summaryrefslogtreecommitdiff
path: root/Master/texmf-dist/source/latex/l3kernel/l3clist.dtx
blob: f88dd61968c9c7929659f8afa3e36a7eb10e6af9 (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
% \iffalse meta-comment
%
%% File: l3clist.dtx
%
% Copyright (C) 2004-2011 Frank Mittelbach, The LaTeX3 Project
%           (C) 2012-2021 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{l3clist} package\\ Comma separated lists^^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 2021-01-09}
%
% \maketitle
%
% \begin{documentation}
%
% Comma lists contain ordered data where items can be added to the left
% or right end of the list.  This data type allows basic list
% manipulations such as adding/removing items, applying a function to
% every item, removing duplicate items, extracting a given item, using
% the comma list with specified separators, and so on.  Sequences
% (defined in \pkg{l3seq}) are safer, faster, and provide more features,
% so they should often be preferred to comma lists.  Comma lists are
% mostly useful when interfacing with \LaTeXe{} or other code that
% expects or provides comma list data.
%
% Several items can be added at once.  To ease input of comma lists from
% data provided by a user outside an \cs{ExplSyntaxOn} \ldots{}
% \cs{ExplSyntaxOff} block, spaces are removed from both sides of each
% comma-delimited argument upon input.  Blank arguments are ignored, to
% allow for trailing commas or repeated commas (which may otherwise
% arise when concatenating comma lists \enquote{by hand}).  In addition,
% a set of braces is removed if the result of space-trimming is braced:
% this allows the storage of any item in a comma list.  For instance,
% \begin{verbatim}
%   \clist_new:N \l_my_clist
%   \clist_put_left:Nn \l_my_clist { ~a~ , ~{b}~ , c~\d }
%   \clist_put_right:Nn \l_my_clist { ~{e~} , , {{f}} , }
% \end{verbatim}
% results in |\l_my_clist| containing |a,b,c~\d,{e~},{{f}}| namely the
% five items |a|, |b|, |c~\d|, |e~| and~|{f}|.  Comma lists normally do
% not contain empty items so the following gives an empty comma list:
% \begin{verbatim}
%   \clist_clear_new:N \l_my_clist
%   \clist_put_right:Nn \l_my_clist { , ~ , , }
%   \clist_if_empty:NTF \l_my_clist { true } { false }
% \end{verbatim}
% and it leaves \texttt{true} in the input stream.  To include an
% \enquote{unsafe} item (empty, or one that contains a comma, or starts
% or ends with a space, or is a single brace group), surround it with
% braces.
%
% Almost all operations on comma lists are
% noticeably slower than those on sequences so converting the data to
% sequences using \cs{seq_set_from_clist:Nn} (see \pkg{l3seq}) may be
% advisable if speed is important.  The exception is that
% \cs{clist_if_in:NnTF} and \cs{clist_remove_duplicates:N} may be faster
% than their sequence analogues for large lists.  However, these
% functions work slowly for \enquote{unsafe} items that must be braced,
% and may produce errors when their argument contains |{|, |}| or |#|
% (assuming the usual \TeX{} category codes apply).  The sequence
% data type should thus certainly be preferred to comma lists to store
% such items.
%
% \section{Creating and initialising comma lists}
%
% \begin{function}{\clist_new:N, \clist_new:c}
%   \begin{syntax}
%     \cs{clist_new:N} \meta{comma list}
%   \end{syntax}
%   Creates a new \meta{comma list} or raises an error if the name is
%   already taken. The declaration is global. The \meta{comma list}
%   initially contains no items.
% \end{function}
%
% \begin{function}[added = 2014-07-05]
%   {
%     \clist_const:Nn, \clist_const:Nx,
%     \clist_const:cn, \clist_const:cx
%   }
%   \begin{syntax}
%     \cs{clist_const:Nn} \meta{clist~var} \Arg{comma list}
%   \end{syntax}
%   Creates a new constant \meta{clist~var} or raises an error
%   if the name is already taken. The value of the
%   \meta{clist~var} is set globally to the
%   \meta{comma list}.
% \end{function}
%
% \begin{function}
%   {\clist_clear:N, \clist_clear:c, \clist_gclear:N, \clist_gclear:c}
%   \begin{syntax}
%     \cs{clist_clear:N} \meta{comma list}
%   \end{syntax}
%   Clears all items from the \meta{comma list}.
% \end{function}
%
% \begin{function}
%   {
%     \clist_clear_new:N,  \clist_clear_new:c,
%     \clist_gclear_new:N, \clist_gclear_new:c
%   }
%   \begin{syntax}
%     \cs{clist_clear_new:N} \meta{comma list}
%   \end{syntax}
%   Ensures that the \meta{comma list} exists globally by applying
%   \cs{clist_new:N} if necessary, then applies
%   \cs[index=clist_clear:N]{clist_(g)clear:N} to leave
%   the list empty.
% \end{function}
%
% \begin{function}
%   {
%     \clist_set_eq:NN,  \clist_set_eq:cN,
%     \clist_set_eq:Nc,  \clist_set_eq:cc,
%     \clist_gset_eq:NN, \clist_gset_eq:cN,
%     \clist_gset_eq:Nc, \clist_gset_eq:cc
%   }
%   \begin{syntax}
%     \cs{clist_set_eq:NN} \meta{comma list_1} \meta{comma list_2}
%   \end{syntax}
%   Sets the content of \meta{comma list_1} equal to that of
%   \meta{comma list_2}.
% \end{function}
%
% \begin{function}[added = 2014-07-17]
%   {
%     \clist_set_from_seq:NN,  \clist_set_from_seq:cN,
%     \clist_set_from_seq:Nc,  \clist_set_from_seq:cc,
%     \clist_gset_from_seq:NN, \clist_gset_from_seq:cN,
%     \clist_gset_from_seq:Nc, \clist_gset_from_seq:cc
%   }
%   \begin{syntax}
%     \cs{clist_set_from_seq:NN} \meta{comma list} \meta{sequence}
%   \end{syntax}
%   Converts the data in the \meta{sequence} into a \meta{comma list}:
%   the original \meta{sequence} is unchanged.
%   Items which contain either spaces or commas are surrounded by braces.
% \end{function}
%
% \begin{function}
%   {
%     \clist_concat:NNN,  \clist_concat:ccc,
%     \clist_gconcat:NNN, \clist_gconcat:ccc
%   }
%   \begin{syntax}
%     \cs{clist_concat:NNN} \meta{comma list_1} \meta{comma list_2} \meta{comma list_3}
%   \end{syntax}
%   Concatenates the content of \meta{comma list_2} and \meta{comma list_3}
%   together and saves the result in \meta{comma list_1}. The items in
%   \meta{comma list_2} are placed at the left side of the new comma list.
% \end{function}
%
% \begin{function}[EXP, pTF, added=2012-03-03]
%   {\clist_if_exist:N, \clist_if_exist:c}
%   \begin{syntax}
%     \cs{clist_if_exist_p:N} \meta{comma list}
%     \cs{clist_if_exist:NTF} \meta{comma list} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests whether the \meta{comma list} is currently defined.  This does
%   not check that the \meta{comma list} really is a comma list.
% \end{function}
%
% \section{Adding data to comma lists}
%
% \begin{function}[added = 2011-09-06]
%   {
%     \clist_set:Nn,  \clist_set:NV,
%     \clist_set:No,  \clist_set:Nx,
%     \clist_set:cn,  \clist_set:cV,
%     \clist_set:co,  \clist_set:cx,
%     \clist_gset:Nn, \clist_gset:NV,
%     \clist_gset:No, \clist_gset:Nx,
%     \clist_gset:cn, \clist_gset:cV,
%     \clist_gset:co, \clist_gset:cx
%   }
%   \begin{syntax}
%     \cs{clist_set:Nn} \meta{comma list} |{|\meta{item_1},\ldots{},\meta{item_n}|}|
%   \end{syntax}
%   Sets \meta{comma list} to contain the \meta{items},
%   removing any previous content from the variable.
%   Blank items are omitted, spaces are removed from both sides of each
%   item, then a set of braces is removed if the resulting space-trimmed
%   item is braced.
%   To store some \meta{tokens} as a single \meta{item} even if the
%   \meta{tokens} contain commas or spaces, add a set of braces:
%   \cs{clist_set:Nn} \meta{comma list} |{| \Arg{tokens} |}|.
% \end{function}
%
% \begin{function}[updated = 2011-09-05]
%   {
%     \clist_put_left:Nn,  \clist_put_left:NV,
%     \clist_put_left:No,  \clist_put_left:Nx,
%     \clist_put_left:cn,  \clist_put_left:cV,
%     \clist_put_left:co,  \clist_put_left:cx,
%     \clist_gput_left:Nn, \clist_gput_left:NV,
%     \clist_gput_left:No, \clist_gput_left:Nx,
%     \clist_gput_left:cn, \clist_gput_left:cV,
%     \clist_gput_left:co, \clist_gput_left:cx
%   }
%   \begin{syntax}
%     \cs{clist_put_left:Nn} \meta{comma list} |{|\meta{item_1},\ldots{},\meta{item_n}|}|
%   \end{syntax}
%   Appends the \meta{items} to the left of the \meta{comma list}.
%   Blank items are omitted, spaces are removed from both sides of each
%   item, then a set of braces is removed if the resulting space-trimmed
%   item is braced.
%   To append some \meta{tokens} as a single \meta{item} even if the
%   \meta{tokens} contain commas or spaces, add a set of braces:
%   \cs{clist_put_left:Nn} \meta{comma list} |{| \Arg{tokens} |}|.
% \end{function}
%
% \begin{function}[updated = 2011-09-05]
%   {
%     \clist_put_right:Nn,  \clist_put_right:NV,
%     \clist_put_right:No,  \clist_put_right:Nx,
%     \clist_put_right:cn,  \clist_put_right:cV,
%     \clist_put_right:co,  \clist_put_right:cx,
%     \clist_gput_right:Nn, \clist_gput_right:NV,
%     \clist_gput_right:No, \clist_gput_right:Nx,
%     \clist_gput_right:cn, \clist_gput_right:cV,
%     \clist_gput_right:co, \clist_gput_right:cx
%   }
%   \begin{syntax}
%     \cs{clist_put_right:Nn} \meta{comma list} |{|\meta{item_1},\ldots{},\meta{item_n}|}|
%   \end{syntax}
%   Appends the \meta{items} to the right of the \meta{comma list}.
%   Blank items are omitted, spaces are removed from both sides of each
%   item, then a set of braces is removed if the resulting space-trimmed
%   item is braced.
%   To append some \meta{tokens} as a single \meta{item} even if the
%   \meta{tokens} contain commas or spaces, add a set of braces:
%   \cs{clist_put_right:Nn} \meta{comma list} |{| \Arg{tokens} |}|.
% \end{function}
%
% \section{Modifying comma lists}
%
% While comma lists are normally used as ordered lists, it may be
% necessary to modify the content. The functions here may be used
% to update comma lists, while retaining the order of the unaffected
% entries.
%
% \begin{function}
%   {
%     \clist_remove_duplicates:N,  \clist_remove_duplicates:c,
%     \clist_gremove_duplicates:N, \clist_gremove_duplicates:c
%   }
%   \begin{syntax}
%     \cs{clist_remove_duplicates:N} \meta{comma list}
%   \end{syntax}
%   Removes duplicate items from the \meta{comma list}, leaving the
%   left most copy of each item in the \meta{comma list}.  The \meta{item}
%   comparison takes place on a token basis, as for \cs{tl_if_eq:nn(TF)}.
%   \begin{texnote}
%     This function iterates through every item in the \meta{comma list} and
%     does a comparison with the \meta{items} already checked. It is therefore
%     relatively slow with large comma lists.
%     Furthermore, it may fail if any of the items in the
%     \meta{comma list} contains |{|, |}|, or |#|
%     (assuming the usual \TeX{} category codes apply).
%   \end{texnote}
% \end{function}
%
% \begin{function}[updated = 2011-09-06]
%   {
%     \clist_remove_all:Nn,  \clist_remove_all:cn,
%     \clist_gremove_all:Nn, \clist_gremove_all:cn
%   }
%   \begin{syntax}
%     \cs{clist_remove_all:Nn} \meta{comma list} \Arg{item}
%   \end{syntax}
%   Removes every occurrence of \meta{item} from the \meta{comma list}.
%   The \meta{item} comparison takes place on a token basis, as for
%   \cs{tl_if_eq:nn(TF)}.
%   \begin{texnote}
%     The function may fail if the \meta{item} contains |{|, |}|, or |#|
%     (assuming the usual \TeX{} category codes apply).
%   \end{texnote}
% \end{function}
%
% \begin{function}[added = 2014-07-18]
%   {
%     \clist_reverse:N, \clist_reverse:c,
%     \clist_greverse:N, \clist_greverse:c
%   }
%   \begin{syntax}
%     \cs{clist_reverse:N} \meta{comma list}
%   \end{syntax}
%   Reverses the order of items stored in the \meta{comma list}.
% \end{function}
%
% \begin{function}[added = 2014-07-18]{\clist_reverse:n}
%   \begin{syntax}
%     \cs{clist_reverse:n} \Arg{comma list}
%   \end{syntax}
%   Leaves the items in the \meta{comma list} in the input stream in
%   reverse order.  Contrarily to other what is done for other
%   \texttt{n}-type \meta{comma list} arguments, braces and spaces are
%   preserved by this process.
%   \begin{texnote}
%     The result is returned within \tn{unexpanded}, which means that the
%     comma list does not expand further when appearing in an
%     \texttt{x}-type or \texttt{e}-type argument expansion.
%   \end{texnote}
% \end{function}
%
% \begin{function}[added = 2017-02-06]
%   {\clist_sort:Nn, \clist_sort:cn, \clist_gsort:Nn, \clist_gsort:cn}
%   \begin{syntax}
%     \cs{clist_sort:Nn} \meta{clist var} \Arg{comparison code}
%   \end{syntax}
%   Sorts the items in the \meta{clist var} according to the
%   \meta{comparison code}, and assigns the result to
%   \meta{clist var}. The details of sorting comparison are
%   described in Section~\ref{sec:l3sort:mech}.
% \end{function}
%
% \section{Comma list conditionals}
%
% \begin{function}[EXP,pTF]{\clist_if_empty:N, \clist_if_empty:c}
%   \begin{syntax}
%     \cs{clist_if_empty_p:N} \meta{comma list}
%     \cs{clist_if_empty:NTF} \meta{comma list} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{comma list} is empty (containing no items).
% \end{function}
%
% \begin{function}[EXP, pTF, added = 2014-07-05]{\clist_if_empty:n}
%   \begin{syntax}
%     \cs{clist_if_empty_p:n} \Arg{comma list}
%     \cs{clist_if_empty:nTF} \Arg{comma list} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{comma list} is empty (containing no items).
%   The rules for space trimming are as for other \texttt{n}-type
%   comma-list functions, hence the comma list |{~,~,,~}| (without
%   outer braces) is empty, while |{~,{},}| (without outer braces)
%   contains one element, which happens to be empty: the comma-list
%   is not empty.
% \end{function}
%
% \begin{function}[updated = 2011-09-06, TF]
%   {
%      \clist_if_in:Nn, \clist_if_in:NV, \clist_if_in:No,
%      \clist_if_in:cn, \clist_if_in:cV, \clist_if_in:co,
%      \clist_if_in:nn, \clist_if_in:nV, \clist_if_in:no
%   }
%   \begin{syntax}
%     \cs{clist_if_in:NnTF} \meta{comma list} \Arg{item} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{item} is present in the \meta{comma list}.
%   In the case of an \texttt{n}-type \meta{comma list}, the usual rules
%   of space trimming and brace stripping apply.  Hence,
%   \begin{verbatim}
%     \clist_if_in:nnTF { a , {b}~ , {b} , c } { b } {true} {false}
%   \end{verbatim}
%   yields \texttt{true}.
%   \begin{texnote}
%     The function may fail if the \meta{item} contains |{|, |}|, or |#|
%     (assuming the usual \TeX{} category codes apply).
%   \end{texnote}
% \end{function}
%
% \section{Mapping to comma lists}
%
% The functions described in this section apply a specified function
% to each item of a comma list.
% All mappings are done at the current group level, \emph{i.e.}~any
% local assignments made by the \meta{function} or \meta{code} discussed
% below remain in effect after the loop.
%
% When the comma list is given explicitly, as an \texttt{n}-type argument,
% spaces are trimmed around each item.
% If the result of trimming spaces is empty, the item is ignored.
% Otherwise, if the item is surrounded by braces, one set is removed,
% and the result is passed to the mapped function. Thus, if the
% comma list that is being mapped is \verb*|{a , {{b} }, ,{}, {c},}|
% then the arguments passed to the mapped function are
% `\verb*|a|', `\verb*|{b} |', an empty argument, and `\verb*|c|'.
%
% When the comma list is given as an \texttt{N}-type argument, spaces
% have already been trimmed on input, and items are simply stripped
% of one set of braces if any. This case is more efficient than using
% \texttt{n}-type comma lists.
%
% \begin{function}[rEXP, updated = 2012-06-29]
%   {\clist_map_function:NN, \clist_map_function:cN, \clist_map_function:nN}
%   \begin{syntax}
%     \cs{clist_map_function:NN} \meta{comma list} \meta{function}
%   \end{syntax}
%   Applies \meta{function} to every \meta{item} stored in the
%   \meta{comma list}. The \meta{function} receives one argument for
%   each iteration. The \meta{items} are returned from left to right.
%   The function \cs{clist_map_inline:Nn} is in general more efficient
%   than \cs{clist_map_function:NN}.
% \end{function}
%
% \begin{function}[updated = 2012-06-29]
%   {\clist_map_inline:Nn, \clist_map_inline:cn, \clist_map_inline:nn}
%   \begin{syntax}
%     \cs{clist_map_inline:Nn} \meta{comma list} \Arg{inline function}
%   \end{syntax}
%   Applies \meta{inline function} to every \meta{item} stored
%   within the \meta{comma list}. The \meta{inline function} should
%   consist of code which receives the \meta{item} as |#1|.
%   The \meta{items} are returned from left to right.
% \end{function}
%
% \begin{function}[updated = 2012-06-29]
%   {\clist_map_variable:NNn, \clist_map_variable:cNn, \clist_map_variable:nNn}
%   \begin{syntax}
%     \cs{clist_map_variable:NNn} \meta{comma list} \meta{variable} \Arg{code}
%   \end{syntax}
%   Stores each \meta{item} of the \meta{comma list} in turn in the
%   (token list) \meta{variable} and applies the \meta{code}.  The
%   \meta{code} will usually make use of the \meta{variable}, but this
%   is not enforced.  The assignments to the \meta{variable} are local.
%   Its value after the loop is the last \meta{item} in the \meta{comma
%   list}, or its original value if there were no \meta{item}.  The
%   \meta{items} are returned from left to right.
% \end{function}
%
% \begin{function}[rEXP, updated = 2012-06-29]{\clist_map_break:}
%   \begin{syntax}
%     \cs{clist_map_break:}
%   \end{syntax}
%   Used to terminate a \cs[no-index]{clist_map_\ldots{}} function before all
%   entries in the \meta{comma list} have been processed. This
%   normally takes place within a conditional statement, for example
%   \begin{verbatim}
%     \clist_map_inline:Nn \l_my_clist
%       {
%         \str_if_eq:nnTF { #1 } { bingo }
%           { \clist_map_break: }
%           {
%             % Do something useful
%           }
%       }
%   \end{verbatim}
%   Use outside of a \cs[no-index]{clist_map_\ldots{}} scenario leads to low
%   level \TeX{} errors.
%   \begin{texnote}
%     When the mapping is broken, additional tokens may be inserted
%     before further items are taken
%     from the input stream. This depends on the design of the mapping
%     function.
%   \end{texnote}
% \end{function}
%
% \begin{function}[updated = 2012-06-29, rEXP]{\clist_map_break:n}
%   \begin{syntax}
%     \cs{clist_map_break:n} \Arg{code}
%   \end{syntax}
%   Used to terminate a \cs[no-index]{clist_map_\ldots{}} function before all
%   entries in the \meta{comma list} have been processed, inserting
%   the \meta{code} after the mapping has ended. This
%   normally takes place within a conditional statement, for example
%   \begin{verbatim}
%     \clist_map_inline:Nn \l_my_clist
%       {
%         \str_if_eq:nnTF { #1 } { bingo }
%           { \clist_map_break:n { <code> } }
%           {
%             % Do something useful
%           }
%       }
%   \end{verbatim}
%   Use outside of a \cs[no-index]{clist_map_\ldots{}} scenario leads to low
%   level \TeX{} errors.
%   \begin{texnote}
%     When the mapping is broken, additional tokens may be inserted
%     before the \meta{code} is
%     inserted into the input stream.
%     This depends on the design of the mapping function.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP, added = 2012-07-13]
%   {\clist_count:N, \clist_count:c, \clist_count:n}
%   \begin{syntax}
%     \cs{clist_count:N} \meta{comma list}
%   \end{syntax}
%   Leaves the number of items in the \meta{comma list} in the input
%   stream as an \meta{integer denotation}. The total number of items
%   in a \meta{comma list} includes those which are duplicates,
%   \emph{i.e.}~every item in a \meta{comma list} is counted.
% \end{function}
%
% \section{Using the content of comma lists directly}
%
% \begin{function}[EXP, added = 2013-05-26]{\clist_use:Nnnn, \clist_use:cnnn}
%   \begin{syntax}
%     \cs{clist_use:Nnnn} \meta{clist~var} \Arg{separator~between~two} \Arg{separator~between~more~than~two} \Arg{separator~between~final~two}
%   \end{syntax}
%   Places the contents of the \meta{clist~var} in the input stream,
%   with the appropriate \meta{separator} between the items.  Namely, if
%   the comma list has more than two items, the \meta{separator between
%     more than two} is placed between each pair of items except the
%   last, for which the \meta{separator between final two} is used.  If
%   the comma list has exactly two items, then they are placed in the input
%   stream separated by the \meta{separator between two}.  If the comma
%   list has a single item, it is placed in the input stream, and a comma
%   list with no items produces no output.  An error is raised if
%   the variable does not exist or if it is invalid.
%
%   For example,
%   \begin{verbatim}
%     \clist_set:Nn \l_tmpa_clist { a , b , , c , {de} , f }
%     \clist_use:Nnnn \l_tmpa_clist { ~and~ } { ,~ } { ,~and~ }
%   \end{verbatim}
%   inserts \enquote{\texttt{a, b, c, de, and f}} in the input
%   stream.  The first separator argument is not used in this case
%   because the comma list has more than $2$ items.
%   \begin{texnote}
%     The result is returned within the \tn{unexpanded}
%     primitive (\cs{exp_not:n}), which means that the \meta{items}
%     do not expand further when appearing in an \texttt{x}-type
%     argument expansion.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP, added = 2013-05-26]{\clist_use:Nn, \clist_use:cn}
%   \begin{syntax}
%     \cs{clist_use:Nn} \meta{clist~var} \Arg{separator}
%   \end{syntax}
%   Places the contents of the \meta{clist~var} in the input stream,
%   with the \meta{separator} between the items. If the comma
%   list has a single item, it is placed in the input stream, and a comma
%   list with no items produces no output.  An error is raised if
%   the variable does not exist or if it is invalid.
%
%   For example,
%   \begin{verbatim}
%     \clist_set:Nn \l_tmpa_clist { a , b , , c , {de} , f }
%     \clist_use:Nn \l_tmpa_clist { ~and~ }
%   \end{verbatim}
%   inserts \enquote{\texttt{a and b and c and de and f}} in the input
%   stream.
%   \begin{texnote}
%     The result is returned within the \tn{unexpanded}
%     primitive (\cs{exp_not:n}), which means that the \meta{items}
%     do not expand further when appearing in an \texttt{x}-type
%     argument expansion.
%   \end{texnote}
% \end{function}
%
% \section{Comma lists as stacks}
%
% Comma lists can be used as stacks, where data is pushed to and popped
% from the top of the comma list. (The left of a comma list is the top, for
% performance reasons.) The stack functions for comma lists are not
% intended to be mixed with the general ordered data functions detailed
% in the previous section: a comma list should either be used as an
% ordered data type or as a stack, but not in both ways.
%
% \begin{function}[noTF, added = 2012-05-14, updated = 2019-02-16]
%   {\clist_get:NN, \clist_get:cN}
%   \begin{syntax}
%     \cs{clist_get:NN} \meta{comma list} \meta{token list variable}
%   \end{syntax}
%   Stores the left-most item from a \meta{comma list} in the
%   \meta{token list variable} without removing it from the
%   \meta{comma list}. The \meta{token list variable} is assigned locally.
%   In the non-branching version, if the \meta{comma list} is empty the
%   \meta{token list variable} is set to the marker value \cs{q_no_value}.
% \end{function}
%
% \begin{function}[updated = 2011-09-06]{\clist_pop:NN, \clist_pop:cN}
%   \begin{syntax}
%     \cs{clist_pop:NN} \meta{comma list} \meta{token list variable}
%   \end{syntax}
%   Pops the left-most item from a \meta{comma list} into the
%   \meta{token list variable}, \emph{i.e.}~removes the item from the
%   comma list and stores it in the \meta{token list variable}.
%   Both of the variables are assigned locally.
% \end{function}
%
% \begin{function}{\clist_gpop:NN, \clist_gpop:cN}
%   \begin{syntax}
%     \cs{clist_gpop:NN} \meta{comma list} \meta{token list variable}
%   \end{syntax}
%   Pops the left-most item from a \meta{comma list} into the
%   \meta{token list variable}, \emph{i.e.}~removes the item from the
%   comma list and stores it in the \meta{token list variable}.
%   The \meta{comma list} is modified globally, while the assignment of
%   the \meta{token list variable} is local.
% \end{function}
%
% \begin{function}[TF, added = 2012-05-14]{\clist_pop:NN, \clist_pop:cN}
%   \begin{syntax}
%     \cs{clist_pop:NNTF} \meta{comma list} \meta{token list variable} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   If the \meta{comma list} is empty, leaves the \meta{false code} in the
%   input stream.  The value of the \meta{token list variable} is
%   not defined in this case and should not be relied upon.  If the
%   \meta{comma list} is non-empty, pops the top item from the
%   \meta{comma list} in the \meta{token list variable}, \emph{i.e.}~removes
%   the item from the \meta{comma list}. Both the \meta{comma list} and the
%   \meta{token list variable} are assigned locally.
% \end{function}
%
% \begin{function}[TF, added = 2012-05-14]{\clist_gpop:NN, \clist_gpop:cN}
%   \begin{syntax}
%     \cs{clist_gpop:NNTF} \meta{comma list} \meta{token list variable} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   If the \meta{comma list} is empty, leaves the \meta{false code} in the
%   input stream.  The value of the \meta{token list variable} is
%   not defined in this case and should not be relied upon.  If the
%   \meta{comma list} is non-empty, pops the top item from the
%   \meta{comma list} in the \meta{token list variable}, \emph{i.e.}~removes
%   the item from the \meta{comma list}. The \meta{comma list} is modified
%   globally, while the \meta{token list variable} is assigned locally.
% \end{function}
%
% \begin{function}
%   {
%     \clist_push:Nn,  \clist_push:NV,  \clist_push:No,  \clist_push:Nx,
%     \clist_push:cn,  \clist_push:cV,  \clist_push:co,  \clist_push:cx,
%     \clist_gpush:Nn, \clist_gpush:NV, \clist_gpush:No, \clist_gpush:Nx,
%     \clist_gpush:cn, \clist_gpush:cV, \clist_gpush:co, \clist_gpush:cx
%   }
%   \begin{syntax}
%     \cs{clist_push:Nn} \meta{comma list} \Arg{items}
%   \end{syntax}
%   Adds the \Arg{items} to the top of the \meta{comma list}.
%   Spaces are removed from both sides of each item as for any
%   \texttt{n}-type comma list.
% \end{function}
%
% \section{Using a single item}
%
% \begin{function}[added = 2014-07-17, EXP]
%   {\clist_item:Nn, \clist_item:cn, \clist_item:nn}
%   \begin{syntax}
%     \cs{clist_item:Nn} \meta{comma list} \Arg{integer expression}
%   \end{syntax}
%   Indexing items in the \meta{comma list} from~$1$ at the top (left), this
%   function evaluates the \meta{integer expression} and leaves the
%   appropriate item from the comma list in the input stream. If the
%   \meta{integer expression} is negative, indexing occurs from the
%   bottom (right) of the comma list. When the \meta{integer expression}
%   is larger than the number of items in the \meta{comma list} (as
%   calculated by \cs{clist_count:N}) then the function expands to
%   nothing.
%   \begin{texnote}
%     The result is returned within the \tn{unexpanded}
%     primitive (\cs{exp_not:n}), which means that the \meta{item}
%     does not expand further when appearing in an \texttt{x}-type
%     argument expansion.
%   \end{texnote}
% \end{function}
%
% \begin{function}[EXP, added = 2016-12-06]
%   {\clist_rand_item:N, \clist_rand_item:n, \clist_rand_item:c}
%   \begin{syntax}
%     \cs{clist_rand_item:N} \meta{clist~var}
%     \cs{clist_rand_item:n} \Arg{comma list}
%   \end{syntax}
%   Selects a pseudo-random item of the \meta{comma list}.  If the
%   \meta{comma list} has no item, the result is empty.
%   \begin{texnote}
%     The result is returned within the \tn{unexpanded}
%     primitive (\cs{exp_not:n}), which means that the \meta{item}
%     does not expand further when appearing in an \texttt{x}-type
%     argument expansion.
%   \end{texnote}
% \end{function}
%
% \section{Viewing comma lists}
%
% \begin{function}[updated = 2015-08-03]{\clist_show:N, \clist_show:c}
%   \begin{syntax}
%     \cs{clist_show:N} \meta{comma list}
%   \end{syntax}
%   Displays the entries in the \meta{comma list} in the terminal.
% \end{function}
%
% \begin{function}[updated = 2013-08-03]{\clist_show:n}
%   \begin{syntax}
%     \cs{clist_show:n} \Arg{tokens}
%   \end{syntax}
%   Displays the entries in the comma list in the terminal.
% \end{function}
%
% \begin{function}[added = 2014-08-22, updated = 2015-08-03]{\clist_log:N, \clist_log:c}
%   \begin{syntax}
%     \cs{clist_log:N} \meta{comma list}
%   \end{syntax}
%   Writes the entries in the \meta{comma list} in the log file.  See
%   also \cs{clist_show:N} which displays the result in the terminal.
% \end{function}
%
% \begin{function}[added = 2014-08-22]{\clist_log:n}
%   \begin{syntax}
%     \cs{clist_log:n} \Arg{tokens}
%   \end{syntax}
%   Writes the entries in the comma list in the log file.  See also
%   \cs{clist_show:n} which displays the result in the terminal.
% \end{function}
%
% \section{Constant and scratch comma lists}
%
% \begin{variable}[added = 2012-07-02]{\c_empty_clist}
%   Constant that is always empty.
% \end{variable}
%
% \begin{variable}[added = 2011-09-06]{\l_tmpa_clist, \l_tmpb_clist}
%   Scratch comma lists for local assignment. These are never used by
%   the kernel code, and so are safe for use with any \LaTeX3-defined
%   function. However, they may be overwritten by other non-kernel
%   code and so should only be used for short-term storage.
% \end{variable}
%
% \begin{variable}[added = 2011-09-06]{\g_tmpa_clist, \g_tmpb_clist}
%   Scratch comma lists for global assignment. These are never used by
%   the kernel code, and so are safe for use with any \LaTeX3-defined
%   function. However, they may be overwritten by other non-kernel
%   code and so should only be used for short-term storage.
% \end{variable}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3clist} implementation}
%
% \TestFiles{m3clist002}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
%    \begin{macrocode}
%<@@=clist>
%    \end{macrocode}
%
% \begin{variable}{\c_empty_clist}
%   An empty comma list is simply an empty token list.
%    \begin{macrocode}
\cs_new_eq:NN \c_empty_clist \c_empty_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_internal_clist}
%   Scratch space for various internal uses. This comma list variable
%   cannot be declared as such because it comes before \cs{clist_new:N}
%    \begin{macrocode}
\tl_new:N \l_@@_internal_clist
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\s_@@_mark,\s_@@_stop}
%   Internal scan marks.
%    \begin{macrocode}
\scan_new:N \s_@@_mark
\scan_new:N \s_@@_stop
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}[EXP]{
%     \@@_use_none_delimit_by_s_stop:w,
%     \@@_use_i_delimit_by_s_stop:nw
%   }
%   Functions to gobble up to a scan mark.
%    \begin{macrocode}
\cs_new:Npn \@@_use_none_delimit_by_s_stop:w #1 \s_@@_stop { }
\cs_new:Npn \@@_use_i_delimit_by_s_stop:nw #1 #2 \s_@@_stop {#1}
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\q_@@_recursion_tail,\q_@@_recursion_stop}
%   Internal recursion quarks.
%    \begin{macrocode}
\quark_new:N \q_@@_recursion_tail
\quark_new:N \q_@@_recursion_stop
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}[EXP]{\@@_if_recursion_tail_break:nN,\@@_if_recursion_tail_stop:n}
%   Functions to query recursion quarks.
%    \begin{macrocode}
\__kernel_quark_new_test:N \@@_if_recursion_tail_break:nN
\__kernel_quark_new_test:N \@@_if_recursion_tail_stop:n
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_tmp:w}
%   A temporary function for various purposes.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_tmp:w { }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Removing spaces around items}
%
% \begin{macro}{\@@_trim_next:w}
%   Called as \cs{exp:w} \cs{@@_trim_next:w} \cs{prg_do_nothing:}
%   \meta{comma list} \ldots{} it expands to \Arg{trimmed item} where
%   the \meta{trimmed item} is the first non-empty result from removing
%   spaces from both ends of comma-delimited items in the \meta{comma
%   list}.  The \cs{prg_do_nothing:} marker avoids losing braces.  The
%   test for blank items is a somewhat optimized \cs{tl_if_empty:oTF}
%   construction; if blank, another item is sought, otherwise trim
%   spaces.
%    \begin{macrocode}
\cs_new:Npn \@@_trim_next:w #1 ,
  {
    \tl_if_empty:oTF { \use_none:nn #1 ? }
      { \@@_trim_next:w \prg_do_nothing: }
      { \tl_trim_spaces_apply:oN {#1} \exp_end: }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[rEXP]{\@@_sanitize:n}
% \begin{macro}{\@@_sanitize:Nn}
%   The auxiliary \cs{@@_sanitize:Nn} receives a delimiter
%   (\cs{c_empty_tl} the first time, afterwards a comma) and that item
%   as arguments.  Unless we are done with the loop it calls
%   \cs{@@_wrap_item:w} to unbrace the item (using a comma delimiter is
%   safe since |#2| came from removing spaces from an argument delimited
%   by a comma) and possibly re-brace it if needed.
%    \begin{macrocode}
\cs_new:Npn \@@_sanitize:n #1
  {
    \exp_after:wN \@@_sanitize:Nn \exp_after:wN \c_empty_tl
    \exp:w \@@_trim_next:w \prg_do_nothing:
    #1 , \q_@@_recursion_tail , \q_@@_recursion_stop
  }
\cs_new:Npn \@@_sanitize:Nn #1#2
  {
    \@@_if_recursion_tail_stop:n {#2}
    #1 \@@_wrap_item:w #2 ,
    \exp_after:wN \@@_sanitize:Nn \exp_after:wN ,
    \exp:w \@@_trim_next:w \prg_do_nothing:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[TF]{\@@_if_wrap:n}
% \begin{macro}{\@@_if_wrap:w}
%   True if the argument must be wrapped to avoid getting altered by some
%   clist operations.  That is the case whenever the argument
%   \begin{itemize}
%   \item starts or end with a space or contains a comma,
%   \item is empty, or
%   \item consists of a single braced group.
%   \end{itemize}
%   All \pkg{l3clist} functions go through the same test when they need
%   to determine whether to brace an item, so it is not a problem that
%   this test has false positives such as \enquote{\cs{s_@@_mark}
%   \texttt{?}}.  If the argument starts or end with a space or contains
%   a comma then one of the three arguments of \cs{@@_if_wrap:w} will
%   have its end delimiter (partly) in one of the three copies of |#1|
%   in \cs{@@_if_wrap:nTF}; this has a knock-on effect meaning that the
%   result of the expansion is not empty; in that case, wrap.
%   Otherwise, the argument is safe unless it starts with a brace group
%   (or is empty) and it is empty or consists of a single
%   \texttt{n}-type argument.
%    \begin{macrocode}
\prg_new_conditional:Npnn \@@_if_wrap:n #1 { TF }
  {
    \tl_if_empty:oTF
      {
        \@@_if_wrap:w
          \s_@@_mark ? #1 ~ \s_@@_mark ? ~ #1
          \s_@@_mark , ~ \s_@@_mark #1 ,
      }
      {
        \tl_if_head_is_group:nTF { #1 { } }
          {
            \tl_if_empty:nTF {#1}
              { \prg_return_true: }
              {
                \tl_if_empty:oTF { \use_none:n #1}
                  { \prg_return_true: }
                  { \prg_return_false: }
              }
          }
          { \prg_return_false: }
      }
      { \prg_return_true: }
  }
\cs_new:Npn \@@_if_wrap:w #1 \s_@@_mark ? ~ #2 ~ \s_@@_mark #3 , { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_wrap_item:w}
%   Safe items are put in \cs{exp_not:n}, otherwise we put an extra set
%   of braces.
%    \begin{macrocode}
\cs_new:Npn \@@_wrap_item:w #1 ,
  { \@@_if_wrap:nTF {#1} { \exp_not:n { {#1} } } { \exp_not:n {#1} } }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Allocation and initialisation}
%
% \begin{macro}{\clist_new:N, \clist_new:c}
% \UnitTested
%   Internally, comma lists are just token lists.
%    \begin{macrocode}
\cs_new_eq:NN \clist_new:N \tl_new:N
\cs_new_eq:NN \clist_new:c \tl_new:c
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {
%     \clist_const:Nn, \clist_const:cn,
%     \clist_const:Nx, \clist_const:cx
%   }
%   Creating and initializing a constant comma list is done by
%   sanitizing all items (stripping spaces and braces).
%    \begin{macrocode}
\cs_new_protected:Npn \clist_const:Nn #1#2
  { \tl_const:Nx #1 { \@@_sanitize:n {#2} } }
\cs_generate_variant:Nn \clist_const:Nn { c , Nx , cx }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\clist_clear:N, \clist_clear:c}
% \UnitTested
% \begin{macro}{\clist_gclear:N, \clist_gclear:c}
% \UnitTested
%   Clearing comma lists is just the same as clearing token lists.
%    \begin{macrocode}
\cs_new_eq:NN \clist_clear:N  \tl_clear:N
\cs_new_eq:NN \clist_clear:c  \tl_clear:c
\cs_new_eq:NN \clist_gclear:N \tl_gclear:N
\cs_new_eq:NN \clist_gclear:c \tl_gclear:c
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_clear_new:N, \clist_clear_new:c}
% \UnitTested
% \begin{macro}{\clist_gclear_new:N, \clist_gclear_new:c}
% \UnitTested
%   Once again a copy from the token list functions.
%    \begin{macrocode}
\cs_new_eq:NN \clist_clear_new:N  \tl_clear_new:N
\cs_new_eq:NN \clist_clear_new:c  \tl_clear_new:c
\cs_new_eq:NN \clist_gclear_new:N \tl_gclear_new:N
\cs_new_eq:NN \clist_gclear_new:c \tl_gclear_new:c
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\clist_set_eq:NN, \clist_set_eq:cN, \clist_set_eq:Nc, \clist_set_eq:cc}
% \UnitTested
% \begin{macro}
%   {
%     \clist_gset_eq:NN, \clist_gset_eq:cN,
%     \clist_gset_eq:Nc, \clist_gset_eq:cc
%   }
% \UnitTested
%   Once again, these are simple copies from the token list functions.
%    \begin{macrocode}
\cs_new_eq:NN \clist_set_eq:NN  \tl_set_eq:NN
\cs_new_eq:NN \clist_set_eq:Nc  \tl_set_eq:Nc
\cs_new_eq:NN \clist_set_eq:cN  \tl_set_eq:cN
\cs_new_eq:NN \clist_set_eq:cc  \tl_set_eq:cc
\cs_new_eq:NN \clist_gset_eq:NN \tl_gset_eq:NN
\cs_new_eq:NN \clist_gset_eq:Nc \tl_gset_eq:Nc
\cs_new_eq:NN \clist_gset_eq:cN \tl_gset_eq:cN
\cs_new_eq:NN \clist_gset_eq:cc \tl_gset_eq:cc
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \clist_set_from_seq:NN, \clist_set_from_seq:cN,
%     \clist_set_from_seq:Nc, \clist_set_from_seq:cc
%   }
% \UnitTested
% \begin{macro}
%   {
%     \clist_gset_from_seq:NN, \clist_gset_from_seq:cN,
%     \clist_gset_from_seq:Nc, \clist_gset_from_seq:cc
%   }
% \UnitTested
% \begin{macro}{\@@_set_from_seq:NNNN, \@@_set_from_seq:n}
%   Setting a comma list from a comma-separated list is done using a
%   simple mapping.  Safe items are put in \cs{exp_not:n}, otherwise we
%   put an extra set of braces.  The first comma must be removed, except
%   in the case of an empty comma-list.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_set_from_seq:NN
  { \@@_set_from_seq:NNNN \clist_clear:N  \__kernel_tl_set:Nx  }
\cs_new_protected:Npn \clist_gset_from_seq:NN
  { \@@_set_from_seq:NNNN \clist_gclear:N \__kernel_tl_gset:Nx }
\cs_new_protected:Npn \@@_set_from_seq:NNNN #1#2#3#4
  {
    \seq_if_empty:NTF #4
      { #1 #3 }
      {
        #2 #3
          {
            \exp_after:wN \use_none:n \exp:w \exp_end_continue_f:w
            \seq_map_function:NN #4 \@@_set_from_seq:n
          }
      }
  }
\cs_new:Npn \@@_set_from_seq:n #1
  {
    ,
    \@@_if_wrap:nTF {#1}
      { \exp_not:n { {#1} } }
      { \exp_not:n {#1} }
  }
\cs_generate_variant:Nn \clist_set_from_seq:NN  {     Nc }
\cs_generate_variant:Nn \clist_set_from_seq:NN  { c , cc }
\cs_generate_variant:Nn \clist_gset_from_seq:NN {     Nc }
\cs_generate_variant:Nn \clist_gset_from_seq:NN { c , cc }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_concat:NNN, \clist_concat:ccc}
% \UnitTested
% \begin{macro}{\clist_gconcat:NNN, \clist_gconcat:ccc}
% \UnitTested
% \begin{macro}{\@@_concat:NNNN}
%   Concatenating comma lists is not quite as easy as it seems, as
%   there needs to be the correct addition of a comma to the output. So
%   a little work to do.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_concat:NNN
  { \@@_concat:NNNN \__kernel_tl_set:Nx }
\cs_new_protected:Npn \clist_gconcat:NNN
  { \@@_concat:NNNN \__kernel_tl_gset:Nx }
\cs_new_protected:Npn \@@_concat:NNNN #1#2#3#4
  {
    #1 #2
      {
        \exp_not:o #3
        \clist_if_empty:NF #3 { \clist_if_empty:NF #4 { , } }
        \exp_not:o #4
      }
  }
\cs_generate_variant:Nn \clist_concat:NNN  { ccc }
\cs_generate_variant:Nn \clist_gconcat:NNN { ccc }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[pTF]{\clist_if_exist:N, \clist_if_exist:c}
%   Copies of the \texttt{cs} functions defined in \pkg{l3basics}.
%    \begin{macrocode}
\prg_new_eq_conditional:NNn \clist_if_exist:N \cs_if_exist:N
  { TF , T , F , p }
\prg_new_eq_conditional:NNn \clist_if_exist:c \cs_if_exist:c
  { TF , T , F , p }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Adding data to comma lists}
%
% \begin{macro}
%   {
%     \clist_set:Nn, \clist_set:NV,
%     \clist_set:No, \clist_set:Nx,
%     \clist_set:cn, \clist_set:cV,
%     \clist_set:co, \clist_set:cx
%   }
% \begin{macro}
%   {
%     \clist_gset:Nn, \clist_gset:NV,
%     \clist_gset:No, \clist_gset:Nx,
%     \clist_gset:cn, \clist_gset:cV,
%     \clist_gset:co, \clist_gset:cx
%   }
%    \begin{macrocode}
\cs_new_protected:Npn \clist_set:Nn #1#2
  { \__kernel_tl_set:Nx #1 { \@@_sanitize:n {#2} } }
\cs_new_protected:Npn \clist_gset:Nn #1#2
  { \__kernel_tl_gset:Nx #1 { \@@_sanitize:n {#2} } }
\cs_generate_variant:Nn \clist_set:Nn  { NV , No , Nx , c , cV , co , cx }
\cs_generate_variant:Nn \clist_gset:Nn { NV , No , Nx , c , cV , co , cx }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \clist_put_left:Nn, \clist_put_left:NV,
%     \clist_put_left:No, \clist_put_left:Nx,
%     \clist_put_left:cn, \clist_put_left:cV,
%     \clist_put_left:co, \clist_put_left:cx
%   }
% \UnitTested
% \begin{macro}
%   {
%     \clist_gput_left:Nn, \clist_gput_left:NV,
%     \clist_gput_left:No, \clist_gput_left:Nx,
%     \clist_gput_left:cn, \clist_gput_left:cV,
%     \clist_gput_left:co, \clist_gput_left:cx
%   }
% \UnitTested
% \begin{macro}{\@@_put_left:NNNn}
%   Everything is based on concatenation after storing in
%   \cs{l_@@_internal_clist}.  This avoids having to worry here about
%   space-trimming and so on.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_put_left:Nn
  { \@@_put_left:NNNn \clist_concat:NNN \clist_set:Nn }
\cs_new_protected:Npn \clist_gput_left:Nn
  { \@@_put_left:NNNn \clist_gconcat:NNN \clist_set:Nn }
\cs_new_protected:Npn \@@_put_left:NNNn #1#2#3#4
  {
    #2 \l_@@_internal_clist {#4}
    #1 #3 \l_@@_internal_clist #3
  }
\cs_generate_variant:Nn \clist_put_left:Nn  {     NV , No , Nx }
\cs_generate_variant:Nn \clist_put_left:Nn  { c , cV , co , cx }
\cs_generate_variant:Nn \clist_gput_left:Nn {     NV , No , Nx }
\cs_generate_variant:Nn \clist_gput_left:Nn { c , cV , co , cx }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \clist_put_right:Nn, \clist_put_right:NV,
%     \clist_put_right:No, \clist_put_right:Nx,
%     \clist_put_right:cn, \clist_put_right:cV,
%     \clist_put_right:co, \clist_put_right:cx
%   }
% \UnitTested
% \begin{macro}
%   {
%     \clist_gput_right:Nn, \clist_gput_right:NV,
%     \clist_gput_right:No, \clist_gput_right:Nx,
%     \clist_gput_right:cn, \clist_gput_right:cV,
%     \clist_gput_right:co, \clist_gput_right:cx
%   }
% \UnitTested
% \begin{macro}{\@@_put_right:NNNn}
%    \begin{macrocode}
\cs_new_protected:Npn \clist_put_right:Nn
  { \@@_put_right:NNNn \clist_concat:NNN \clist_set:Nn }
\cs_new_protected:Npn \clist_gput_right:Nn
  { \@@_put_right:NNNn \clist_gconcat:NNN \clist_set:Nn }
\cs_new_protected:Npn \@@_put_right:NNNn #1#2#3#4
  {
    #2 \l_@@_internal_clist {#4}
    #1 #3 #3 \l_@@_internal_clist
  }
\cs_generate_variant:Nn \clist_put_right:Nn  {     NV , No , Nx }
\cs_generate_variant:Nn \clist_put_right:Nn  { c , cV , co , cx }
\cs_generate_variant:Nn \clist_gput_right:Nn {     NV , No , Nx }
\cs_generate_variant:Nn \clist_gput_right:Nn { c , cV , co , cx }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Comma lists as stacks}
%
% \begin{macro}{\clist_get:NN, \clist_get:cN}
% \UnitTested
% \begin{macro}{\@@_get:wN}
%   Getting an item from the left of a comma list is pretty easy: just
%   trim off the first item using the comma.  No need to trim spaces as
%   comma-list \emph{variables} are assumed to have \enquote{cleaned-up}
%   items.  (Note that grabbing a comma-delimited item removes an outer
%   pair of braces if present, exactly as needed to uncover the
%   underlying item.)
%    \begin{macrocode}
\cs_new_protected:Npn \clist_get:NN #1#2
  {
    \if_meaning:w #1 \c_empty_clist
      \tl_set:Nn #2 { \q_no_value }
    \else:
      \exp_after:wN \@@_get:wN #1 , \s_@@_stop #2
    \fi:
  }
\cs_new_protected:Npn \@@_get:wN #1 , #2 \s_@@_stop #3
  { \tl_set:Nn #3 {#1} }
\cs_generate_variant:Nn \clist_get:NN { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_pop:NN, \clist_pop:cN}
% \UnitTested
% \begin{macro}{\clist_gpop:NN, \clist_gpop:cN}
% \UnitTested
% \begin{macro}
%   {\@@_pop:NNN, \@@_pop:wwNNN, \@@_pop:wN}
%   An empty clist leads to \cs{q_no_value}, otherwise grab until the
%   first comma and assign to the variable.  The second argument of
%   \cs{@@_pop:wwNNN} is a comma list ending in a comma and
%   \cs{s_@@_mark}, unless the original clist contained exactly one item:
%   then the argument is just \cs{s_@@_mark}.  The next auxiliary picks
%   either \cs{exp_not:n} or \cs{use_none:n} as |#2|, ensuring that the
%   result can safely be an empty comma list.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_pop:NN
  { \@@_pop:NNN \__kernel_tl_set:Nx }
\cs_new_protected:Npn \clist_gpop:NN
  { \@@_pop:NNN \__kernel_tl_gset:Nx }
\cs_new_protected:Npn \@@_pop:NNN #1#2#3
  {
    \if_meaning:w #2 \c_empty_clist
      \tl_set:Nn #3 { \q_no_value }
    \else:
      \exp_after:wN \@@_pop:wwNNN #2 , \s_@@_mark \s_@@_stop #1#2#3
    \fi:
  }
\cs_new_protected:Npn \@@_pop:wwNNN #1 , #2 \s_@@_stop #3#4#5
  {
    \tl_set:Nn #5 {#1}
    #3 #4
      {
        \@@_pop:wN \prg_do_nothing:
          #2 \exp_not:o
          , \s_@@_mark \use_none:n
        \s_@@_stop
      }
  }
\cs_new:Npn \@@_pop:wN #1 , \s_@@_mark #2 #3 \s_@@_stop { #2 {#1} }
\cs_generate_variant:Nn \clist_pop:NN  { c }
\cs_generate_variant:Nn \clist_gpop:NN { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[TF]{\clist_get:NN, \clist_get:cN}
% \begin{macro}[TF]{\clist_pop:NN, \clist_pop:cN}
% \begin{macro}[TF]{\clist_gpop:NN, \clist_gpop:cN}
% \begin{macro}{\@@_pop_TF:NNN}
%   The same, as branching code: very similar to the above.
%    \begin{macrocode}
\prg_new_protected_conditional:Npnn \clist_get:NN #1#2 { T , F , TF }
  {
    \if_meaning:w #1 \c_empty_clist
      \prg_return_false:
    \else:
      \exp_after:wN \@@_get:wN #1 , \s_@@_stop #2
      \prg_return_true:
    \fi:
  }
\prg_generate_conditional_variant:Nnn \clist_get:NN { c } { T , F , TF }
\prg_new_protected_conditional:Npnn \clist_pop:NN #1#2 { T , F , TF }
  { \@@_pop_TF:NNN \__kernel_tl_set:Nx #1 #2 }
\prg_new_protected_conditional:Npnn \clist_gpop:NN #1#2 { T , F , TF }
  { \@@_pop_TF:NNN \__kernel_tl_gset:Nx #1 #2 }
\cs_new_protected:Npn \@@_pop_TF:NNN #1#2#3
  {
    \if_meaning:w #2 \c_empty_clist
      \prg_return_false:
    \else:
      \exp_after:wN \@@_pop:wwNNN #2 , \s_@@_mark \s_@@_stop #1#2#3
      \prg_return_true:
    \fi:
  }
\prg_generate_conditional_variant:Nnn \clist_pop:NN { c } { T , F , TF }
\prg_generate_conditional_variant:Nnn \clist_gpop:NN { c } { T , F , TF }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{
%   \clist_push:Nn, \clist_push:NV, \clist_push:No, \clist_push:Nx,
%   \clist_push:cn, \clist_push:cV, \clist_push:co, \clist_push:cx
% }
% \UnitTested
% \begin{macro}{
%   \clist_gpush:Nn, \clist_gpush:NV, \clist_gpush:No, \clist_gpush:Nx,
%   \clist_gpush:cn, \clist_gpush:cV, \clist_gpush:co, \clist_gpush:cx
% }
% \UnitTested
%   Pushing to a comma list is the same as adding on the left.
%    \begin{macrocode}
\cs_new_eq:NN \clist_push:Nn  \clist_put_left:Nn
\cs_new_eq:NN \clist_push:NV  \clist_put_left:NV
\cs_new_eq:NN \clist_push:No  \clist_put_left:No
\cs_new_eq:NN \clist_push:Nx  \clist_put_left:Nx
\cs_new_eq:NN \clist_push:cn  \clist_put_left:cn
\cs_new_eq:NN \clist_push:cV  \clist_put_left:cV
\cs_new_eq:NN \clist_push:co  \clist_put_left:co
\cs_new_eq:NN \clist_push:cx  \clist_put_left:cx
\cs_new_eq:NN \clist_gpush:Nn \clist_gput_left:Nn
\cs_new_eq:NN \clist_gpush:NV \clist_gput_left:NV
\cs_new_eq:NN \clist_gpush:No \clist_gput_left:No
\cs_new_eq:NN \clist_gpush:Nx \clist_gput_left:Nx
\cs_new_eq:NN \clist_gpush:cn \clist_gput_left:cn
\cs_new_eq:NN \clist_gpush:cV \clist_gput_left:cV
\cs_new_eq:NN \clist_gpush:co \clist_gput_left:co
\cs_new_eq:NN \clist_gpush:cx \clist_gput_left:cx
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Modifying comma lists}
%
% \begin{variable}{\l_@@_internal_remove_clist, \l_@@_internal_remove_seq}
%   An internal comma list and a sequence for the removal routines.
%    \begin{macrocode}
\clist_new:N \l_@@_internal_remove_clist
\seq_new:N \l_@@_internal_remove_seq
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\clist_remove_duplicates:N, \clist_remove_duplicates:c}
% \UnitTested
% \begin{macro}{\clist_gremove_duplicates:N, \clist_gremove_duplicates:c}
% \UnitTested
% \begin{macro}{\@@_remove_duplicates:NN}
%   Removing duplicates means making a new list then copying it.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_remove_duplicates:N
  { \@@_remove_duplicates:NN \clist_set_eq:NN }
\cs_new_protected:Npn \clist_gremove_duplicates:N
  { \@@_remove_duplicates:NN \clist_gset_eq:NN }
\cs_new_protected:Npn \@@_remove_duplicates:NN #1#2
  {
    \clist_clear:N \l_@@_internal_remove_clist
    \clist_map_inline:Nn #2
      {
        \clist_if_in:NnF \l_@@_internal_remove_clist {##1}
          { \clist_put_right:Nn \l_@@_internal_remove_clist {##1} }
      }
    #1 #2 \l_@@_internal_remove_clist
  }
\cs_generate_variant:Nn \clist_remove_duplicates:N  { c }
\cs_generate_variant:Nn \clist_gremove_duplicates:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_remove_all:Nn, \clist_remove_all:cn}
% \UnitTested
% \begin{macro}{\clist_gremove_all:Nn, \clist_gremove_all:cn}
% \UnitTested
% \begin{macro}{\@@_remove_all:NNNn}
% \begin{macro}{\@@_remove_all:w}
% \begin{macro}{\@@_remove_all:}
%   The method used here for safe items is very similar to
%   \cs{tl_replace_all:Nnn}.  However, if the item contains commas or
%   leading/trailing spaces, or is empty, or consists of a single brace
%   group, we know that it can only appear within braces so the code
%   would fail; instead just convert to a sequence and do the removal
%   with \pkg{l3seq} code (it involves somewhat elaborate code to do
%   most of the work expandably but the final token list comparisons
%   non-expandably).
%
%   For \enquote{safe} items, build a function delimited by the
%   \meta{item} that should be removed,
%   surrounded with commas, and call that function followed by
%   the expanded comma list, and another copy of the \meta{item}.
%   The loop is controlled by the argument grabbed by
%   \cs{@@_remove_all:w}: when the item was found,
%   the \cs{s_@@_mark} delimiter used is the one inserted by
%   \cs{@@_tmp:w}, and \cs{@@_use_none_delimit_by_s_stop:w}
%   is deleted. At the end, the final \meta{item} is
%   grabbed, and the argument of \cs{@@_tmp:w} contains
%   \cs{s_@@_mark}: in that case, \cs{@@_remove_all:w}
%   removes the second \cs{s_@@_mark} (inserted by \cs{@@_tmp:w}),
%   and lets \cs{@@_use_none_delimit_by_s_stop:w} act.
%
%   No brace is lost because items are always grabbed with a leading comma.
%   The result of the first assignment has an extra leading comma,
%   which we remove in a second assignment.
%   Two exceptions: if the clist lost all of its elements, the result
%   is empty, and we shouldn't remove anything; if the clist started up
%   empty, the first step happens to turn it into a single comma, and
%   the second step removes it.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_remove_all:Nn
  { \@@_remove_all:NNNn \clist_set_from_seq:NN \__kernel_tl_set:Nx }
\cs_new_protected:Npn \clist_gremove_all:Nn
  { \@@_remove_all:NNNn \clist_gset_from_seq:NN \__kernel_tl_gset:Nx }
\cs_new_protected:Npn \@@_remove_all:NNNn #1#2#3#4
  {
    \@@_if_wrap:nTF {#4}
      {
        \seq_set_from_clist:NN \l_@@_internal_remove_seq #3
        \seq_remove_all:Nn \l_@@_internal_remove_seq {#4}
        #1 #3 \l_@@_internal_remove_seq
      }
      {
        \cs_set:Npn \@@_tmp:w ##1 , #4 ,
          {
            ##1
            , \s_@@_mark , \@@_use_none_delimit_by_s_stop:w ,
            \@@_remove_all:
          }
        #2 #3
          {
            \exp_after:wN \@@_remove_all:
            #3 , \s_@@_mark , #4 , \s_@@_stop
          }
        \clist_if_empty:NF #3
          {
            #2 #3
              {
                \exp_args:No \exp_not:o
                  { \exp_after:wN \use_none:n #3 }
              }
          }
      }
  }
\cs_new:Npn \@@_remove_all:
  { \exp_after:wN \@@_remove_all:w \@@_tmp:w , }
\cs_new:Npn \@@_remove_all:w #1 , \s_@@_mark , #2 , { \exp_not:n {#1} }
\cs_generate_variant:Nn \clist_remove_all:Nn  { c }
\cs_generate_variant:Nn \clist_gremove_all:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \clist_reverse:N, \clist_reverse:c,
%     \clist_greverse:N, \clist_greverse:c
%   }
%   Use \cs{clist_reverse:n} in an \texttt{x}-expanding assignment.  The
%   extra work that \cs{clist_reverse:n} does to preserve braces and
%   spaces would not be needed for the well-controlled case of
%   \texttt{N}-type comma lists, but the slow-down is not too bad.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_reverse:N #1
  { \__kernel_tl_set:Nx #1 { \exp_args:No \clist_reverse:n {#1} } }
\cs_new_protected:Npn \clist_greverse:N #1
  { \__kernel_tl_gset:Nx #1 { \exp_args:No \clist_reverse:n {#1} } }
\cs_generate_variant:Nn \clist_reverse:N { c }
\cs_generate_variant:Nn \clist_greverse:N { c }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\clist_reverse:n}
% \begin{macro}[EXP]{\@@_reverse:wwNww, \@@_reverse_end:ww}
%   The reversed token list is built one item at a time, and stored
%   between \cs{s_@@_stop} and \cs{s_@@_mark}, in the form of |?| followed by
%   zero or more instances of \enquote{\meta{item}\texttt{,}}.  We start from a comma
%   list \enquote{\meta{item_1}\texttt{,\ldots,}\meta{item_n}}.  During the loop,
%   the auxiliary \cs{@@_reverse:wwNww} receives \enquote{\texttt{?}\meta{item_i}} as
%   |#1|, \enquote{\meta{item_{i+1}}\texttt{,\ldots,}\meta{item_n}} as |#2|,
%   \cs{@@_reverse:wwNww} as |#3|, what remains until \cs{s_@@_stop} as
%   |#4|, and \enquote{\meta{item_{i-1}}\texttt{,\ldots,}\meta{item_1}\texttt{,}} as |#5|.
%   The auxiliary moves |#1| just before |#5|, with a comma, and calls
%   itself (|#3|).  After the last item is moved, \cs{@@_reverse:wwNww}
%   receives \enquote{\cs{s_@@_mark} \cs{@@_reverse:wwNww} \texttt{!}} as its argument
%   |#1|, thus \cs{@@_reverse_end:ww} as its argument |#3|.  This second
%   auxiliary cleans up until the marker~|!|, removes the trailing comma
%   (introduced when the first item was moved after \cs{s_@@_stop}), and
%   leaves its argument~|#1| within \cs{exp_not:n}.  There is also a
%   need to remove a leading comma, hence \cs{exp_not:o} and
%   \cs{use_none:n}.
%    \begin{macrocode}
\cs_new:Npn \clist_reverse:n #1
  {
    \@@_reverse:wwNww ? #1 ,
      \s_@@_mark \@@_reverse:wwNww ! ,
      \s_@@_mark \@@_reverse_end:ww
      \s_@@_stop ? \s_@@_mark
  }
\cs_new:Npn \@@_reverse:wwNww
    #1 , #2 \s_@@_mark #3 #4 \s_@@_stop ? #5 \s_@@_mark
  { #3 ? #2 \s_@@_mark #3 #4 \s_@@_stop #1 , #5 \s_@@_mark }
\cs_new:Npn \@@_reverse_end:ww #1 ! #2 , \s_@@_mark
  { \exp_not:o { \use_none:n #2 } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\clist_sort:Nn, \clist_sort:cn, \clist_gsort:Nn, \clist_gsort:cn}
%   Implemented in \pkg{l3sort}.
% \end{macro}
%
% \subsection{Comma list conditionals}
%
% \begin{macro}[pTF]{\clist_if_empty:N, \clist_if_empty:c}
% \UnitTested
%   Simple copies from the token list variable material.
%    \begin{macrocode}
\prg_new_eq_conditional:NNn \clist_if_empty:N \tl_if_empty:N
  { p , T , F , TF }
\prg_new_eq_conditional:NNn \clist_if_empty:c \tl_if_empty:c
  { p , T , F , TF }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP, pTF]{\clist_if_empty:n}
% \begin{macro}[EXP]{\@@_if_empty_n:w}
% \begin{macro}[EXP]{\@@_if_empty_n:wNw}
%   As usual, we insert a token (here |?|) before grabbing
%   any argument: this avoids losing braces. The argument
%   of \cs{tl_if_empty:oTF} is empty if |#1| is |?| followed
%   by blank spaces (besides, this particular variant of
%   the emptiness test is optimized). If the item of the
%   comma list is blank, grab the next one. As soon as one
%   item is non-blank, exit: the second auxiliary grabs
%   \cs{prg_return_false:} as |#2|, unless every item in
%   the comma list was blank and the loop actually got broken
%   by the trailing \cs{s_@@_mark} \cs{prg_return_false:} item.
%    \begin{macrocode}
\prg_new_conditional:Npnn \clist_if_empty:n #1 { p , T , F , TF }
  {
    \@@_if_empty_n:w ? #1
    , \s_@@_mark \prg_return_false:
    , \s_@@_mark \prg_return_true:
    \s_@@_stop
  }
\cs_new:Npn \@@_if_empty_n:w #1 ,
  {
    \tl_if_empty:oTF { \use_none:nn #1 ? }
      { \@@_if_empty_n:w ? }
      { \@@_if_empty_n:wNw }
  }
\cs_new:Npn \@@_if_empty_n:wNw #1 \s_@@_mark #2#3 \s_@@_stop {#2}
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[TF]
%   {
%     \clist_if_in:Nn, \clist_if_in:NV, \clist_if_in:No,
%     \clist_if_in:cn, \clist_if_in:cV, \clist_if_in:co,
%     \clist_if_in:nn, \clist_if_in:nV, \clist_if_in:no
%   }
% \begin{macro}{\@@_if_in_return:nnN}
% \UnitTested
%   For \enquote{safe} items, we simply surround the comma list, and the
%   item, with commas, then use the same code as for \cs{tl_if_in:Nn}.
%   For \enquote{unsafe} items we follow the same route as
%   \cs{seq_if_in:Nn}, mapping through the list a comparison function.
%   If found, return \texttt{true} and remove \cs{prg_return_false:}.
%    \begin{macrocode}
\prg_new_protected_conditional:Npnn \clist_if_in:Nn #1#2 { T  , F , TF }
  {
    \exp_args:No \@@_if_in_return:nnN #1 {#2} #1
  }
\prg_new_protected_conditional:Npnn \clist_if_in:nn #1#2 { T  , F , TF }
  {
    \clist_set:Nn \l_@@_internal_clist {#1}
    \exp_args:No \@@_if_in_return:nnN \l_@@_internal_clist {#2}
      \l_@@_internal_clist
  }
\cs_new_protected:Npn \@@_if_in_return:nnN #1#2#3
  {
    \@@_if_wrap:nTF {#2}
      {
        \cs_set:Npx \@@_tmp:w ##1
          {
            \exp_not:N \tl_if_eq:nnT {##1}
            \exp_not:n
              {
                {#2}
                { \clist_map_break:n { \prg_return_true: \use_none:n } }
              }
          }
        \clist_map_function:NN #3 \@@_tmp:w
        \prg_return_false:
      }
      {
        \cs_set:Npn \@@_tmp:w ##1 ,#2, { }
        \tl_if_empty:oTF
          { \@@_tmp:w ,#1, {} {} ,#2, }
          { \prg_return_false: } { \prg_return_true: }
      }
  }
\prg_generate_conditional_variant:Nnn \clist_if_in:Nn
  { NV , No , c , cV , co } { T , F , TF }
\prg_generate_conditional_variant:Nnn \clist_if_in:nn
  { nV , no } { T , F , TF }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Mapping to comma lists}
%
% \begin{macro}{\clist_map_function:NN, \clist_map_function:cN}
% \UnitTested
% \begin{macro}{\@@_map_function:Nw}
%   If the variable is empty, the mapping is skipped (otherwise,
%   that comma-list would be seen as consisting of one empty item).
%   Then loop over the comma-list, grabbing one comma-delimited
%   item at a time. The end is marked by \cs{q_@@_recursion_tail}.
%   The auxiliary function \cs{@@_map_function:Nw} is also used
%   in \cs{clist_map_inline:Nn}.
%    \begin{macrocode}
\cs_new:Npn \clist_map_function:NN #1#2
  {
    \clist_if_empty:NF #1
      {
        \exp_last_unbraced:NNo \@@_map_function:Nw #2 #1
          , \q_@@_recursion_tail ,
        \prg_break_point:Nn \clist_map_break: { }
      }
  }
\cs_new:Npn \@@_map_function:Nw #1#2 ,
  {
    \@@_if_recursion_tail_break:nN {#2} \clist_map_break:
    #1 {#2}
    \@@_map_function:Nw #1
  }
\cs_generate_variant:Nn \clist_map_function:NN { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_map_function:nN}
% \UnitTested
% \begin{macro}{\@@_map_function_n:Nn}
% \begin{macro}{\@@_map_unbrace:Nw}
%   The \texttt{n}-type mapping function is a bit more awkward,
%   since spaces must be trimmed from each item.
%   Space trimming is again based on \cs{@@_trim_next:w}.
%   The auxiliary \cs{@@_map_function_n:Nn} receives as arguments the
%   function, and the next non-empty item (after space trimming but
%   before brace removal).  One level of braces is removed by
%   \cs{@@_map_unbrace:Nw}.
%    \begin{macrocode}
\cs_new:Npn \clist_map_function:nN #1#2
  {
    \exp_after:wN \@@_map_function_n:Nn \exp_after:wN #2
    \exp:w \@@_trim_next:w \prg_do_nothing: #1 , \q_@@_recursion_tail ,
    \prg_break_point:Nn \clist_map_break: { }
  }
\cs_new:Npn \@@_map_function_n:Nn #1 #2
  {
    \@@_if_recursion_tail_break:nN {#2} \clist_map_break:
    \@@_map_unbrace:Nw #1 #2,
    \exp_after:wN \@@_map_function_n:Nn \exp_after:wN #1
    \exp:w \@@_trim_next:w \prg_do_nothing:
  }
\cs_new:Npn \@@_map_unbrace:Nw #1 #2, { #1 {#2} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_map_inline:Nn, \clist_map_inline:cn}
% \UnitTested
% \begin{macro}{\clist_map_inline:nn}
% \UnitTested
%   Inline mapping is done by creating a suitable function
%   \enquote{on the fly}: this is done globally to avoid
%   any issues with \TeX{}'s groups.  We use a different
%   function for each level of nesting.
%
%   Since the mapping is non-expandable,  we can perform
%   the space-trimming  needed by the \texttt{n} version
%   simply  by storing the comma-list in a variable.  We
%   don't need  a different comma-list  for each nesting
%   level: the comma-list is expanded before the mapping
%   starts.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_map_inline:Nn #1#2
  {
    \clist_if_empty:NF #1
      {
        \int_gincr:N \g__kernel_prg_map_int
        \cs_gset_protected:cpn
          { @@_map_ \int_use:N \g__kernel_prg_map_int :w } ##1 {#2}
        \exp_last_unbraced:Nco \@@_map_function:Nw
          { @@_map_ \int_use:N \g__kernel_prg_map_int :w }
          #1 , \q_@@_recursion_tail ,
        \prg_break_point:Nn \clist_map_break:
          { \int_gdecr:N \g__kernel_prg_map_int }
      }
  }
\cs_new_protected:Npn \clist_map_inline:nn #1
  {
    \clist_set:Nn \l_@@_internal_clist {#1}
    \clist_map_inline:Nn \l_@@_internal_clist
  }
\cs_generate_variant:Nn \clist_map_inline:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_map_variable:NNn, \clist_map_variable:cNn}
% \UnitTested
% \begin{macro}{\clist_map_variable:nNn}
% \begin{macro}{\@@_map_variable:Nnw}
%   As for other comma-list mappings, filter out the case of
%   an empty list. Same approach as \cs{clist_map_function:Nn},
%   additionally we store each item in the given variable.
%   As for inline mappings, space trimming for the \texttt{n}
%   variant is done by storing the comma list in a variable.
%   The quark test is done before assigning the item to the variable:
%   this avoids storing a quark which the user wouldn't expect.
%   The strange \cs{use:n} avoids unlikely problems when |#2| would
%   contain \cs{q_@@_recursion_stop}.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_map_variable:NNn #1#2#3
  {
    \clist_if_empty:NF #1
      {
        \exp_args:Nno \use:nn
          { \@@_map_variable:Nnw #2 {#3} }
          #1
          , \q_@@_recursion_tail , \q_@@_recursion_stop
        \prg_break_point:Nn \clist_map_break: { }
      }
  }
\cs_new_protected:Npn \clist_map_variable:nNn #1
  {
    \clist_set:Nn \l_@@_internal_clist {#1}
    \clist_map_variable:NNn \l_@@_internal_clist
  }
\cs_new_protected:Npn \@@_map_variable:Nnw #1#2#3,
  {
    \@@_if_recursion_tail_stop:n {#3}
    \tl_set:Nn #1 {#3}
    \use:n {#2}
    \@@_map_variable:Nnw #1 {#2}
  }
\cs_generate_variant:Nn \clist_map_variable:NNn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_map_break:, \clist_map_break:n}
%   The break statements use the general \cs{prg_map_break:Nn} mechanism.
%    \begin{macrocode}
\cs_new:Npn \clist_map_break:
  { \prg_map_break:Nn \clist_map_break: { } }
\cs_new:Npn \clist_map_break:n
  { \prg_map_break:Nn \clist_map_break: }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\clist_count:N, \clist_count:c, \clist_count:n}
% \begin{macro}{\@@_count:n}
% \begin{macro}{\@@_count:w}
%   Counting the items in a comma list is done using the same approach as for
%   other token count functions: turn each entry into a \texttt{+1} then use
%   integer evaluation to actually do the mathematics.
%   In the case of an \texttt{n}-type comma-list, we could of course use
%   \cs{clist_map_function:nN}, but that is very slow, because it carefully
%   removes spaces. Instead, we loop manually, and skip blank items
%   (but not |{}|, hence the extra spaces).
%    \begin{macrocode}
\cs_new:Npn \clist_count:N #1
  {
    \int_eval:n
      {
        0
        \clist_map_function:NN #1 \@@_count:n
      }
  }
\cs_generate_variant:Nn \clist_count:N { c }
\cs_new:Npx \clist_count:n #1
  {
    \exp_not:N \int_eval:n
      {
        0
        \exp_not:N \@@_count:w \c_space_tl
        #1 \exp_not:n { , \q_@@_recursion_tail , \q_@@_recursion_stop }
      }
  }
\cs_new:Npn \@@_count:n #1 { + 1 }
\cs_new:Npx \@@_count:w #1 ,
  {
    \exp_not:n { \exp_args:Nf \@@_if_recursion_tail_stop:n } {#1}
    \exp_not:N \tl_if_blank:nF {#1} { + 1 }
    \exp_not:N \@@_count:w \c_space_tl
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Using comma lists}
%
% \begin{macro}[EXP]{\clist_use:Nnnn, \clist_use:cnnn}
% \begin{macro}[EXP]
%   {\@@_use:wwn, \@@_use:nwwwwnwn, \@@_use:nwwn}
% \begin{macro}[EXP]{\clist_use:Nn, \clist_use:cn}
%   First check that the variable exists.  Then count the items in the
%   comma list.  If it has none, output nothing.  If it has one item,
%   output that item, brace stripped (note that space-trimming has
%   already been done when the comma list was assigned).  If it has two,
%   place the \meta{separator~between~two} in the middle.
%
%   Otherwise, \cs{@@_use:nwwwwnwn} takes the following arguments; 1:
%   a \meta{separator}, 2, 3, 4: three items from the comma list (or
%   quarks), 5: the rest of the comma list, 6: a \meta{continuation}
%   function (\texttt{use_ii} or \texttt{use_iii} with its
%   \meta{separator} argument), 7: junk, and 8: the temporary result,
%   which is built in a brace group following \cs{q_@@_stop}.  The
%   \meta{separator} and the first of the three items are placed in the
%   result, then we use the \meta{continuation}, placing the remaining
%   two items after it.  When we begin this loop, the three items really
%   belong to the comma list, the first \cs{q_@@_mark} is taken as a
%   delimiter to the \texttt{use_ii} function, and the continuation is
%   \texttt{use_ii} itself.  When we reach the last two items of the
%   original token list, \cs{q_@@_mark} is taken as a third item, and now
%   the second \cs{q_@@_mark} serves as a delimiter to \texttt{use_ii},
%   switching to the other \meta{continuation}, \texttt{use_iii}, which
%   uses the \meta{separator between final two}.
%    \begin{macrocode}
\cs_new:Npn \clist_use:Nnnn #1#2#3#4
  {
    \clist_if_exist:NTF #1
      {
        \int_case:nnF { \clist_count:N #1 }
          {
            { 0 } { }
            { 1 } { \exp_after:wN \@@_use:wwn #1 , , { } }
            { 2 } { \exp_after:wN \@@_use:wwn #1 , {#2} }
          }
          {
            \exp_after:wN \@@_use:nwwwwnwn
            \exp_after:wN { \exp_after:wN } #1 ,
            \s_@@_mark , { \@@_use:nwwwwnwn {#3} }
            \s_@@_mark , { \@@_use:nwwn {#4} }
            \s_@@_stop { }
          }
      }
      {
        \__kernel_msg_expandable_error:nnn
          { kernel } { bad-variable } {#1}
      }
  }
\cs_generate_variant:Nn \clist_use:Nnnn { c }
\cs_new:Npn \@@_use:wwn #1 , #2 , #3 { \exp_not:n { #1 #3 #2 } }
\cs_new:Npn \@@_use:nwwwwnwn
    #1#2 , #3 , #4 , #5 \s_@@_mark , #6#7 \s_@@_stop #8
  { #6 {#3} , {#4} , #5 \s_@@_mark , {#6} #7 \s_@@_stop { #8 #1 #2 } }
\cs_new:Npn \@@_use:nwwn #1#2 , #3 \s_@@_stop #4
  { \exp_not:n { #4 #1 #2 } }
\cs_new:Npn \clist_use:Nn #1#2
  { \clist_use:Nnnn #1 {#2} {#2} {#2} }
\cs_generate_variant:Nn \clist_use:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Using a single item}
%
% \begin{macro}{\clist_item:Nn, \clist_item:cn}
% \begin{macro}{\@@_item:nnnN, \@@_item:ffoN, \@@_item:ffnN}
% \begin{macro}{\@@_item_N_loop:nw}
%   To avoid needing to test the end of the list at each step,
%   we first compute the \meta{length} of the list. If the item number
%   is~$0$, less than $-\meta{length}$, or more than $\meta{length}$,
%   the result is empty. If it is negative, but not less than $-\meta{length}$,
%   add $\meta{length}+1$ to the item number before performing the loop.
%   The loop itself is very simple, return the item if the counter
%   reached~$1$, otherwise, decrease the counter and repeat.
%    \begin{macrocode}
\cs_new:Npn \clist_item:Nn #1#2
  {
    \@@_item:ffoN
      { \clist_count:N #1 }
      { \int_eval:n {#2} }
      #1
      \@@_item_N_loop:nw
  }
\cs_new:Npn \@@_item:nnnN #1#2#3#4
  {
    \int_compare:nNnTF {#2} < 0
      {
        \int_compare:nNnTF {#2} < { - #1 }
          { \@@_use_none_delimit_by_s_stop:w }
          { \exp_args:Nf #4 { \int_eval:n { #2 + 1 + #1 } } }
      }
      {
        \int_compare:nNnTF {#2} > {#1}
          { \@@_use_none_delimit_by_s_stop:w }
          { #4 {#2} }
      }
    { } , #3 , \s_@@_stop
  }
\cs_generate_variant:Nn \@@_item:nnnN { ffo, ff }
\cs_new:Npn \@@_item_N_loop:nw #1 #2,
  {
    \int_compare:nNnTF {#1} = 0
      { \@@_use_i_delimit_by_s_stop:nw { \exp_not:n {#2} } }
      { \exp_args:Nf \@@_item_N_loop:nw { \int_eval:n { #1 - 1 } } }
  }
\cs_generate_variant:Nn \clist_item:Nn { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_item:nn}
% \begin{macro}{
%     \@@_item_n:nw,
%     \@@_item_n_loop:nw,
%     \@@_item_n_end:n,
%     \@@_item_n_strip:n,
%     \@@_item_n_strip:w}
%   This starts in the same way as \cs{clist_item:Nn} by counting the items
%   of the comma list. The final item should be space-trimmed before being
%   brace-stripped, hence we insert a couple of odd-looking
%   \cs{prg_do_nothing:} to avoid losing braces. Blank items are ignored.
%    \begin{macrocode}
\cs_new:Npn \clist_item:nn #1#2
  {
    \@@_item:ffnN
      { \clist_count:n {#1} }
      { \int_eval:n {#2} }
      {#1}
      \@@_item_n:nw
  }
\cs_new:Npn \@@_item_n:nw #1
  { \@@_item_n_loop:nw {#1} \prg_do_nothing: }
\cs_new:Npn \@@_item_n_loop:nw #1 #2,
  {
    \exp_args:No \tl_if_blank:nTF {#2}
      { \@@_item_n_loop:nw {#1} \prg_do_nothing: }
      {
        \int_compare:nNnTF {#1} = 0
          { \exp_args:No \@@_item_n_end:n {#2} }
          {
            \exp_args:Nf \@@_item_n_loop:nw
              { \int_eval:n { #1 - 1 } }
              \prg_do_nothing:
          }
      }
  }
\cs_new:Npn \@@_item_n_end:n #1 #2 \s_@@_stop
  { \tl_trim_spaces_apply:nN {#1} \@@_item_n_strip:n }
\cs_new:Npn \@@_item_n_strip:n #1 { \@@_item_n_strip:w #1 , }
\cs_new:Npn \@@_item_n_strip:w #1 , { \exp_not:n {#1} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\clist_rand_item:n, \clist_rand_item:N, \clist_rand_item:c}
% \begin{macro}{\@@_rand_item:nn}
%   The |N|-type function is not implemented through the |n|-type
%   function for efficiency: for instance comma-list variables do not
%   require space-trimming of their items.  Even testing for emptyness
%   of an |n|-type comma-list is slow, so we count items first and use
%   that both for the emptyness test and the pseudo-random integer.
%   Importantly, \cs{clist_item:Nn} and \cs{clist_item:nn} only evaluate
%   their argument once.
%    \begin{macrocode}
\cs_new:Npn \clist_rand_item:n #1
  { \exp_args:Nf \@@_rand_item:nn { \clist_count:n {#1} } {#1} }
\cs_new:Npn \@@_rand_item:nn #1#2
  {
    \int_compare:nNnF {#1} = 0
      { \clist_item:nn {#2} { \int_rand:nn { 1 } {#1} } }
  }
\cs_new:Npn \clist_rand_item:N #1
  {
    \clist_if_empty:NF #1
      { \clist_item:Nn #1 { \int_rand:nn { 1 } { \clist_count:N #1 } } }
  }
\cs_generate_variant:Nn \clist_rand_item:N { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Viewing comma lists}
%
% \begin{macro}{\clist_show:N, \clist_show:c, \clist_log:N, \clist_log:c, \@@_show:NN}
%   Apply the general \cs{__kernel_chk_defined:NT} and
%   \cs{msg_show:nnnnnn}.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_show:N { \@@_show:NN \msg_show:nnxxxx }
\cs_generate_variant:Nn \clist_show:N { c }
\cs_new_protected:Npn \clist_log:N { \@@_show:NN \msg_log:nnxxxx }
\cs_generate_variant:Nn \clist_log:N { c }
\cs_new_protected:Npn \@@_show:NN #1#2
  {
    \__kernel_chk_defined:NT #2
      {
        #1 { LaTeX/kernel } { show-clist }
          { \token_to_str:N #2 }
          { \clist_map_function:NN #2 \msg_show_item:n }
          { } { }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\clist_show:n, \clist_log:n, \@@_show:Nn}
%   A variant of the above: no existence check, empty first argument for
%   the message.
%    \begin{macrocode}
\cs_new_protected:Npn \clist_show:n { \@@_show:Nn \msg_show:nnxxxx }
\cs_new_protected:Npn \clist_log:n { \@@_show:Nn \msg_log:nnxxxx }
\cs_new_protected:Npn \@@_show:Nn #1#2
  {
    #1 { LaTeX/kernel } { show-clist }
      { } { \clist_map_function:nN {#2} \msg_show_item:n } { } { }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Scratch comma lists}
%
% \begin{variable}{\l_tmpa_clist, \l_tmpb_clist}
% \begin{variable}{\g_tmpa_clist, \g_tmpb_clist}
%   Temporary comma list variables.
%    \begin{macrocode}
\clist_new:N \l_tmpa_clist
\clist_new:N \l_tmpb_clist
\clist_new:N \g_tmpa_clist
\clist_new:N \g_tmpb_clist
%    \end{macrocode}
% \end{variable}
% \end{variable}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex