13 } }
\cs_new_protected:Npn \char_set_catcode_comment:N #1
  { \char_set_catcode:nn { `#1 } { 14 } }
\cs_new_protected:Npn \char_set_catcode_invalid:N #1
  { \char_set_catcode:nn { `#1 } { 15 } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \char_set_catcode_escape:n           ,
%     \char_set_catcode_group_begin:n      ,
%     \char_set_catcode_group_end:n        ,
%     \char_set_catcode_math_toggle:n      ,
%     \char_set_catcode_alignment:n        ,
%     \char_set_catcode_end_line:n         ,
%     \char_set_catcode_parameter:n        ,
%     \char_set_catcode_math_superscript:n ,
%     \char_set_catcode_math_subscript:n   ,
%     \char_set_catcode_ignore:n           ,
%     \char_set_catcode_space:n            ,
%     \char_set_catcode_letter:n           ,
%     \char_set_catcode_other:n            ,
%     \char_set_catcode_active:n           ,
%     \char_set_catcode_comment:n          ,
%     \char_set_catcode_invalid:n
%   }
%    \begin{macrocode}
\cs_new_protected:Npn \char_set_catcode_escape:n #1
  { \char_set_catcode:nn {#1} { 0 } }
\cs_new_protected:Npn \char_set_catcode_group_begin:n #1
  { \char_set_catcode:nn {#1} { 1 } }
\cs_new_protected:Npn \char_set_catcode_group_end:n #1
  { \char_set_catcode:nn {#1} { 2 } }
\cs_new_protected:Npn \char_set_catcode_math_toggle:n #1
  { \char_set_catcode:nn {#1} { 3 } }
\cs_new_protected:Npn \char_set_catcode_alignment:n #1
  { \char_set_catcode:nn {#1} { 4 } }
\cs_new_protected:Npn \char_set_catcode_end_line:n #1
  { \char_set_catcode:nn {#1} { 5 } }
\cs_new_protected:Npn \char_set_catcode_parameter:n #1
  { \char_set_catcode:nn {#1} { 6 } }
\cs_new_protected:Npn \char_set_catcode_math_superscript:n #1
  { \char_set_catcode:nn {#1} { 7 } }
\cs_new_protected:Npn \char_set_catcode_math_subscript:n #1
  { \char_set_catcode:nn {#1} { 8 } }
\cs_new_protected:Npn \char_set_catcode_ignore:n #1
  { \char_set_catcode:nn {#1} { 9 } }
\cs_new_protected:Npn \char_set_catcode_space:n #1
  { \char_set_catcode:nn {#1} { 10 } }
\cs_new_protected:Npn \char_set_catcode_letter:n #1
  { \char_set_catcode:nn {#1} { 11 } }
\cs_new_protected:Npn \char_set_catcode_other:n #1
  { \char_set_catcode:nn {#1} { 12 } }
\cs_new_protected:Npn \char_set_catcode_active:n #1
  { \char_set_catcode:nn {#1} { 13 } }
\cs_new_protected:Npn \char_set_catcode_comment:n #1
  { \char_set_catcode:nn {#1} { 14 } }
\cs_new_protected:Npn \char_set_catcode_invalid:n #1
  { \char_set_catcode:nn {#1} { 15 } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\char_set_mathcode:nn}
% \begin{macro}{\char_value_mathcode:n}
% \begin{macro}{\char_show_value_mathcode:n}
% \begin{macro}{\char_set_lccode:nn}
% \begin{macro}{\char_value_lccode:n}
% \begin{macro}{\char_show_value_lccode:n}
% \begin{macro}{\char_set_uccode:nn}
% \begin{macro}{\char_value_uccode:n}
% \begin{macro}{\char_show_value_uccode:n}
% \begin{macro}{\char_set_sfcode:nn}
% \begin{macro}{\char_value_sfcode:n}
% \begin{macro}{\char_show_value_sfcode:n}
%   Pretty repetitive, but necessary!
%    \begin{macrocode}
\cs_new_protected:Npn \char_set_mathcode:nn #1#2
  { \tex_mathcode:D \int_eval:n {#1} = \int_eval:n {#2} \exp_stop_f: }
\cs_new:Npn \char_value_mathcode:n #1
  { \tex_the:D \tex_mathcode:D \int_eval:n {#1} \exp_stop_f: }
\cs_new_protected:Npn \char_show_value_mathcode:n #1
  { \exp_args:Nf \tl_show:n { \char_value_mathcode:n {#1} } }
\cs_new_protected:Npn \char_set_lccode:nn #1#2
  { \tex_lccode:D \int_eval:n {#1} = \int_eval:n {#2} \exp_stop_f: }
\cs_new:Npn \char_value_lccode:n #1
  { \tex_the:D \tex_lccode:D \int_eval:n {#1} \exp_stop_f: }
\cs_new_protected:Npn \char_show_value_lccode:n #1
  { \exp_args:Nf \tl_show:n { \char_value_lccode:n {#1} } }
\cs_new_protected:Npn \char_set_uccode:nn #1#2
  { \tex_uccode:D \int_eval:n {#1} = \int_eval:n {#2} \exp_stop_f: }
\cs_new:Npn \char_value_uccode:n #1
  { \tex_the:D \tex_uccode:D \int_eval:n {#1} \exp_stop_f: }
\cs_new_protected:Npn \char_show_value_uccode:n #1
  { \exp_args:Nf \tl_show:n { \char_value_uccode:n {#1} } }
\cs_new_protected:Npn \char_set_sfcode:nn #1#2
  { \tex_sfcode:D \int_eval:n {#1} = \int_eval:n {#2} \exp_stop_f: }
\cs_new:Npn \char_value_sfcode:n #1
  { \tex_the:D \tex_sfcode:D \int_eval:n {#1} \exp_stop_f: }
\cs_new_protected:Npn \char_show_value_sfcode:n #1
  { \exp_args:Nf \tl_show:n { \char_value_sfcode:n {#1} } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{variable}{\l_char_active_seq, \l_char_special_seq}
%   Two sequences for dealing with special characters. The first is characters
%   which may be active, the second longer list is for \enquote{special}
%   characters more generally. Both lists are escaped so that for example
%   bulk code assignments can be carried out. In both cases, the order is
%   by \textsc{ascii} character code (as is done in for example
%   \cs{ExplSyntaxOn}).
%    \begin{macrocode}
\seq_new:N \l_char_special_seq
\seq_set_split:Nnn \l_char_special_seq { }
  { \  \" \# \$ \% \& \\ \^ \_ \{ \} \~ }
\seq_new:N \l_char_active_seq
\seq_set_split:Nnn \l_char_active_seq { }
  { \" \$ \& \^ \_ \~ }
%    \end{macrocode}
% \end{variable}
%
% \subsection{Creating character tokens}
%
% \begin{macro}
%   {
%     \char_set_active_eq:NN, \char_gset_active_eq:NN,
%     \char_set_active_eq:Nc, \char_gset_active_eq:Nc,
%     \char_set_active_eq:nN, \char_gset_active_eq:nN,
%     \char_set_active_eq:nc, \char_gset_active_eq:nc
%   }
%   Four simple functions with very similar definitions, so set up using
%   an auxiliary.
%   These are similar to \LuaTeX{}'s \tn{letcharcode} primitive.
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_active:N \^^@
  \cs_set_protected:Npn \@@_tmp:nN #1#2
    {
      \cs_new_protected:cpn { #1 :nN } ##1
        {
          \group_begin:
            \char_set_lccode:nn { `\^^@ } { ##1 }
          \tex_lowercase:D { \group_end: #2 ^^@ }
        }
      \cs_new_protected:cpx { #1 :NN } ##1
        { \exp_not:c { #1 : nN } { `##1 } }
    }
  \@@_tmp:nN { char_set_active_eq }  \cs_set_eq:NN
  \@@_tmp:nN { char_gset_active_eq } \cs_gset_eq:NN
\group_end:
\cs_generate_variant:Nn \char_set_active_eq:NN  { Nc }
\cs_generate_variant:Nn \char_gset_active_eq:NN { Nc }
\cs_generate_variant:Nn \char_set_active_eq:nN  { nc }
\cs_generate_variant:Nn \char_gset_active_eq:nN { nc }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_int_to_roman:w}
%   For efficiency in 8-bit engines, we use the faster primitive approach
%   to making roman numerals.
%    \begin{macrocode}
\cs_new_eq:NN \@@_int_to_roman:w \tex_romannumeral:D
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\char_generate:nn}
% \begin{macro}[EXP]{\@@_generate_aux:nn}
% \begin{macro}[EXP]{\@@_generate_aux:nnw, \@@_generate_auxii:nnw}
% \begin{variable}{\l_@@_tmp_tl}
% \begin{macro}[EXP]{\@@_generate_invalid_catcode:}
%   The aim here is to generate characters of (broadly) arbitrary category
%   code. Where possible, that is done using engine support (\XeTeX{},
%   \LuaTeX{}). There are though various issues which are covered below. At
%   the interface layer, turn the two arguments into integers up-front so
%   this is only done once.
%    \begin{macrocode}
\cs_new:Npn \char_generate:nn #1#2
  {
    \exp:w \exp_after:wN \@@_generate_aux:w
      \int_value:w \int_eval:n {#1} \exp_after:wN ;
      \int_value:w \int_eval:n {#2} ;
  }
%    \end{macrocode}
%   Before doing any actual conversion, first some special case filtering.
%   Spaces are out here as \LuaTeX{} emulation only makes normal (charcode
%   $32$ spaces). However, |^^@| is filtered out separately as that can't be
%   done with macro emulation either, so is flagged up separately. That
%   done, hand off to the engine-dependent part.
%    \begin{macrocode}
\cs_new:Npn \@@_generate_aux:w #1 ; #2 ;
  {
    \if_int_compare:w #2 = 10 \exp_stop_f:
      \if_int_compare:w #1 =  0 \exp_stop_f:
        \__kernel_msg_expandable_error:nn { kernel } { char-null-space }
      \else:
        \__kernel_msg_expandable_error:nn { kernel } { char-space }
      \fi:
    \else:
      \if_int_odd:w 0
          \if_int_compare:w #2 < 1  \exp_stop_f: 1 \fi:
          \if_int_compare:w #2 = 5  \exp_stop_f: 1 \fi:
          \if_int_compare:w #2 = 9  \exp_stop_f: 1 \fi:
          \if_int_compare:w #2 > 13 \exp_stop_f: 1 \fi: \exp_stop_f:
        \__kernel_msg_expandable_error:nn { kernel }
          { char-invalid-catcode }
      \else:
        \if_int_odd:w 0
          \if_int_compare:w #1 < 0 \exp_stop_f: 1 \fi:
          \if_int_compare:w #1 > \c_max_char_int 1 \fi: \exp_stop_f:
          \__kernel_msg_expandable_error:nn { kernel }
            { char-out-of-range }
        \else:
          \@@_generate_aux:nnw {#1} {#2}
        \fi:
      \fi:
    \fi:
    \exp_end:
  }
\tl_new:N \l_@@_tmp_tl
%    \end{macrocode}
%   Engine-dependent definitions are now needed for the implementation. For
%   \LuaTeX{} and \XeTeX{} there is engine-level support.
%   They can do cases that macro emulation can't. All of those are filtered
%   out here using a primitive-based boolean expression to avoid fixing
%   the category code of the null character used in the false branch
%   (for 8-bit engines).
%   The final level is the basic definition at the engine level: the arguments
%   here are integers so there is no need to worry about them too much.
%   Older versions of  \XeTeX{} cannot generate active characters so we filter
%   that:
%   at some future stage that may change: the slightly odd ordering of
%   auxiliaries reflects that.
%    \begin{macrocode}
\group_begin:
%<*package>
  \char_set_catcode_active:N \^^L
  \cs_set:Npn ^^L { }
%</package>
  \char_set_catcode_other:n { 0 }
  \if_int_odd:w 0
      \sys_if_engine_luatex:T { 1 }
      \sys_if_engine_xetex:T { 1 } \exp_stop_f:
    \sys_if_engine_luatex:TF
      {
        \cs_new:Npn \@@_generate_aux:nnw #1#2#3 \exp_end:
          {
            #3
            \exp_after:wN \exp_after:wN \exp_after:wN \exp_end:
            \lua_now:e { l3kernel.charcat(#1, #2) }
          }
      }
      {
        \cs_new:Npn \@@_generate_aux:nnw #1#2#3 \exp_end:
          {
            #3
            \exp_after:wN \exp_end:
            \tex_Ucharcat:D #1 \exp_stop_f: #2 \exp_stop_f:
          }
        \cs_if_exist:NF \tex_expanded:D
          {
            \cs_new_eq:NN \@@_generate_auxii:nnw \@@_generate_aux:nnw
            \cs_gset:Npn \@@_generate_aux:nnw #1#2#3 \exp_end:
              {
                #3
                \if_int_compare:w #2 = 13 \exp_stop_f:
                  \__kernel_msg_expandable_error:nn { kernel } { char-active }
                \else:
                  \@@_generate_auxii:nnw {#1} {#2}
                \fi:
                \exp_end:
              }
          }
      }
  \else:
%    \end{macrocode}
%   For engines where \tn{Ucharcat} isn't available or emulated, we have
%   to work in macros, and cover only the $8$-bit range. The first stage is
%   to build up a |tl| containing |^^@| with each category code that can
%   be accessed in this way, with an error set up for the other cases. This
%   is all done such that it can be quickly accessed using a |\if_case:w|
%   low-level conditional. There are a few things to notice here.
%   As |^^L| is |\outer| we need to locally set it to avoid a problem.
%   To get open/close braces into the list, they are set up using |\if_false:|
%   pairing and are then |x|-type expanded together into the desired form.
%    \begin{macrocode}
      \tl_set:Nn \l_@@_tmp_tl { \exp_not:N \or: }
      \char_set_catcode_group_begin:n { 0 } % {
      \tl_put_right:Nn \l_@@_tmp_tl { ^^@ \if_false: } }
      \char_set_catcode_group_end:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { { \fi: \exp_not:N \or: ^^@ } % }
      \tl_set:Nx \l_@@_tmp_tl { \l_@@_tmp_tl }
      \char_set_catcode_math_toggle:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \char_set_catcode_alignment:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: }
      \char_set_catcode_parameter:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \char_set_catcode_math_superscript:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \char_set_catcode_math_subscript:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: }
%    \end{macrocode}
%   For making spaces, there needs to be an |o|-type expansion of a |\use:n|
%   (or some other tokenization) to avoid dropping the space. We also
%   set up active tokens although they are (currently) filtered out by the
%   interface layer (\tn{Ucharcat} cannot make active tokens).
%    \begin{macrocode}
      \char_set_catcode_space:n { 0 }
      \tl_put_right:No \l_@@_tmp_tl { \use:n { \or: } ^^@ }
      \char_set_catcode_letter:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \char_set_catcode_other:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
      \char_set_catcode_active:n { 0 }
      \tl_put_right:Nn \l_@@_tmp_tl { \or: ^^@ }
%    \end{macrocode}
%   Convert the above temporary list into a series of constant token
%   lists, one for each character code, using \tn{tex_lowercase:D} to
%   convert |^^@| in each case. The \texttt{x}-type expansion ensures
%   that \tn{tex_lowercase:D} receives the contents of the token list.
%   In package mode, |^^L| is awkward hence this is done in three parts.
%   Notice that at this stage |^^@| is active.
%    \begin{macrocode}
      \cs_set_protected:Npn \@@_tmp:n #1
        {
          \char_set_lccode:nn { 0 } {#1}
          \char_set_lccode:nn { 32 } {#1}
          \exp_args:Nx \tex_lowercase:D
            {
              \tl_const:Nn
                \exp_not:c { c_@@_ \@@_int_to_roman:w #1 _tl }
                { \exp_not:o \l_@@_tmp_tl }
            }
        }
%<*package>
      \int_step_function:nnN { 0 } { 11 }  \@@_tmp:n
      \group_begin:
        \tl_replace_once:Nnn \l_@@_tmp_tl { ^^@ } { \ERROR }
        \@@_tmp:n { 12 }
      \group_end:
      \int_step_function:nnN { 13 } { 255 } \@@_tmp:n
%</package>
%<*initex>
      \int_step_function:nnN { 0 } { 255 }  \@@_tmp:n
%</initex>
%    \end{macrocode}
%   As \TeX{} is very unhappy if if finds an alignment character inside
%   a primitive \tn{halign} even when skipping false branches, some
%   precautions are required. \TeX{} is happy if the token is hidden
%   between braces within \cs{if_false:} \dots{} \cs{fi:}.
%    \begin{macrocode}
      \cs_new:Npn \@@_generate_aux:nnw #1#2#3 \exp_end:
        {
          #3
          \if_false: { \fi:
          \exp_after:wN \exp_after:wN
          \exp_after:wN \exp_end:
          \exp_after:wN \exp_after:wN
          \if_case:w #2
            \exp_last_unbraced:Nv \exp_stop_f:
              { c_@@_ \@@_int_to_roman:w #1 _tl }
          \or: }
          \fi:
        }
  \fi:
\group_end:
%    \end{macrocode}
% \end{macro}
% \end{variable}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\c_catcode_other_space_tl}
%   Create a space with category code $12$: an \enquote{other} space.
%    \begin{macrocode}
\tl_const:Nx \c_catcode_other_space_tl { \char_generate:nn { `\  } { 12 } }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Generic tokens}
%
%    \begin{macrocode}
%<@@=token>
%    \end{macrocode}
%
% \begin{macro}{\token_to_meaning:N, \token_to_meaning:c}
% \begin{macro}{\token_to_str:N, \token_to_str:c}
%   These are all defined in \pkg{l3basics}, as they are needed
%   \enquote{early}. This is just a reminder!
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \c_group_begin_token,
%     \c_group_end_token,
%     \c_math_toggle_token,
%     \c_alignment_token,
%     \c_parameter_token,
%     \c_math_superscript_token,
%     \c_math_subscript_token,
%     \c_space_token,
%     \c_catcode_letter_token,
%     \c_catcode_other_token
%   }
%   We define these useful tokens. For the brace and space tokens things have
%   to be done by hand: the formal argument spec.~for \cs{cs_new_eq:NN} does
%   not cover them so we do things by hand. (As currently coded it would
%   \emph{work} with \cs{cs_new_eq:NN} but that's not really a great idea to
%   show off: we want people to stick to the defined interfaces and that
%   includes us.) So that these few odd names go into the log when appropriate
%   there is a need to hand-apply the \cs{__kernel_chk_if_free_cs:N} check.
%    \begin{macrocode}
\group_begin:
  \__kernel_chk_if_free_cs:N \c_group_begin_token
  \tex_global:D \tex_let:D \c_group_begin_token {
  \__kernel_chk_if_free_cs:N \c_group_end_token
  \tex_global:D \tex_let:D \c_group_end_token }
  \char_set_catcode_math_toggle:N \*
  \cs_new_eq:NN \c_math_toggle_token *
  \char_set_catcode_alignment:N \*
  \cs_new_eq:NN \c_alignment_token *
  \cs_new_eq:NN \c_parameter_token #
  \cs_new_eq:NN \c_math_superscript_token ^
  \char_set_catcode_math_subscript:N \*
  \cs_new_eq:NN \c_math_subscript_token *
  \__kernel_chk_if_free_cs:N \c_space_token
  \use:n { \tex_global:D \tex_let:D \c_space_token = ~ } ~
  \cs_new_eq:NN \c_catcode_letter_token a
  \cs_new_eq:NN \c_catcode_other_token 1
\group_end:
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\c_catcode_active_tl}
%   Not an implicit token!
%    \begin{macrocode}
\group_begin:
  \char_set_catcode_active:N \*
  \tl_const:Nn \c_catcode_active_tl { \exp_not:N * }
\group_end:
%    \end{macrocode}
% \end{variable}
%
% \subsection{Token conditionals}
%
% \begin{macro}[pTF]{\token_if_group_begin:N}
%   Check if token is a begin group token. We use the constant
%   \cs{c_group_begin_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_group_begin:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_group_begin_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_group_end:N}
%   Check if token is a end group token. We use the constant
%   \cs{c_group_end_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_group_end:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_group_end_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_math_toggle:N}
%   Check if token is a math shift token. We use the constant
%   \cs{c_math_toggle_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_math_toggle:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_math_toggle_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_alignment:N}
%   Check if token is an alignment tab token. We use the constant
%   \cs{c_alignment_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_alignment:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_alignment_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_parameter:N}
%   Check if token is a parameter token. We use the constant
%   \cs{c_parameter_token} for this. We have to trick \TeX{} a bit to
%   avoid an error message: within a group we prevent
%   \cs{c_parameter_token} from behaving like a macro parameter character.
%   The definitions of \cs{prg_new_conditional:Npnn} are global, so they
%   remain after the group.
%    \begin{macrocode}
\group_begin:
\cs_set_eq:NN \c_parameter_token \scan_stop:
\prg_new_conditional:Npnn \token_if_parameter:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_parameter_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
\group_end:
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_math_superscript:N}
%   Check if token is a math superscript token. We use the constant
%   \cs{c_math_superscript_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_math_superscript:N #1
  { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_math_superscript_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_math_subscript:N}
%   Check if token is a math subscript token. We use the constant
%   \cs{c_math_subscript_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_math_subscript:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_math_subscript_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_space:N}
%   Check if token is a space token. We use the constant
%   \cs{c_space_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_space:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_space_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_letter:N}
%   Check if token is a letter token. We use the constant
%   \cs{c_catcode_letter_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_letter:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_catcode_letter_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_other:N}
%   Check if token is an other char token. We use the constant
%   \cs{c_catcode_other_token} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_other:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_catcode_other_token
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_active:N}
%   Check if token is an active char token. We use the constant
%   \cs{c_catcode_active_tl} for this. A technical point is that
%   \cs{c_catcode_active_tl} is in fact a macro expanding to
%   |\exp_not:N *|, where |*| is active.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_active:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \c_catcode_active_tl
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_eq_meaning:NN}
%   Check if the tokens |#1| and |#2| have same meaning.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_eq_meaning:NN #1#2 { p , T ,  F , TF }
  {
    \if_meaning:w  #1  #2
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_eq_catcode:NN}
%  Check if the tokens |#1| and |#2| have same category code.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_eq_catcode:NN #1#2 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \exp_not:N #2
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_eq_charcode:NN}
%  Check if the tokens |#1| and |#2| have same character code.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_eq_charcode:NN #1#2 { p , T ,  F , TF }
  {
    \if_charcode:w \exp_not:N #1 \exp_not:N #2
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_macro:N}
% \begin{macro}{\@@_if_macro_p:w}
%   When a token is a macro, \cs{token_to_meaning:N} always outputs
%   something like |\long macro:#1->#1| so we could naively check to
%   see if the meaning contains |->|. However, this can fail the five
%   \tn[no-index]{...mark} primitives, whose meaning has the form
%   |...mark:|\meta{user material}. The problem is that the
%   \meta{user material} can contain |->|.
%
%   However, only characters, macros, and marks can contain the colon
%   character. The idea is thus to grab until the first |:|, and analyse
%   what is left. However, macros can have any combination of |\long|,
%   |\protected| or |\outer| (not used in \LaTeX3) before the string
%   |macro:|. We thus only select the part of the meaning between
%   the first |ma| and the first following |:|. If this string is
%   |cro|, then we have a macro. If the string is |rk|, then we have
%   a mark. The string can also be |cro parameter character | for a
%   colon with a weird category code (namely the usual category code
%   of |#|). Otherwise, it is empty.
%
%   This relies on the fact that |\long|, |\protected|, |\outer|
%   cannot contain |ma|, regardless of the escape character, even if
%   the escape character is |m|\ldots{}
%
%   Both |ma| and |:| must be of category code $12$ (other), so are
%   detokenized.
%
%    \begin{macrocode}
\use:x
  {
    \prg_new_conditional:Npnn \exp_not:N \token_if_macro:N ##1
      { p , T ,  F , TF }
      {
        \exp_not:N \exp_after:wN \exp_not:N \@@_if_macro_p:w
        \exp_not:N \token_to_meaning:N ##1 \tl_to_str:n { ma : }
          \exp_not:N \q_stop
      }
    \cs_new:Npn \exp_not:N  \@@_if_macro_p:w
      ##1 \tl_to_str:n { ma } ##2 \c_colon_str ##3 \exp_not:N \q_stop
  }
      {
        \str_if_eq:nnTF { #2 } { cro }
          { \prg_return_true: }
          { \prg_return_false: }
      }
%    \end{macrocode}
%  \end{macro}
%  \end{macro}
%
% \begin{macro}[pTF]{\token_if_cs:N}
%   Check if token has same catcode as a control sequence. This
%   follows the same pattern as for \cs{token_if_letter:N} \emph{etc.}
%   We use \cs{scan_stop:} for this.
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_cs:N #1 { p , T ,  F , TF }
  {
    \if_catcode:w \exp_not:N #1 \scan_stop:
      \prg_return_true: \else: \prg_return_false: \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_expandable:N}
%   Check if token is expandable. We use the fact that \TeX{}
%   temporarily converts \cs{exp_not:N} \meta{token} into \cs{scan_stop:}
%   if \meta{token} is expandable.  An \texttt{undefined} token is not
%   considered as expandable.  No problem nesting the conditionals,
%   since the third |#1| is only skipped if it is non-expandable (hence
%   not part of \TeX{}'s conditional apparatus).
%    \begin{macrocode}
\prg_new_conditional:Npnn \token_if_expandable:N #1 { p , T ,  F , TF }
  {
    \exp_after:wN \if_meaning:w \exp_not:N #1 #1
      \prg_return_false:
    \else:
      \if_cs_exist:N #1
        \prg_return_true:
      \else:
        \prg_return_false:
      \fi:
    \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_delimit_by_char":w,
%     \@@_delimit_by_count:w,
%     \@@_delimit_by_dimen:w,
%     \@@_delimit_by_macro:w,
%     \@@_delimit_by_muskip:w,
%     \@@_delimit_by_skip:w,
%     \@@_delimit_by_toks:w,
%   }
%   These auxiliary functions are used below to define some
%   conditionals which detect whether the \tn{meaning} of their
%   argument begins with a particular string.  Each auxiliary takes an
%   argument delimited by a string, a second one delimited by
%   \cs{q_stop}, and returns the first one and its delimiter.
%   This result is eventually compared to another string.
%    \begin{macrocode}
\group_begin:
\cs_set_protected:Npn \@@_tmp:w #1
  {
    \use:x
      {
        \cs_new:Npn \exp_not:c { @@_delimit_by_ #1 :w }
            ####1 \tl_to_str:n {#1} ####2 \exp_not:N \q_stop
          { ####1 \tl_to_str:n {#1} }
      }
  }
\@@_tmp:w { char" }
\@@_tmp:w { count }
\@@_tmp:w { dimen }
\@@_tmp:w { macro }
\@@_tmp:w { muskip }
\@@_tmp:w { skip }
\@@_tmp:w { toks }
\group_end:
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]
%   {
%     \token_if_chardef:N,               \token_if_mathchardef:N,
%                                        \token_if_long_macro:N,
%     \token_if_protected_macro:N,       \token_if_protected_long_macro:N,
%     \token_if_dim_register:N,          \token_if_int_register:N,
%     \token_if_muskip_register:N,
%     \token_if_skip_register:N,         \token_if_toks_register:N,
%   }
%   Each of these conditionals tests whether its argument's
%   \tn{meaning} starts with a given string.  This is essentially done
%   by having an auxiliary grab an argument delimited by the string and
%   testing whether the argument was empty.  Of course, a copy of this
%   string must first be added to the end of the \tn{meaning} to avoid
%   a runaway argument in case it does not contain the string.  Two
%   complications arise.  First, the escape character is not fixed, and
%   cannot be included in the delimiter of the auxiliary function (this
%   function cannot be defined on the fly because tests must remain
%   expandable): instead the first argument of the auxiliary (plus the
%   delimiter to avoid complications with trailing spaces) is compared
%   using \cs{str_if_eq:eeTF} to the result of applying
%   \cs{token_to_str:N} to a control sequence.  Second, the
%   \tn{meaning} of primitives such as \tn{dimen} or \tn{dimendef}
%   starts in the same way as registers such as
%   \tn{dimen}\texttt{123}, so they must be tested for.
%
%   Characters used as delimiters must have catcode~$12$
%   and are obtained through \cs{tl_to_str:n}.  This requires doing all
%   definitions within \texttt{x}-expansion.  The temporary function
%   \cs{@@_tmp:w} used to define each conditional receives three
%   arguments: the name of the conditional, the auxiliary's delimiter
%   (also used to name the auxiliary), and the string to which one
%   compares the auxiliary's result.  Note that the \tn{meaning} of a
%   protected long macro starts with |\protected\long macro|, with no
%   space after |\protected| but a space after |\long|, hence the
%   mixture of \cs{token_to_str:N} and \cs{tl_to_str:n}.
%
%   For the first five conditionals, \cs{cs_if_exist:cT} turns out to
%   be \texttt{false}, and the code boils down to a string comparison
%   between the result of the auxiliary on the \tn{meaning} of the
%   conditional's argument~|####1|, and~|#3|.  Both are evaluated at
%   run-time, as this is important to get the correct escape character.
%
%   The other five conditionals have additional code that compares the
%   argument~|####1| to two \TeX{} primitives which would wrongly be
%   recognized as registers otherwise.  Despite using \TeX{}'s
%   primitive conditional construction, this does not break
%   when~|####1| is itself a conditional, because branches of the
%   conditionals are only skipped if |####1|~is one of the two
%   primitives that are tested for (which are not \TeX{} conditionals).
%    \begin{macrocode}
\group_begin:
\cs_set_protected:Npn \@@_tmp:w #1#2#3
  {
    \use:x
      {
        \prg_new_conditional:Npnn \exp_not:c { token_if_ #1 :N } ####1
          { p , T ,  F , TF }
          {
            \cs_if_exist:cT { tex_ #2 :D }
              {
                \exp_not:N \if_meaning:w ####1 \exp_not:c { tex_ #2 :D }
                \exp_not:N \prg_return_false:
                \exp_not:N \else:
                \exp_not:N \if_meaning:w ####1 \exp_not:c { tex_ #2 def:D }
                \exp_not:N \prg_return_false:
                \exp_not:N \else:
              }
            \exp_not:N \str_if_eq:eeTF
              {
                \exp_not:N \exp_after:wN
                \exp_not:c { @@_delimit_by_ #2 :w }
                \exp_not:N \token_to_meaning:N ####1
                ? \tl_to_str:n {#2} \exp_not:N \q_stop
              }
              { \exp_not:n {#3} }
              { \exp_not:N \prg_return_true: }
              { \exp_not:N \prg_return_false: }
            \cs_if_exist:cT { tex_ #2 :D }
              {
                \exp_not:N \fi:
                \exp_not:N \fi:
              }
          }
      }
  }
\@@_tmp:w { chardef } { char" } { \token_to_str:N \char" }
\@@_tmp:w { mathchardef } { char" } { \token_to_str:N \mathchar" }
\@@_tmp:w { long_macro } { macro } { \tl_to_str:n { \long } macro }
\@@_tmp:w { protected_macro } { macro }
  { \tl_to_str:n { \protected } macro }
\@@_tmp:w { protected_long_macro } { macro }
  { \token_to_str:N \protected \tl_to_str:n { \long } macro }
\@@_tmp:w { dim_register } { dimen } { \token_to_str:N \dimen }
\@@_tmp:w { int_register } { count } { \token_to_str:N \count }
\@@_tmp:w { muskip_register } { muskip } { \token_to_str:N \muskip }
\@@_tmp:w { skip_register } { skip } { \token_to_str:N \skip }
\@@_tmp:w { toks_register } { toks } { \token_to_str:N \toks }
\group_end:
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[pTF]{\token_if_primitive:N}
% \begin{macro}{\@@_if_primitive:NNw,
%     \@@_if_primitive_space:w,
%     \@@_if_primitive_nullfont:N,
%     \@@_if_primitive_loop:N,
%     \@@_if_primitive:Nw,
%     \@@_if_primitive_undefined:N}
%^^A See http://groups.google.com/group/comp.text.tex/browse_thread/thread/0a72666873f8753d#
%
%   We filter out macros first, because they cause endless trouble later
%   otherwise.
%
%   Primitives are almost distinguished by the fact that the result
%   of \cs{token_to_meaning:N} is formed from letters only. Every other
%   token has either a space (e.g., |the letter A|), a digit
%   (e.g., |\count123|) or a double quote (e.g., |\char"A|).
%
%   Ten exceptions: on the one hand, \cs{tex_undefined:D} is not a
%   primitive, but its meaning is |undefined|, only letters;
%   on the other hand, \tn{space}, \tn{italiccorr},
%   \tn{hyphen}, \tn{firstmark}, \tn{topmark},
%   \tn{botmark}, \tn{splitfirstmark}, \tn{splitbotmark},
%   and \tn{nullfont} are primitives, but have non-letters
%   in their meaning.
%
%   We start by removing the two first (non-space) characters from
%   the meaning. This removes the escape character (which may be
%   nonexistent depending on \tn{endlinechar}), and takes care
%   of three of the exceptions: \tn{space}, \tn{italiccorr}
%   and \tn{hyphen}, whose meaning is at most two characters.
%   This leaves a string terminated by some |:|, and \cs{q_stop}.
%
%   The meaning of each one of the five \tn[no-index]{...mark} primitives
%   has the form \meta{letters}|:|\meta{user material}. In other words,
%   the first non-letter is a colon. We remove everything after the first
%   colon.
%
%   We are now left with a string, which we must analyze. For primitives,
%   it contains only letters. For non-primitives, it contains either
%   |"|, or a space, or a digit. Two exceptions remain: \cs{tex_undefined:D},
%   which is not a primitive, and \tn{nullfont}, which is a primitive.
%
%   Spaces cannot be grabbed in an undelimited way, so we check them
%   separately. If there is a space, we test for \tn{nullfont}.
%   Otherwise, we go through characters one by one, and stop at the
%   first character less than |`A| (this is not quite a test for
%   \enquote{only letters}, but is close enough to work in this context).
%   If this first character is |:| then we have a primitive, or
%   \cs{tex_undefined:D}, and if it is |"| or a digit, then the token
%   is not a primitive.
%
%    \begin{macrocode}
\tex_chardef:D \c_@@_A_int = `A ~ %
\use:x
  {
    \prg_new_conditional:Npnn \exp_not:N \token_if_primitive:N ##1
      { p , T , F , TF }
      {
        \exp_not:N \token_if_macro:NTF ##1
          \exp_not:N \prg_return_false:
          {
            \exp_not:N \exp_after:wN \exp_not:N \@@_if_primitive:NNw
            \exp_not:N \token_to_meaning:N ##1
              \tl_to_str:n { : : : } \exp_not:N \q_stop ##1
          }
      }
    \cs_new:Npn \exp_not:N \@@_if_primitive:NNw
      ##1##2 ##3 \c_colon_str ##4 \exp_not:N \q_stop
      {
        \exp_not:N \tl_if_empty:oTF
          { \exp_not:N \@@_if_primitive_space:w ##3 ~ }
          {
            \exp_not:N \@@_if_primitive_loop:N ##3
              \c_colon_str \exp_not:N \q_stop
          }
          { \exp_not:N \@@_if_primitive_nullfont:N }
      }
  }
\cs_new:Npn \@@_if_primitive_space:w #1 ~ { }
\cs_new:Npn \@@_if_primitive_nullfont:N #1
  {
    \if_meaning:w \tex_nullfont:D #1
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
\cs_new:Npn \@@_if_primitive_loop:N #1
  {
    \if_int_compare:w `#1 < \c_@@_A_int %
      \exp_after:wN \@@_if_primitive:Nw
      \exp_after:wN #1
    \else:
      \exp_after:wN \@@_if_primitive_loop:N
    \fi:
  }
\cs_new:Npn \@@_if_primitive:Nw #1 #2 \q_stop
  {
    \if:w : #1
      \exp_after:wN \@@_if_primitive_undefined:N
    \else:
      \prg_return_false:
      \exp_after:wN \use_none:n
    \fi:
  }
\cs_new:Npn \@@_if_primitive_undefined:N #1
  {
    \if_cs_exist:N #1
      \prg_return_true:
    \else:
      \prg_return_false:
    \fi:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Peeking ahead at the next token}
%
%    \begin{macrocode}
%<@@=peek>
%    \end{macrocode}
%
% Peeking ahead is implemented using a two part mechanism. The
% outer level provides a defined interface to the lower level material.
% This allows a large amount of code to be shared. There are four
% cases:
% \begin{enumerate}
%   \item peek at the next token;
%   \item peek at the next non-space token;
%   \item peek at the next token and remove it;
%   \item peek at the next non-space token and remove it.
% \end{enumerate}
%
% \begin{variable}{\l_peek_token}
% \begin{variable}{\g_peek_token}
%   Storage tokens which are publicly documented: the token peeked.
%    \begin{macrocode}
\cs_new_eq:NN \l_peek_token ?
\cs_new_eq:NN \g_peek_token ?
%    \end{macrocode}
% \end{variable}
% \end{variable}
%
% \begin{variable}{\l_@@_search_token}
%   The token to search for as an implicit token:
%   \emph{cf.}~\cs{l_@@_search_tl}.
%    \begin{macrocode}
\cs_new_eq:NN \l_@@_search_token ?
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_search_tl}
%   The token to search for as an explicit token:
%   \emph{cf.}~\cs{l_@@_search_token}.
%    \begin{macrocode}
\tl_new:N \l_@@_search_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}
%   {\@@_true:w, \@@_true_aux:w, \@@_false:w, \@@_tmp:w}
%   Functions used by the branching and space-stripping code.
%    \begin{macrocode}
\cs_new:Npn \@@_true:w  { }
\cs_new:Npn \@@_true_aux:w  { }
\cs_new:Npn \@@_false:w { }
\cs_new:Npn \@@_tmp:w { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\peek_after:Nw}
% \begin{macro}{\peek_gafter:Nw}
%   Simple wrappers for \tn{futurelet}: no arguments absorbed
%   here.
%    \begin{macrocode}
\cs_new_protected:Npn \peek_after:Nw
  { \tex_futurelet:D \l_peek_token }
\cs_new_protected:Npn \peek_gafter:Nw
  { \tex_global:D \tex_futurelet:D \g_peek_token }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_true_remove:w}
%   A function to remove the next token and then regain control.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_true_remove:w
  {
    \tex_afterassignment:D \@@_true_aux:w
    \cs_set_eq:NN \@@_tmp:w
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\peek_remove_spaces:n, \@@_remove_spaces:}
%   Repeatedly use \cs{@@_true_remove:w} to remove a space and call
%   \cs{@@_true_aux:w}.
%    \begin{macrocode}
\cs_new_protected:Npn \peek_remove_spaces:n #1
  {
    \cs_set:Npx \@@_false:w { \exp_not:n {#1} }
    \group_align_safe_begin:
    \cs_set:Npn \@@_true_aux:w { \peek_after:Nw \@@_remove_spaces: }
    \@@_true_aux:w
  }
\cs_new_protected:Npn \@@_remove_spaces:
  {
    \if_meaning:w \l_peek_token \c_space_token
      \exp_after:wN \@@_true_remove:w
    \else:
      \group_align_safe_end:
      \exp_after:wN \@@_false:w
    \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_token_generic_aux:NNNTF}
%   The generic functions store the test token in both implicit and
%   explicit modes, and the \texttt{true} and \texttt{false} code as
%   token lists, more or less. The two branches have to be absorbed here
%   as the input stream needs to be cleared for the peek function itself.
%   Here, |#1| is \cs{@@_true_remove:w} when removing the token and
%   \cs{@@_true_aux:w} otherwise.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_token_generic_aux:NNNTF #1#2#3#4#5
  {
    \group_align_safe_begin:
    \cs_set_eq:NN \l_@@_search_token #3
    \tl_set:Nn \l_@@_search_tl {#3}
    \cs_set:Npx \@@_true_aux:w
      {
        \exp_not:N \group_align_safe_end:
        \exp_not:n {#4}
      }
    \cs_set_eq:NN \@@_true:w #1
    \cs_set:Npx \@@_false:w
      {
        \exp_not:N \group_align_safe_end:
        \exp_not:n {#5}
      }
    \peek_after:Nw #2
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[TF]{\@@_token_generic:NN, \@@_token_remove_generic:NN}
%   For token removal there needs to be a call to the auxiliary
%   function which does the work.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_token_generic:NNTF
  { \@@_token_generic_aux:NNNTF \@@_true_aux:w }
\cs_new_protected:Npn \@@_token_generic:NNT #1#2#3
  { \@@_token_generic:NNTF #1 #2 {#3} { } }
\cs_new_protected:Npn \@@_token_generic:NNF #1#2#3
  { \@@_token_generic:NNTF #1 #2 { } {#3} }
\cs_new_protected:Npn \@@_token_remove_generic:NNTF
  { \@@_token_generic_aux:NNNTF \@@_true_remove:w }
\cs_new_protected:Npn \@@_token_remove_generic:NNT #1#2#3
  { \@@_token_remove_generic:NNTF #1 #2 {#3} { } }
\cs_new_protected:Npn \@@_token_remove_generic:NNF #1#2#3
  { \@@_token_remove_generic:NNTF #1 #2 { } {#3} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_execute_branches_meaning:}
%   The meaning test is straight forward.
%    \begin{macrocode}
\cs_new:Npn \@@_execute_branches_meaning:
  {
    \if_meaning:w \l_peek_token \l_@@_search_token
      \exp_after:wN \@@_true:w
    \else:
      \exp_after:wN \@@_false:w
    \fi:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_execute_branches_catcode:, \@@_execute_branches_charcode:}
% \begin{macro}
%   {
%     \@@_execute_branches_catcode_aux:    ,
%     \@@_execute_branches_catcode_auxii:N ,
%     \@@_execute_branches_catcode_auxiii:
%   }
%   The catcode and charcode tests are very similar, and in order to use
%   the same auxiliaries we do something a little bit odd, firing
%   \cs{if_catcode:w} and \cs{if_charcode:w} before finding the operands
%   for those tests, which are only given in the |auxii:N| and
%   |auxiii:| auxiliaries.  For our purposes, three kinds of tokens may
%   follow the peeking function:
%   \begin{itemize}
%     \item control sequences which are not equal to a non-active
%       character token (\emph{e.g.}, macro, primitive);
%     \item active characters which are not equal to a non-active
%       character token (\emph{e.g.}, macro, primitive);
%     \item explicit non-active character tokens, or control sequences
%       or active characters set equal to a non-active character token.
%   \end{itemize}
%   The first two cases are not distinguishable simply using \TeX{}'s
%   \tn{futurelet}, because we can only access the \tn{meaning} of
%   tokens in that way.  In those cases, detected thanks to a
%   comparison with \cs{scan_stop:}, we grab the following token, and
%   compare it explicitly with the explicit search token stored in
%   \cs{l_@@_search_tl}.  The \cs{exp_not:N} prevents outer macros
%   (coming from non-\LaTeX3 code) from blowing up.  In the third case,
%   \cs{l_peek_token} is good enough for the test, and we compare it
%   again with the explicit search token.  Just like the peek token, the
%   search token may be of any of the three types above, hence the need
%   to use the explicit token that was given to the peek function.
%    \begin{macrocode}
\cs_new:Npn \@@_execute_branches_catcode:
  { \if_catcode:w \@@_execute_branches_catcode_aux: }
\cs_new:Npn \@@_execute_branches_charcode:
  { \if_charcode:w \@@_execute_branches_catcode_aux: }
\cs_new:Npn \@@_execute_branches_catcode_aux:
  {
        \if_catcode:w \exp_not:N \l_peek_token \scan_stop:
          \exp_after:wN \exp_after:wN
          \exp_after:wN \@@_execute_branches_catcode_auxii:N
          \exp_after:wN \exp_not:N
        \else:
          \exp_after:wN \@@_execute_branches_catcode_auxiii:
        \fi:
  }
\cs_new:Npn \@@_execute_branches_catcode_auxii:N #1
  {
        \exp_not:N #1
        \exp_after:wN \exp_not:N \l_@@_search_tl
      \exp_after:wN \@@_true:w
    \else:
      \exp_after:wN \@@_false:w
    \fi:
    #1
  }
\cs_new:Npn \@@_execute_branches_catcode_auxiii:
  {
        \exp_not:N \l_peek_token
        \exp_after:wN \exp_not:N \l_@@_search_tl
      \exp_after:wN \@@_true:w
    \else:
      \exp_after:wN \@@_false:w
    \fi:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[TF]
%   {
%     \peek_catcode:N,
%     \peek_catcode_remove:N,
%     \peek_charcode:N,
%     \peek_charcode_remove:N,
%     \peek_meaning:N,
%     \peek_meaning_remove:N,
%   }
%   The public functions themselves cannot be defined using
%   \cs{prg_new_conditional:Npnn}.  Instead, the |TF|, |T|, |F| variants
%   are defined in terms of corresponding variants of
%   \cs{@@_token_generic:NNTF} or \cs{@@_token_remove_generic:NNTF},
%   with first argument one of \cs{@@_execute_branches_catcode:},
%   \cs{@@_execute_branches_charcode:}, or
%   \cs{@@_execute_branches_meaning:}.
%    \begin{macrocode}
\tl_map_inline:nn { { catcode } { charcode } { meaning } }
  {
    \tl_map_inline:nn { { } { _remove } }
      {
        \tl_map_inline:nn { { TF } { T } { F } }
          {
            \cs_new_protected:cpx { peek_ #1 ##1 :N ####1 }
              {
                \exp_not:c { @@_token ##1 _generic:NN ####1 }
                \exp_not:c { @@_execute_branches_ #1 : }
              }
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[TF]
%   {
%     \peek_catcode_ignore_spaces:N, \peek_catcode_remove_ignore_spaces:N,
%     \peek_charcode_ignore_spaces:N, \peek_charcode_remove_ignore_spaces:N,
%     \peek_meaning_ignore_spaces:N, \peek_meaning_remove_ignore_spaces:N
%   }
%   To ignore spaces, remove them using \cs{peek_remove_spaces:n}
%   before running the tests.
%    \begin{macrocode}
\tl_map_inline:nn
  {
    { catcode } { catcode_remove }
    { charcode } { charcode_remove }
    { meaning } { meaning_remove }
  }
  {
    \cs_new_protected:cpx { peek_#1_ignore_spaces:NTF } ##1##2##3
      {
        \peek_remove_spaces:n
          { \exp_not:c { peek_#1:NTF } ##1 {##2} {##3} }
      }
    \cs_new_protected:cpx { peek_#1_ignore_spaces:NT } ##1##2
      {
        \peek_remove_spaces:n
          { \exp_not:c { peek_#1:NT } ##1 {##2} }
      }
    \cs_new_protected:cpx { peek_#1_ignore_spaces:NF } ##1##2
      {
        \peek_remove_spaces:n
          { \exp_not:c { peek_#1:NF } ##1 {##2} }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[TF]{\peek_N_type:}
% \begin{macro}
%   {\@@_execute_branches_N_type:, \@@_N_type:w, \@@_N_type_aux:nnw}
%   All tokens are \texttt{N}-type tokens, except in four cases:
%   begin-group tokens, end-group tokens, space tokens with character
%   code~$32$, and outer tokens.  Since \cs{l_peek_token} might be
%   outer, we cannot use the convenient \cs{bool_if:nTF} function, and
%   must resort to the old trick of using \tn{ifodd} to expand a set of
%   tests.  The \texttt{false} branch of this test is taken if the token
%   is one of the first three kinds of non-\texttt{N}-type tokens
%   (explicit or implicit), thus we call \cs{@@_false:w}.  In the
%   \texttt{true} branch, we must detect outer tokens, without impacting
%   performance too much for non-outer tokens.  The first filter is to
%   search for \texttt{outer} in the \tn{meaning} of \cs{l_peek_token}.
%   If that is absent, \cs{use_none_delimit_by_q_stop:w} cleans up, and
%   we call \cs{@@_true:w}.  Otherwise, the token can be a non-outer
%   macro or a primitive mark whose parameter or replacement text
%   contains \texttt{outer}, it can be the primitive \tn{outer}, or it
%   can be an outer token.  Macros and marks would have \texttt{ma} in
%   the part before the first occurrence of \texttt{outer}; the meaning
%   of \tn{outer} has nothing after \texttt{outer}, contrarily to outer
%   macros; and that covers all cases, calling \cs{@@_true:w} or
%   \cs{@@_false:w} as appropriate.  Here, there is no \meta{search
%     token}, so we feed a dummy \cs{scan_stop:} to the
%   \cs{@@_token_generic:NNTF} function.
%    \begin{macrocode}
\group_begin:
  \cs_set_protected:Npn \@@_tmp:w #1 \q_stop
    {
      \cs_new_protected:Npn \@@_execute_branches_N_type:
        {
          \if_int_odd:w
              \if_catcode:w \exp_not:N \l_peek_token {   0 \exp_stop_f: \fi:
              \if_catcode:w \exp_not:N \l_peek_token }   0 \exp_stop_f: \fi:
              \if_meaning:w \l_peek_token \c_space_token 0 \exp_stop_f: \fi:
              1 \exp_stop_f:
            \exp_after:wN \@@_N_type:w
              \token_to_meaning:N \l_peek_token
              \q_mark \@@_N_type_aux:nnw
              #1 \q_mark \use_none_delimit_by_q_stop:w
              \q_stop
            \exp_after:wN \@@_true:w
          \else:
            \exp_after:wN \@@_false:w
          \fi:
        }
      \cs_new_protected:Npn \@@_N_type:w ##1 #1 ##2 \q_mark ##3
        { ##3 {##1} {##2} }
    }
  \exp_after:wN \@@_tmp:w \tl_to_str:n { outer } \q_stop
\group_end:
\cs_new_protected:Npn \@@_N_type_aux:nnw #1 #2 #3 \fi:
  {
    \fi:
    \tl_if_in:noTF {#1} { \tl_to_str:n {ma} }
      { \@@_true:w }
      { \tl_if_empty:nTF {#2} { \@@_true:w } { \@@_false:w } }
  }
\cs_new_protected:Npn \peek_N_type:TF
  {
    \@@_token_generic:NNTF
      \@@_execute_branches_N_type: \scan_stop:
  }
\cs_new_protected:Npn \peek_N_type:T
  { \@@_token_generic:NNT \@@_execute_branches_N_type: \scan_stop: }
\cs_new_protected:Npn \peek_N_type:F
  { \@@_token_generic:NNF \@@_execute_branches_N_type: \scan_stop: }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%    \begin{macrocode}
%</initex|package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex