summaryrefslogtreecommitdiff
path: root/macros/latex/contrib/l3kernel/l3keys.dtx
blob: 10286cfc3a1d28d29d55db58d6f32abc0711dc10 (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
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
% \iffalse meta-comment
%
%% File: l3keys.dtx
%
% Copyright (C) 2006-2021 The LaTeX 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{l3keys} package\\ Key--value interfaces^^A
% }
%
% \author{^^A
%  The \LaTeX{} Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released 2021-10-17}
%
% \maketitle
%
% \begin{documentation}
%
% The key--value method is a popular system for creating large numbers
% of settings for controlling function or package behaviour.  The
% system normally results in input of the form
% \begin{verbatim}
%   \MyModuleSetup{
%     key-one = value one,
%     key-two = value two
%   }
% \end{verbatim}
% or
% \begin{verbatim}
%   \MyModuleMacro[
%     key-one = value one,
%     key-two = value two
%   ]{argument}
% \end{verbatim}
% for the user.
%
% The high level functions here are intended as a method to create
% key--value controls. Keys are themselves created using a key--value
% interface, minimising the number of functions and arguments
% required. Each key is created by setting one or more \emph{properties}
% of the key:
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       key-one .code:n   = code including parameter #1,
%       key-two .tl_set:N = \l_mymodule_store_tl
%     }
% \end{verbatim}
% These values can then be set as with other key--value approaches:
% \begin{verbatim}
%   \keys_set:nn { mymodule }
%     {
%       key-one = value one,
%       key-two = value two
%     }
% \end{verbatim}
%
% At a document level, \cs{keys_set:nn} is used within a
% document function, for example
% \begin{verbatim}
%   \DeclareDocumentCommand \MyModuleSetup { m }
%     { \keys_set:nn { mymodule } { #1 }  }
%   \DeclareDocumentCommand \MyModuleMacro { o m }
%     {
%       \group_begin:
%         \keys_set:nn { mymodule } { #1 }
%         % Main code for \MyModuleMacro
%       \group_end:
%     }
% \end{verbatim}
%
% Key names may contain any tokens, as they are handled internally
% using \cs{tl_to_str:n}. As discussed in
% section~\ref{sec:l3keys:subdivision}, it is suggested that the character
% |/| is reserved for sub-division of keys into logical
% groups. Functions and variables are \emph{not} expanded when creating
% key names, and so
% \begin{verbatim}
%   \tl_set:Nn \l_mymodule_tmp_tl { key }
%   \keys_define:nn { mymodule }
%     {
%       \l_mymodule_tmp_tl .code:n = code
%     }
% \end{verbatim}
% creates a key called |\l_mymodule_tmp_tl|, and not one called
% \texttt{key}.
%
% \section{Creating keys}
%
% \begin{function}[updated = 2017-11-14]{\keys_define:nn}
%   \begin{syntax}
%     \cs{keys_define:nn} \Arg{module} \Arg{keyval list}
%   \end{syntax}
%   Parses the \meta{keyval list} and defines the keys listed there for
%   \meta{module}. The \meta{module} name is treated as a string.
%   In practice the
%   \meta{module} should be chosen to be unique to the module in question
%   (unless deliberately adding keys to an existing module).
%
%   The \meta{keyval list} should consist of one or more key names along
%   with an associated key \emph{property}. The properties of a key
%   determine how it acts. The individual properties are described
%   in the following text; a typical use of \cs{keys_define:nn} might
%   read
%   \begin{verbatim}
%     \keys_define:nn { mymodule }
%       {
%         keyname .code:n = Some~code~using~#1,
%         keyname .value_required:n = true
%       }
%   \end{verbatim}
%   where the properties of the key begin from the |.| after the key
%   name.
% \end{function}
%
% The various properties available take either no arguments at
% all, or require one or more arguments. This is indicated in the
% name of the property using an argument specification. In the following
% discussion, each property is illustrated attached to an
% arbitrary \meta{key}, which when used may be supplied with a
% \meta{value}. All key \emph{definitions} are local.
%
% Key properties are applied in the reading order and so the ordering
% is significant. Key properties which define \enquote{actions}, such
% as |.code:n|, |.tl_set:N|, \emph{etc.}, override one another.
% Some other properties are mutually exclusive, notably |.value_required:n|
% and |.value_forbidden:n|, and so they replace one another. However,
% properties covering non-exclusive behaviours may be given in any order. Thus
% for example the following definitions are equivalent.
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       keyname .code:n           = Some~code~using~#1,
%       keyname .value_required:n = true
%     }
%   \keys_define:nn { mymodule }
%     {
%       keyname .value_required:n = true,
%       keyname .code:n           = Some~code~using~#1
%     }
% \end{verbatim}
% Note that with the exception of the special |.undefine:| property, all
% key properties define the key within the current \TeX{} scope.
%
% \begin{function}[updated = 2013-07-08]
%   {.bool_set:N, .bool_set:c, .bool_gset:N, .bool_gset:c}
%   \begin{syntax}
%     \meta{key} .bool_set:N = \meta{boolean}
%   \end{syntax}
%   Defines \meta{key} to set \meta{boolean} to \meta{value} (which
%   must be either \texttt{true} or \texttt{false}).  If the variable
%   does not exist, it will be created globally at the point that
%   the key is set up.
% \end{function}
%
% \begin{function}[added = 2011-08-28, updated = 2013-07-08]
%   {
%     .bool_set_inverse:N, .bool_set_inverse:c,
%     .bool_gset_inverse:N, .bool_gset_inverse:c
%   }
%   \begin{syntax}
%     \meta{key} .bool_set_inverse:N = \meta{boolean}
%   \end{syntax}
%   Defines \meta{key} to set \meta{boolean} to the logical
%   inverse of \meta{value} (which  must be either \texttt{true} or
%   \texttt{false}).
%   If the \meta{boolean} does not exist, it will be created globally
%   at the point that the key is set up.
% \end{function}
%
% \begin{function}{.choice:}
%   \begin{syntax}
%     \meta{key} .choice:
%   \end{syntax}
%   Sets \meta{key} to act as a choice key. Each valid choice
%   for \meta{key} must then be created, as discussed in
%   section~\ref{sec:l3keys:choice}.
% \end{function}
%
% \begin{function}[added = 2011-08-21, updated = 2013-07-10]
%   {.choices:nn, .choices:Vn, .choices:on, .choices:xn}
%   \begin{syntax}
%     \meta{key} .choices:nn = \Arg{choices} \Arg{code}
%   \end{syntax}
%   Sets \meta{key} to act as a choice key, and defines a series \meta{choices}
%   which are implemented using the \meta{code}. Inside \meta{code},
%   \cs{l_keys_choice_tl} will be the name of the choice made, and
%   \cs{l_keys_choice_int} will be the position of the choice in the list
%   of \meta{choices} (indexed from~$1$).
%   Choices are discussed in detail in section~\ref{sec:l3keys:choice}.
% \end{function}
%
% \begin{function}[added = 2011-09-11]
%   {.clist_set:N, .clist_set:c, .clist_gset:N, .clist_gset:c}
%   \begin{syntax}
%     \meta{key} .clist_set:N = \meta{comma list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{comma list variable} to \meta{value}.
%   Spaces around commas and empty items will be stripped.
%   If the variable does not exist, it
%   is created globally at the point that the key is set up.
% \end{function}
%
% \begin{function}[updated = 2013-07-10]{.code:n}
%   \begin{syntax}
%     \meta{key} .code:n = \Arg{code}
%   \end{syntax}
%   Stores the \meta{code} for execution when \meta{key} is used.
%   The \meta{code} can include one parameter (|#1|), which will be the
%   \meta{value} given for the \meta{key}.
% \end{function}
%
% \begin{function}[added = 2020-01-11]
%   {
%     .cs_set:Np, .cs_set:cp,
%     .cs_set_protected:Np, .cs_set_protected:cp,
%     .cs_gset:Np, .cs_gset:cp,
%     .cs_gset_protected:Np, .cs_gset_protected:cp,
%   }
%   \begin{syntax}
%     \meta{key} .cs_set:Np = \meta{control sequence} \meta{arg.~spec.}
%   \end{syntax}
%   Defines \meta{key} to set \meta{control sequence} to have \meta{arg.~spec.}
%   and replacement text \meta{value}.
% \end{function}
%
% \begin{function}[updated = 2013-07-09]
%   {.default:n, .default:V, .default:o, .default:x}
%   \begin{syntax}
%     \meta{key} .default:n = \Arg{default}
%   \end{syntax}
%   Creates a \meta{default} value for \meta{key}, which is used if no
%   value is given. This will be used if only the key name is given,
%   but not if a blank \meta{value} is given:
%   \begin{verbatim}
%     \keys_define:nn { mymodule }
%       {
%         key .code:n    = Hello~#1,
%         key .default:n = World
%       }
%     \keys_set:nn { mymodule }
%       {
%         key = Fred, % Prints 'Hello Fred'
%         key,        % Prints 'Hello World'
%         key = ,     % Prints 'Hello '
%       }
%   \end{verbatim}
%   The default does not affect keys where values are required or
%   forbidden. Thus a required value cannot be supplied by a default
%   value, and giving a default value for a key which cannot take a value
%   does not trigger an error.
% \end{function}
%
% \begin{function}[updated = 2020-01-17]
%   {.dim_set:N, .dim_set:c, .dim_gset:N, .dim_gset:c}
%   \begin{syntax}
%     \meta{key} .dim_set:N = \meta{dimension}
%   \end{syntax}
%   Defines \meta{key} to set \meta{dimension} to \meta{value} (which
%   must a dimension expression).  If the variable does not exist, it
%   is created globally at the point that the key is set up. The key will
%   require a value at point-of-use unless a default is set.
% \end{function}
%
% \begin{function}[updated = 2020-01-17]
%   {.fp_set:N, .fp_set:c, .fp_gset:N, .fp_gset:c}
%   \begin{syntax}
%     \meta{key} .fp_set:N = \meta{floating point}
%   \end{syntax}
%   Defines \meta{key} to set \meta{floating point} to \meta{value}
%   (which must a floating point expression).  If the variable does not exist,
%   it is created globally at the point that the key is set up. The key will
%   require a value at point-of-use unless a default is set.
% \end{function}
%
% \begin{function}[added = 2013-07-14]
%   {.groups:n}
%   \begin{syntax}
%     \meta{key} .groups:n = \Arg{groups}
%   \end{syntax}
%   Defines \meta{key} as belonging to the \meta{groups} declared. Groups
%   provide a \enquote{secondary axis} for selectively setting keys, and are
%   described in Section~\ref{sec:l3keys:selective}.
% \end{function}
%
% \begin{function}[added = 2016-11-22]{.inherit:n}
%   \begin{syntax}
%     \meta{key} .inherit:n = \Arg{parents}
%   \end{syntax}
%   Specifies that the \meta{key} path should inherit the keys listed
%   as \meta{parents}. For example, after setting
%   \begin{verbatim}
%     \keys_define:nn { foo } { test .code:n = \tl_show:n {#1} }
%     \keys_define:nn { } { bar .inherit:n = foo }
%   \end{verbatim}
%   setting
%   \begin{verbatim}
%     \keys_set:nn { bar } { test = a }
%   \end{verbatim}
%   will be equivalent to
%   \begin{verbatim}
%     \keys_set:nn { foo } { test = a }
%   \end{verbatim}
% \end{function}
%
% \begin{function}[updated = 2013-07-09]
%   {.initial:n, .initial:V, .initial:o, .initial:x}
%   \begin{syntax}
%     \meta{key} .initial:n = \Arg{value}
%   \end{syntax}
%   Initialises the \meta{key} with the \meta{value}, equivalent to
%   \begin{quote}
%     \cs{keys_set:nn} \Arg{module} \{ \meta{key} = \meta{value} \}
%   \end{quote}
% \end{function}
%
% \begin{function}[updated = 2020-01-17]
%   {.int_set:N, .int_set:c, .int_gset:N, .int_gset:c}
%   \begin{syntax}
%     \meta{key} .int_set:N = \meta{integer}
%   \end{syntax}
%   Defines \meta{key} to set \meta{integer} to \meta{value} (which
%   must be an integer expression).  If the variable does not exist, it
%   is created globally at the point that the key is set up. The key will
%   require a value at point-of-use unless a default is set.
% \end{function}
%
% \begin{function}[updated = 2013-07-10]{.meta:n}
%   \begin{syntax}
%     \meta{key} .meta:n = \Arg{keyval list}
%   \end{syntax}
%   Makes \meta{key} a meta-key, which will set \meta{keyval list} in
%   one go.  The \meta{keyval list} can refer as |#1| to the value given
%   at the time the \meta{key} is used (or, if no value is given, the
%   \meta{key}'s default value).
% \end{function}
%
% \begin{function}[added = 2013-07-10]{.meta:nn}
%   \begin{syntax}
%     \meta{key} .meta:nn = \Arg{path} \Arg{keyval list}
%   \end{syntax}
%   Makes \meta{key} a meta-key, which will set \meta{keyval list} in
%   one go using the \meta{path} in place of the current one.  The
%   \meta{keyval list} can refer as |#1| to the value given at the time
%   the \meta{key} is used (or, if no value is given, the \meta{key}'s
%   default value).
% \end{function}
%
% \begin{function}[added = 2011-08-21]{.multichoice:}
%   \begin{syntax}
%     \meta{key} .multichoice:
%   \end{syntax}
%   Sets \meta{key} to act as a multiple choice key. Each valid choice
%   for \meta{key} must then be created, as discussed in
%   section~\ref{sec:l3keys:choice}.
% \end{function}
%
% \begin{function}[added = 2011-08-21, updated = 2013-07-10]
%   {.multichoices:nn, .multichoices:Vn, .multichoices:on, .multichoices:xn}
%   \begin{syntax}
%     \meta{key} .multichoices:nn \Arg{choices} \Arg{code}
%   \end{syntax}
%   Sets \meta{key} to act as a multiple choice key, and defines a series
%   \meta{choices}
%   which are implemented using the \meta{code}. Inside \meta{code},
%   \cs{l_keys_choice_tl} will be the name of the choice made, and
%   \cs{l_keys_choice_int} will be the position of the choice in the list
%   of \meta{choices} (indexed from~$1$).
%   Choices are discussed in detail in section~\ref{sec:l3keys:choice}.
% \end{function}
%
% \begin{function}[added = 2019-05-05, updated = 2020-01-17]
%   {.muskip_set:N, .muskip_set:c, .muskip_gset:N, .muskip_gset:c}
%   \begin{syntax}
%     \meta{key} .muskip_set:N = \meta{muskip}
%   \end{syntax}
%   Defines \meta{key} to set \meta{muskip} to \meta{value} (which
%   must be a muskip expression). If the variable does not exist, it
%   is created globally at the point that the key is set up. The key will
%   require a value at point-of-use unless a default is set.
% \end{function}
%
% \begin{function}[added = 2019-01-31]
%   {.prop_put:N, .prop_put:c, .prop_gput:N, .prop_gput:c}
%   \begin{syntax}
%     \meta{key} .prop_put:N = \meta{property list}
%   \end{syntax}
%   Defines \meta{key} to put the \meta{value} onto the \meta{property list}
%   stored under the \meta{key}.
%   If the variable does not exist, it
%   is created globally at the point that the key is set up.
% \end{function}
%
% \begin{function}[updated = 2020-01-17]
%   {.skip_set:N, .skip_set:c, .skip_gset:N, .skip_gset:c}
%   \begin{syntax}
%     \meta{key} .skip_set:N = \meta{skip}
%   \end{syntax}
%   Defines \meta{key} to set \meta{skip} to \meta{value} (which
%   must be a skip expression). If the variable does not exist, it
%   is created globally at the point that the key is set up. The key will
%   require a value at point-of-use unless a default is set.
% \end{function}
%
% \begin{function}{.tl_set:N, .tl_set:c, .tl_gset:N, .tl_gset:c}
%   \begin{syntax}
%     \meta{key} .tl_set:N = \meta{token list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{token list variable} to \meta{value}.
%   If the variable does not exist, it is created globally
%   at the point that the key is set up.
% \end{function}
%
% \begin{function}{.tl_set_x:N, .tl_set_x:c, .tl_gset_x:N, .tl_gset_x:c}
%   \begin{syntax}
%     \meta{key} .tl_set_x:N = \meta{token list variable}
%   \end{syntax}
%   Defines \meta{key} to set \meta{token list variable} to \meta{value},
%   which will be subjected to an \texttt{x}-type expansion
%   (\emph{i.e.}~using \cs{tl_set:Nx}). If the variable does not exist,
%   it is created globally at the point that the key is set up.
% \end{function}
%
% \begin{function}[added = 2015-07-14]{.undefine:}
%   \begin{syntax}
%     \meta{key} .undefine:
%   \end{syntax}
%   Removes the definition of the \meta{key} within the current scope.
% \end{function}
%
% \begin{function}[added = 2015-07-14]{.value_forbidden:n}
%   \begin{syntax}
%     \meta{key} .value_forbidden:n = \texttt{true\string|false}
%   \end{syntax}
%   Specifies that \meta{key} cannot receive a \meta{value} when used.
%   If a \meta{value} is given then an error will be issued. Setting
%   the property \texttt{false} cancels the restriction.
% \end{function}
%
% \begin{function}[added = 2015-07-14]{.value_required:n}
%   \begin{syntax}
%      \meta{key} .value_required:n = \texttt{true\string|false}
%   \end{syntax}
%   Specifies that \meta{key} must receive a \meta{value} when used.
%   If a \meta{value} is not given then an error will be issued. Setting
%   the property \texttt{false} cancels the restriction.
% \end{function}
%
% \section{Sub-dividing keys}
% \label{sec:l3keys:subdivision}
%
% When creating large numbers of keys, it may be desirable to divide
% them into several sub-groups for a given module. This can be achieved
% either by adding a sub-division to the module name:
% \begin{verbatim}
%   \keys_define:nn { mymodule / subgroup }
%     { key .code:n = code }
% \end{verbatim}
% or to the key name:
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     { subgroup / key .code:n = code }
% \end{verbatim}
% As illustrated, the best choice of token for sub-dividing keys in
% this way is |/|. This is because of the method that is
% used to represent keys internally. Both of the above code fragments
% set the same key, which has full name \texttt{mymodule/subgroup/key}.
%
% As illustrated in the next section, this subdivision is
% particularly relevant to making multiple choices.
%
% \section{Choice and multiple choice keys}
% \label{sec:l3keys:choice}
%
% The \pkg{l3keys} system supports two types of choice key, in which a series
% of pre-defined input values are linked to varying implementations. Choice
% keys are usually created so that the various values are mutually-exclusive:
% only one can apply at any one time. \enquote{Multiple} choice keys are also
% supported: these allow a selection of values to be chosen at the same time.
%
% Mutually-exclusive choices are created by setting the \texttt{.choice:}
% property:
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     { key .choice: }
% \end{verbatim}
% For keys which are set up as choices, the valid choices are generated
% by creating sub-keys of the choice key. This can be carried out in
% two ways.
%
% In many cases, choices execute similar code which is dependant only
% on the name of the choice or the position of the choice in the
% list of all possibilities. Here, the keys can share the same code, and can
% be rapidly created using the  \texttt{.choices:nn} property.
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       key .choices:nn =
%         { choice-a, choice-b, choice-c }
%         {
%           You~gave~choice~'\tl_use:N \l_keys_choice_tl',~
%           which~is~in~position~\int_use:N \l_keys_choice_int \c_space_tl
%           in~the~list.
%         }
%     }
% \end{verbatim}
% The index \cs{l_keys_choice_int} in the list of choices starts at~$1$.
%
% \begin{variable}{\l_keys_choice_int, \l_keys_choice_tl}
%   Inside the code block for a choice generated using \texttt{.choices:nn},
%   the variables \cs{l_keys_choice_tl} and \cs{l_keys_choice_int} are
%   available to indicate the name of the current choice, and its position in
%   the comma list.  The position is indexed from~$1$. Note that, as with
%   standard key code generated using \texttt{.code:n}, the value passed to
%   the key (i.e.~the choice name) is also available as |#1|.
% \end{variable}
%
% On the other hand, it is sometimes useful to create choices which
% use entirely different code from one another. This can be achieved
% by setting the \texttt{.choice:} property of a key, then manually
% defining sub-keys.
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       key .choice:,
%       key / choice-a .code:n = code-a,
%       key / choice-b .code:n = code-b,
%       key / choice-c .code:n = code-c,
%     }
% \end{verbatim}
%
% It is possible to mix the two methods, but manually-created choices
% should \emph{not} use \cs{l_keys_choice_tl} or \cs{l_keys_choice_int}.
% These variables do not have defined behaviour when used outside of
% code created using \texttt{.choices:nn}
% (\emph{i.e.}~anything might happen).
%
% It is possible to allow choice keys to take values which have not previously
% been defined by adding code for the special \texttt{unknown} choice. The
% general behavior of the \texttt{unknown} key is described in
% Section~\ref{sec:l3keys:unknown}. A typical example in the case of a choice
% would be to issue a custom error message:
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       key .choice:,
%       key / choice-a .code:n = code-a,
%       key / choice-b .code:n = code-b,
%       key / choice-c .code:n = code-c,
%       key / unknown  .code:n =
%         \msg_error:nnxxx { mymodule } { unknown-choice }
%           { key }                              % Name of choice key
%           { choice-a , choice-b ,  choice-c }  % Valid choices
%           { \exp_not:n {#1} }                  % Invalid choice given
%       %
%       %
%     }
% \end{verbatim}
%
% Multiple choices are created in a very similar manner to mutually-exclusive
% choices, using the properties \texttt{.multichoice:} and
% \texttt{.multichoices:nn}. As with mutually exclusive choices, multiple
% choices are define as sub-keys. Thus both
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       key .multichoices:nn =
%         { choice-a, choice-b, choice-c }
%         {
%           You~gave~choice~'\tl_use:N \l_keys_choice_tl',~
%           which~is~in~position~
%           \int_use:N \l_keys_choice_int \c_space_tl
%           in~the~list.
%         }
%     }
% \end{verbatim}
% and
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       key .multichoice:,
%       key / choice-a .code:n = code-a,
%       key / choice-b .code:n = code-b,
%       key / choice-c .code:n = code-c,
%     }
% \end{verbatim}
% are valid.
%
% When a multiple choice key is set
% \begin{verbatim}
%   \keys_set:nn { mymodule }
%     {
%       key = { a , b , c } % 'key' defined as a multiple choice
%     }
% \end{verbatim}
% each choice is applied in turn, equivalent to a \texttt{clist} mapping or
% to applying each value individually:
% \begin{verbatim}
%   \keys_set:nn { mymodule }
%     {
%       key = a ,
%       key = b ,
%       key = c ,
%     }
% \end{verbatim}
% Thus each separate choice will have passed to it the
% \cs{l_keys_choice_tl} and \cs{l_keys_choice_int} in exactly
% the same way as described for \texttt{.choices:nn}.
%
% \section{Setting keys}
%
% \begin{function}[updated = 2017-11-14]
%   {\keys_set:nn, \keys_set:nV, \keys_set:nv, \keys_set:no}
%   \begin{syntax}
%     \cs{keys_set:nn} \Arg{module} \Arg{keyval list}
%   \end{syntax}
%   Parses the \meta{keyval list}, and sets those keys which are defined
%   for \meta{module}. The behaviour on finding an unknown key can be set
%   by defining a special \texttt{unknown} key: this is illustrated
%   later.
% \end{function}
%
% \begin{variable}[updated = 2020-02-08]
%   {\l_keys_key_str, \l_keys_path_str, \l_keys_value_tl}
%   For each key processed, information of the full \emph{path} of the
%   key, the \emph{name} of the key and the \emph{value} of the key is
%   available within three token list variables. These may be used within
%   the code of the key.
%
%   The \emph{value} is everything after the \texttt{=}, which may be
%   empty if no value was given. This is stored in \cs{l_keys_value_tl}, and
%   is not processed in any way by \cs{keys_set:nn}.
%
%   The \emph{path} of the key is a \enquote{full} description of the key,
%   and is unique for each key. It consists of the module and full key name,
%   thus for example
%   \begin{verbatim}
%     \keys_set:nn { mymodule } { key-a = some-value }
%   \end{verbatim}
%   has path \texttt{mymodule/key-a} while
%   \begin{verbatim}
%     \keys_set:nn { mymodule } { subset  / key-a = some-value }
%   \end{verbatim}
%   has path \texttt{mymodule/subset/key-a}. This information is stored in
%   \cs{l_keys_path_str}.
%
%   The \emph{name} of the key is the part of the path after the last
%   \texttt{/}, and thus is not unique. In the preceding examples, both keys
%   have name \texttt{key-a} despite having different paths.  This information
%   is stored in \cs{l_keys_key_str}.
% \end{variable}
%
% \section{Handling of unknown keys}
% \label{sec:l3keys:unknown}
%
% If a key has not previously been defined (is unknown), \cs{keys_set:nn}
% looks for a special \texttt{unknown} key for the same module, and if this is
% not defined raises an error indicating that the key name was unknown. This
% mechanism can be used for example to issue custom error texts.
% \begin{verbatim}
%   \keys_define:nn { mymodule }
%     {
%       unknown .code:n =
%         You~tried~to~set~key~'\l_keys_key_str'~to~'#1'.
%     }
% \end{verbatim}
%
% \begin{function}[added = 2011-08-23, updated = 2019-01-29]
%   {
%     \keys_set_known:nn, \keys_set_known:nV,
%     \keys_set_known:nv, \keys_set_known:no,
%     \keys_set_known:nnN, \keys_set_known:nVN,
%     \keys_set_known:nvN, \keys_set_known:noN,
%     \keys_set_known:nnnN, \keys_set_known:nVnN,
%     \keys_set_known:nvnN, \keys_set_known:nonN
%   }
%   \begin{syntax}
%     \cs{keys_set_known:nn} \Arg{module} \Arg{keyval list}
%     \cs{keys_set_known:nnN} \Arg{module} \Arg{keyval list} \meta{tl}
%     \cs{keys_set_known:nnnN} \Arg{module} \Arg{keyval list} \Arg{root} \meta{tl}
%   \end{syntax}
%   These functions set keys which are known for the \meta{module}, and
%   simply ignore other keys. The \cs{keys_set_known:nn} function parses the
%   \meta{keyval list}, and sets those keys which are defined for
%   \meta{module}. Any keys which are unknown are not processed further by
%   the parser. In addition, \cs{keys_set_known:nnN} stores the key--value
%   pairs in the \meta{tl} in comma-separated form (\emph{i.e.}~an edited
%   version of the \meta{keyval list}). When a \meta{root} is given
%   (\cs{keys_set_known:nnnN}), the key--value entries are returned
%   relative to this point in the key tree. When it is absent, only the
%   key name and value are provided. The correct list is returned by
%   nested calls.
% \end{function}
%
% \section{Selective key setting}
% \label{sec:l3keys:selective}
%
% In some cases it may be useful to be able to select only some keys for
% setting, even though these keys have the same path. For example, with
% a set of keys defined using
% \begin{verbatim}
%   \keys define:nn { mymodule }
%     {
%       key-one   .code:n   = { \my_func:n {#1} } ,
%       key-two   .tl_set:N = \l_my_a_tl          ,
%       key-three .tl_set:N = \l_my_b_tl          ,
%       key-four  .fp_set:N = \l_my_a_fp          ,
%     }
% \end{verbatim}
% the use of \cs{keys_set:nn} attempts to set all four keys. However, in
% some contexts it may only be sensible to set some keys, or to control the
% order of setting. To do this, keys may be assigned to \emph{groups}:
% arbitrary sets which are independent of the key tree. Thus modifying the
% example to read
% \begin{verbatim}
%   \keys define:nn { mymodule }
%     {
%       key-one   .code:n   = { \my_func:n {#1} } ,
%       key-one   .groups:n = { first }           ,
%       key-two   .tl_set:N = \l_my_a_tl          ,
%       key-two   .groups:n = { first }           ,
%       key-three .tl_set:N = \l_my_b_tl          ,
%       key-three .groups:n = { second }          ,
%       key-four  .fp_set:N = \l_my_a_fp          ,
%     }
% \end{verbatim}
% assigns \texttt{key-one} and \texttt{key-two} to group \texttt{first},
% \texttt{key-three} to group \texttt{second}, while \texttt{key-four} is
% not assigned to a group.
%
% Selective key setting may be achieved either by selecting one or more
% groups to be made \enquote{active}, or by marking one or more groups to
% be ignored in key setting.
%
% \begin{function}[added = 2013-07-14, updated = 2019-01-29]
%   {
%     \keys_set_filter:nnn, \keys_set_filter:nnV,
%     \keys_set_filter:nnv, \keys_set_filter:nno,
%     \keys_set_filter:nnnN, \keys_set_filter:nnVN,
%     \keys_set_filter:nnvN, \keys_set_filter:nnoN,
%     \keys_set_filter:nnnnN, \keys_set_filter:nnVnN,
%     \keys_set_filter:nnvnN, \keys_set_filter:nnonN,
%   }
%   \begin{syntax}
%     \cs{keys_set_filter:nnn} \Arg{module} \Arg{groups} \Arg{keyval list}
%     \cs{keys_set_filter:nnnN} \Arg{module} \Arg{groups} \Arg{keyval list} \meta{tl}
%     \cs{keys_set_filter:nnnnN} \Arg{module} \Arg{groups} \Arg{keyval list} \meta{root} \meta{tl}
%   \end{syntax}
%   Activates key filtering in an \enquote{opt-out} sense: keys assigned to any
%   of the \meta{groups} specified are ignored. The \meta{groups} are
%   given as a comma-separated list. Unknown keys are not assigned to any
%   group and are thus always set. The key--value pairs for each
%   key which is filtered out are stored in the \meta{tl} in a
%   comma-separated form (\emph{i.e.}~an edited version of the \meta{keyval
%   list}). The \cs{keys_set_filter:nnn} version skips this stage.
%
%   Use of \cs{keys_set_filter:nnnN} can be nested, with the correct residual
%   \meta{keyval list} returned at each stage. In the version which takes
%   a \meta{root} argument, the key list is returned relative to that point
%   in the key tree. In the cases without a \meta{root} argument, only
%   the key names and values are returned.
% \end{function}
%
% \begin{function}[added = 2013-07-14, updated = 2017-05-27]
%   {
%     \keys_set_groups:nnn, \keys_set_groups:nnV,
%     \keys_set_groups:nnv, \keys_set_groups:nno
%   }
%   \begin{syntax}
%     \cs{keys_set_groups:nnn} \Arg{module} \Arg{groups} \Arg{keyval list}
%   \end{syntax}
%   Activates key filtering in an \enquote{opt-in} sense: only keys assigned to
%   one or more of the \meta{groups} specified are set. The \meta{groups} are
%   given as a comma-separated list. Unknown keys are not assigned to any
%   group and are thus never set.
% \end{function}
%
% \section{Utility functions for keys}
%
% \begin{function}[EXP, pTF, updated = 2017-11-14]{\keys_if_exist:nn}
%   \begin{syntax}
%     \cs{keys_if_exist_p:nn} \Arg{module} \Arg{key} \\
%     \cs{keys_if_exist:nnTF} \Arg{module} \Arg{key} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{key} exists for \meta{module}, \emph{i.e.}~if any code
%   has been defined for \meta{key}.
% \end{function}
%
% \begin{function}[added = 2011-08-21,EXP,pTF, updated = 2017-11-14]
%   {\keys_if_choice_exist:nnn}
%   \begin{syntax}
%     \cs{keys_if_choice_exist_p:nnn} \Arg{module} \Arg{key} \Arg{choice} \\
%     \cs{keys_if_choice_exist:nnnTF} \Arg{module} \Arg{key} \Arg{choice} \Arg{true code} \Arg{false code}
%   \end{syntax}
%   Tests if the \meta{choice} is defined for the \meta{key} within the
%   \meta{module}, \emph{i.e.}~if any code has been defined for
%   \meta{key}/\meta{choice}. The test is \texttt{false} if the \meta{key}
%   itself is not defined.
% \end{function}
%
% \begin{function}[updated = 2015-08-09]{\keys_show:nn}
%   \begin{syntax}
%     \cs{keys_show:nn} \Arg{module} \Arg{key}
%   \end{syntax}
%   Displays in the terminal
%   the information associated to the \meta{key} for a \meta{module},
%   including the function which is used to actually implement it.
% \end{function}
%
% \begin{function}[added = 2014-08-22, updated = 2015-08-09]{\keys_log:nn}
%   \begin{syntax}
%     \cs{keys_log:nn} \Arg{module} \Arg{key}
%   \end{syntax}
%   Writes in the log file the information associated to the \meta{key}
%   for a \meta{module}.  See also \cs{keys_show:nn} which displays the
%   result in the terminal.
% \end{function}
%
% \section{Low-level interface for parsing key--val lists}
%
% To re-cap from earlier, a key--value list is input of the form
% \begin{verbatim}
%   KeyOne = ValueOne ,
%   KeyTwo = ValueTwo ,
%   KeyThree
% \end{verbatim}
% where each key--value pair is separated by a comma from the rest of
% the list, and each key--value pair does not necessarily contain an
% equals sign or a value! Processing this type of input correctly
% requires a number of careful steps, to correctly account for
% braces, spaces and the category codes of separators.
%
% While the functions described earlier are used as a high-level interface
% for processing such input, in special circumstances you may wish to use
% a lower-level approach.
% The low-level parsing system converts a \meta{key--value list}
% into \meta{keys} and associated \meta{values}. After the parsing phase
% is completed, the resulting keys and values (or keys alone) are
% available for further processing. This processing is not carried out by the
% low-level parser itself, and so the parser requires the names of
% two functions along with the key--value list. One function is
% needed to process key--value pairs (it receives two arguments),
% and a second function is required for keys given without any value
% (it is called with a single argument).
%
% The parser does not double |#| tokens or expand any input. Active
% tokens |=| and |,| appearing at the outer level of braces are converted
% to category \enquote{other} (12) so that the parser does not \enquote{miss}
% any due to category code changes. Spaces are removed from the ends
% of the keys and values. Keys and values which are given in braces
% have exactly one set removed (after space trimming), thus
% \begin{verbatim}
%    key = {value here},
% \end{verbatim}
% and
% \begin{verbatim}
%   key = value here,
% \end{verbatim}
% are treated identically.
%
% \begin{function}[rEXP, added=2020-12-19, updated = 2021-05-10]{\keyval_parse:nnn}
%   \begin{syntax}
%     \cs{keyval_parse:nnn} \Arg{code_1} \Arg{code_2} \Arg{key--value list}
%   \end{syntax}
%   Parses the \meta{key--value list} into a series of \meta{keys} and
%   associated \meta{values}, or keys alone (if no \meta{value} was
%   given).  \meta{code_1} receives each \meta{key} (with no \meta{value}) as a
%   trailing brace group, whereas \meta{code_2} is appended by two brace groups,
%   the \meta{key} and \meta{value}.
%   The order of the \meta{keys} in the \meta{key--value list}
%   is preserved. Thus
%   \begin{verbatim}
%     \keyval_parse:nnn
%       { \use_none:nn  { code 1 } }
%       { \use_none:nnn { code 2 } }
%       { key1 = value1 , key2 = value2, key3 = , key4 }
%   \end{verbatim}
%   is converted into an input stream
%   \begin{verbatim}
%     \use_none:nnn { code 2 } { key1 } { value1 }
%     \use_none:nnn { code 2 } { key2 } { value2 }
%     \use_none:nnn { code 2 } { key3 } { }
%     \use_none:nn  { code 1 } { key4 }
%   \end{verbatim}
%   Note that there is a difference between an empty value (an equals
%   sign followed by nothing) and a missing value (no equals sign at
%   all). Spaces are trimmed from the ends of the \meta{key} and \meta{value},
%   then one \emph{outer} set of braces is removed from the \meta{key}
%   and \meta{value} as part of the processing. If you need exactly the output
%   shown above, you'll need to either \texttt{x}-type or \texttt{e}-type expand
%   the function.
%   \begin{texnote}
%     The result of each list element is returned within \cs{exp_not:n}, which
%     means that the converted input stream does not expand further when
%     appearing in an \texttt{x}-type or \texttt{e}-type argument expansion.
%   \end{texnote}
% \end{function}
%
% \begin{function}[rEXP, updated = 2021-05-10]{\keyval_parse:NNn}
%   \begin{syntax}
%     \cs{keyval_parse:NNn} \meta{function_1} \meta{function_2} \Arg{key--value list}
%   \end{syntax}
%   Parses the \meta{key--value list} into a series of \meta{keys} and
%   associated \meta{values}, or keys alone (if no \meta{value} was
%   given).  \meta{function_1} should take one argument, while
%   \meta{function_2} should absorb two arguments. After
%   \cs{keyval_parse:NNn} has parsed the \meta{key--value list},
%   \meta{function_1} is used to process keys given with no value
%   and \meta{function_2} is used to process keys given with a
%   value. The order of the \meta{keys} in the \meta{key--value list}
%   is preserved. Thus
%   \begin{verbatim}
%     \keyval_parse:NNn \function:n \function:nn
%       { key1 = value1 , key2 = value2, key3 = , key4 }
%   \end{verbatim}
%   is converted into an input stream
%   \begin{verbatim}
%     \function:nn { key1 } { value1 }
%     \function:nn { key2 } { value2 }
%     \function:nn { key3 } { }
%     \function:n  { key4 }
%   \end{verbatim}
%   Note that there is a difference between an empty value (an equals
%   sign followed by nothing) and a missing value (no equals sign at
%   all). Spaces are trimmed from the ends of the \meta{key} and \meta{value},
%   then one \emph{outer} set of braces is removed from the \meta{key}
%   and \meta{value} as part of the processing.
%
%   This shares the implementation of \cs{keyval_parse:nnn}, the difference is
%   only semantically.
%   \begin{texnote}
%     The result is returned within \cs{exp_not:n}, which means that the
%     converted input stream does not expand further when appearing in an
%     \texttt{x}-type or \texttt{e}-type argument expansion.
%   \end{texnote}
% \end{function}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3keys} Implementation}
%
%    \begin{macrocode}
%<*package>
%    \end{macrocode}
%
% \subsection{Low-level interface}
%
% The low-level key parser's implementation is based heavily on \pkg{expkv}.
% Compared to \pkg{keyval} it adds a number of additional \enquote{safety}
% requirements and allows to process the parsed list of key--value pairs in a
% variety of ways.  The net result is that this code needs around one and a half
% the amount of time as \pkg{keyval} to parse the same list of keys. To optimise
% speed as far as reasonably practical, a number of lower-level approaches are
% taken rather than using the higher-level \pkg{expl3} interfaces.
%
%    \begin{macrocode}
%<@@=keyval>
%    \end{macrocode}
%
% \begin{variable}{\s_@@_nil,\s_@@_mark,\s_@@_stop,\s_@@_tail}
%    \begin{macrocode}
\scan_new:N \s_@@_nil
\scan_new:N \s_@@_mark
\scan_new:N \s_@@_stop
\scan_new:N \s_@@_tail
%    \end{macrocode}
% \end{variable}
%
%   This temporary macro will be used since some of the definitions will need an
%   active comma or equals sign. Inside of this macro |#1| will be the active
%   comma and |#2| will be the active equals sign.
%    \begin{macrocode}
\group_begin:
  \cs_set_protected:Npn \@@_tmp:w #1#2
    {
%    \end{macrocode}
%
% \begin{macro}[EXP]{\keyval_parse:nnn, \keyval_parse:NNn}
%   The main function starts the first of two loops. The outer loop splits the
%   key--value list at active commas, the inner loop will do so at other commas.
%   The use of \cs{s_@@_mark} here prevents loss of braces from the key
%   argument.
%    \begin{macrocode}
      \cs_if_exist:NTF \tex_expanded:D
        {
          \cs_new:Npn \keyval_parse:nnn ##1 ##2 ##3
            {
              \__kernel_exp_not:w \tex_expanded:D
                {
                  {
                    \@@_loop_active:nnw {##1} {##2}
                      \s_@@_mark ##3 #1 \s_@@_tail #1
                  }
                }
            }
        }
        {
          \cs_new:Npn \keyval_parse:nnn ##1 ##2 ##3
            {
              \group_align_safe_begin:
              \@@_loop_active:nnw {##1} {##2}
                \s_@@_mark ##3 #1 \s_@@_tail #1
              \group_align_safe_end:
            }
        }
      \cs_new_eq:NN \keyval_parse:NNn \keyval_parse:nnn
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_loop_active:nnw}
%   First a fast test for the end of the loop is done, it'll gobble everything
%   up to a \cs{s_@@_tail}. The loop ending macro will gobble everything to the
%   last comma in this definition.
%   If the end isn't reached yet, start the second loop splitting at other
%   commas, the next iteration of this first loop will be inserted by the end of
%   \cs{@@_loop_other:nnw}.
%    \begin{macrocode}
      \cs_new:Npn \@@_loop_active:nnw ##1 ##2 ##3 #1
        {
          \@@_if_recursion_tail:w ##3
            \@@_end_loop_active:w \s_@@_tail
          \@@_loop_other:nnw {##1} {##2} ##3 , \s_@@_tail ,
        }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_split_other:w, \@@_split_active:w}
%   These two macros allow to split at the first equals sign of category 12 or
%   13. At the same time they also execute branching by inserting the first
%   token following \cs{s_@@_mark} that followed the equals sign. Hence they
%   also test for the presence of such an equals sign simultaneously.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_other:w ##1 = ##2 \s_@@_mark ##3
        { ##3 ##1 \s_@@_stop \s_@@_mark ##2 }
      \cs_new:Npn \@@_split_active:w ##1 #2 ##2 \s_@@_mark ##3
        { ##3 ##1 \s_@@_stop \s_@@_mark ##2 }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_loop_other:nnw}
%   The second loop uses the same test for its end as the first loop, next it
%   splits at the first active equals sign using \cs{@@_split_active:w}.  The
%   \cs{s_@@_nil} prevents accidental brace stripping and acts as a delimiter in
%   the next steps. First testing for an active equals sign will reduce the
%   number of necessary expansion steps for the expected average use case of
%   other equals signs and hence perform better on average.
%    \begin{macrocode}
      \cs_new:Npn \@@_loop_other:nnw ##1 ##2 ##3 ,
        {
          \@@_if_recursion_tail:w ##3
            \@@_end_loop_other:w \s_@@_tail
          \@@_split_active:w ##3 \s_@@_nil
            \s_@@_mark \@@_split_active_auxi:w
            #2 \s_@@_mark \@@_clean_up_active:w
          {##1} {##2}
          \s_@@_mark
        }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_split_active_auxi:w}
% \begin{macro}[EXP]{\@@_split_active_auxii:w}
% \begin{macro}[EXP]{\@@_split_active_auxiii:w}
% \begin{macro}[EXP]{\@@_split_active_auxiv:w}
% \begin{macro}[EXP]{\@@_split_active_auxv:w}
%   After \cs{@@_split_active:w} the following will only be called if there was
%   at least one active equals sign in the current key--value pair. Therefore
%   this is the execution branch for a key--value pair with an active equals
%   sign. |##1| will be everything up to the first active equals sign. First it
%   tests for other equals signs in the key name, which will eventually throw an
%   error via \cs{@@_misplaced_equal_after_active_error:w}. If none was found we
%   forward the key to \cs{@@_split_active_auxii:w}.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_active_auxi:w ##1 \s_@@_stop
        {
          \@@_split_other:w ##1 \s_@@_nil
            \s_@@_mark \@@_misplaced_equal_after_active_error:w
            = \s_@@_mark \@@_split_active_auxii:w
        }
%    \end{macrocode}
%   \cs{@@_split_active_auxii:w} gets the correct key name with a leading
%   \cs{s_@@_mark} as |##1|. It has to sanitise the remainder of the previous
%   test and trims the key name which will be forwarded to
%   \cs{@@_split_active_auxiii:w}.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_active_auxii:w
          ##1 \s_@@_nil \s_@@_mark \@@_misplaced_equal_after_active_error:w
          \s_@@_stop \s_@@_mark
          ##2 \s_@@_nil #2 \s_@@_mark \@@_clean_up_active:w
        { \@@_trim:nN {##1} \@@_split_active_auxiii:w ##2 \s_@@_nil }
%    \end{macrocode}
%   Next we test for a misplaced active equals sign in the value, if none is
%   found \cs{@@_split_active_auxiv:w} will be called.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_active_auxiii:w ##1 ##2 \s_@@_nil
        {
          \@@_split_active:w ##2 \s_@@_nil
            \s_@@_mark \@@_misplaced_equal_in_split_error:w
            #2 \s_@@_mark \@@_split_active_auxiv:w
            {##1}
        }
%    \end{macrocode}
%   This runs the last test after sanitising the remainder of the previous one.
%   This time test for a misplaced equals sign of category 12 in the value.
%   Finally the last auxiliary macro will be called.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_active_auxiv:w
          ##1 \s_@@_nil \s_@@_mark \@@_misplaced_equal_in_split_error:w
          \s_@@_stop \s_@@_mark
        {
          \@@_split_other:w ##1 \s_@@_nil
            \s_@@_mark \@@_misplaced_equal_in_split_error:w
            = \s_@@_mark \@@_split_active_auxv:w
        }
%    \end{macrocode}
%   This last macro in this execution branch sanitises the last test, trims the
%   value and passes it to \cs{@@_pair:nnnn}.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_active_auxv:w
          ##1 \s_@@_nil \s_@@_mark \@@_misplaced_equal_in_split_error:w
          \s_@@_stop \s_@@_mark
        { \@@_trim:nN { ##1 } \@@_pair:nnnn }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_clean_up_active:w}
%   The following is the branch taken if the key--value pair doesn't contain an
%   active equals sign. The remainder of that test will be cleaned up by
%   \cs{@@_clean_up_active:w} which will then split at an equals sign of
%   category other.
%    \begin{macrocode}
      \cs_new:Npn \@@_clean_up_active:w
          ##1 \s_@@_nil \s_@@_mark \@@_split_active_auxi:w \s_@@_stop \s_@@_mark
        {
          \@@_split_other:w ##1 \s_@@_nil
            \s_@@_mark \@@_split_other_auxi:w
            = \s_@@_mark \@@_clean_up_other:w
        }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_split_other_auxi:w}
% \begin{macro}[EXP]{\@@_split_other_auxii:w}
% \begin{macro}[EXP]{\@@_split_other_auxiii:w}
%   This is executed if the key--value pair doesn't contain an active equals
%   sign but at least one other. |##1| of \cs{@@_split_other_auxi:w} will
%   contain the complete key name, which is trimmed and forwarded to the next
%   auxiliary macro.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_other_auxi:w ##1 \s_@@_stop
        { \@@_trim:nN { ##1 } \@@_split_other_auxii:w }
%    \end{macrocode}
%   We know that the value doesn't contain misplaced active equals signs but we
%   have to test for others. Also we need to sanitise the previous test, which
%   is done here and not earlier to avoid superfluous argument grabbing.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_other_auxii:w
          ##1 ##2 \s_@@_nil = \s_@@_mark \@@_clean_up_other:w
        {
          \@@_split_other:w ##2 \s_@@_nil
            \s_@@_mark \@@_misplaced_equal_in_split_error:w
            = \s_@@_mark \@@_split_other_auxiii:w
            { ##1 }
        }
%    \end{macrocode}
%   \cs{@@_split_other_auxiii:w} sanitises the test for other equals signs,
%   trims the value and forwards it to \cs{@@_pair:nnnn}.
%    \begin{macrocode}
      \cs_new:Npn \@@_split_other_auxiii:w
          ##1 \s_@@_nil \s_@@_mark \@@_misplaced_equal_in_split_error:w
          \s_@@_stop \s_@@_mark
        { \@@_trim:nN { ##1 } \@@_pair:nnnn }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_clean_up_other:w}
%   \cs{@@_clean_up_other:w} is the last branch that might exist. It is called
%   if no equals sign was found, hence the only possibilities left are a blank
%   list element, which is to be skipped, or a lonely key. If it's no empty list
%   element this will trim the key name and forward it to \cs{@@_key:nn}.
%    \begin{macrocode}
      \cs_new:Npn \@@_clean_up_other:w
          ##1 \s_@@_nil \s_@@_mark \@@_split_other_auxi:w \s_@@_stop \s_@@_mark
        {
          \@@_if_blank:w ##1 \s_@@_nil \s_@@_stop \@@_blank_true:w
            \s_@@_mark \s_@@_stop
            \@@_trim:nN { ##1 } \@@_key:nn
        }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_misplaced_equal_after_active_error:w}
% \begin{macro}[EXP]{\@@_misplaced_equal_in_split_error:w}
%   All these two macros do is gobble the remainder of the current other loop
%   execution and throw an error. Afterwards they have to insert the next loop
%   iteration.
%    \begin{macrocode}
      \cs_new:Npn \@@_misplaced_equal_after_active_error:w
          \s_@@_mark ##1 \s_@@_stop \s_@@_mark ##2 \s_@@_nil
           = \s_@@_mark \@@_split_active_auxii:w
           \s_@@_mark ##3 \s_@@_nil
           #2 \s_@@_mark \@@_clean_up_active:w
        {
          \msg_expandable_error:nn
            { keyval } { misplaced-equals-sign }
          \@@_loop_other:nnw
        }
      \cs_new:Npn \@@_misplaced_equal_in_split_error:w
          \s_@@_mark ##1 \s_@@_stop \s_@@_mark ##2 \s_@@_nil
          ##3 \s_@@_mark ##4 ##5
        {
          \msg_expandable_error:nn
            { keyval } { misplaced-equals-sign }
          \@@_loop_other:nnw
        }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_end_loop_other:w, \@@_end_loop_active:w}
%   All that's left for the parsing loops are the macros which end the
%   recursion. Both just gobble the remaining tokens of the respective loop
%   including the next recursion call. \cs{@@_end_loop_other:w} also has to
%   insert the next iteration of the active loop.
%    \begin{macrocode}
      \cs_new:Npn \@@_end_loop_other:w
          \s_@@_tail
          \@@_split_active:w
          \s_@@_mark \s_@@_tail
          \s_@@_nil \s_@@_mark
          \@@_split_active_auxi:w
          #2 \s_@@_mark \@@_clean_up_active:w
        { \@@_loop_active:nnw }
      \cs_new:Npn \@@_end_loop_active:w
          \s_@@_tail
          \@@_loop_other:nnw ##1 \s_@@_mark \s_@@_tail , \s_@@_tail ,
        { }
%    \end{macrocode}
% \end{macro}
%
% The parsing loops are done, so here ends the definition of \cs{@@_tmp:w},
% which will finally set up the macros.
%    \begin{macrocode}
    }
  \char_set_catcode_active:n { `\, }
  \char_set_catcode_active:n { `\= }
  \@@_tmp:w , =
\group_end:
%    \end{macrocode}
%
% \begin{macro}[EXP]{\@@_pair:nnnn, \@@_key:nn}
%   These macros will be called on the parsed keys and values of the key--value
%   list. All arguments are completely trimmed. They test for blank key names
%   and call the functions passed to \cs{keyval_parse:nnn} inside of
%   \cs{exp_not:n} with the correct arguments. Afterwards they insert the next
%   iteration of the other loop.
%    \begin{macrocode}
\group_begin:
  \cs_set_protected:Npn \@@_tmp:w #1#2
    {
      \cs_new:Npn \@@_pair:nnnn ##1 ##2 ##3 ##4
        {
          \@@_if_blank:w \s_@@_mark ##2 \s_@@_nil \s_@@_stop \@@_blank_key_error:w
            \s_@@_mark \s_@@_stop
          #1
          \exp_not:n { ##4 {##2} {##1} }
          #2
          \@@_loop_other:nnw {##3} {##4}
        }
      \cs_new:Npn \@@_key:nn ##1 ##2
        {
          \@@_if_blank:w \s_@@_mark ##1 \s_@@_nil \s_@@_stop \@@_blank_key_error:w
            \s_@@_mark \s_@@_stop
          #1
          \exp_not:n { ##2 {##1} }
          #2
          \@@_loop_other:nnw {##2}
        }
    }
  \cs_if_exist:NTF \tex_expanded:D
    { \@@_tmp:w { } { } }
    { \@@_tmp:w \group_align_safe_end: \group_align_safe_begin: }
\group_end:
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_if_empty:w,\@@_if_blank:w,\@@_if_recursion_tail:w}
%   All these tests work by gobbling tokens until a certain combination is met,
%   which makes them pretty fast. The test for a blank argument should be called
%   with an arbitrary token following the argument. Each of these utilize the
%   fact that the argument will contain a leading \cs{s_@@_mark}.
%    \begin{macrocode}
\cs_new:Npn \@@_if_empty:w #1 \s_@@_mark \s_@@_stop { }
\cs_new:Npn \@@_if_blank:w \s_@@_mark #1 { \@@_if_empty:w \s_@@_mark }
\cs_new:Npn \@@_if_recursion_tail:w \s_@@_mark #1 \s_@@_tail { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_blank_true:w,\@@_blank_key_error:w}
%   These macros will be called if the tests above didn't gobble them, they
%   execute the branching.
%    \begin{macrocode}
\cs_new:Npn \@@_blank_true:w \s_@@_mark \s_@@_stop \@@_trim:nN #1 \@@_key:nn
  { \@@_loop_other:nnw }
\cs_new:Npn \@@_blank_key_error:w #1 \@@_loop_other:nnw
  {
    \msg_expandable_error:nn { keyval } { blank-key-name }
    \@@_loop_other:nnw
  }
%    \end{macrocode}
% \end{macro}
%
% Two messages for the low level parsing system.
%    \begin{macrocode}
\msg_new:nnn { keyval } { misplaced-equals-sign }
  { Misplaced~'='~in~key-value~input~\msg_line_context: }
\msg_new:nnn { keyval } { blank-key-name }
  { Blank~key~name~in~key-value~input~\msg_line_context: }
\prop_gput:Nnn \g_msg_module_name_prop { keyval } { LaTeX3 }
\prop_gput:Nnn \g_msg_module_type_prop { keyval } { }
%    \end{macrocode}
%
% \begin{macro}[EXP]{\@@_trim:nN}
% \begin{macro}[EXP]
%   {\@@_trim_auxi:w,\@@_trim_auxii:w,\@@_trim_auxiii:w,\@@_trim_auxiv:w}
% And an adapted version of \cs{__tl_trim_spaces:nn} which is a bit faster for
% our use case, as it can strip the braces at the end. This is pretty much the
% same concept, so I won't comment on it here. The speed gain by using this
% instead of \cs{tl_trim_spaces_apply:nN} is about 10\,\% of the total time for
% \cs{keyval_parse:NNn} with one key and one key--value pair, so I think it's
% worth it.
%    \begin{macrocode}
\group_begin:
  \cs_set_protected:Npn \@@_tmp:w #1
    {
      \cs_new:Npn \@@_trim:nN ##1
        {
          \@@_trim_auxi:w
            ##1
            \s_@@_nil
            \s_@@_mark #1 { }
            \s_@@_mark \@@_trim_auxii:w
            \@@_trim_auxiii:w
            #1 \s_@@_nil
            \@@_trim_auxiv:w
        }
      \cs_new:Npn \@@_trim_auxi:w ##1 \s_@@_mark #1 ##2 \s_@@_mark ##3
        {
          ##3
          \@@_trim_auxi:w
          \s_@@_mark
          ##2
          \s_@@_mark #1 {##1}
        }
      \cs_new:Npn \@@_trim_auxii:w \@@_trim_auxi:w \s_@@_mark \s_@@_mark ##1
        {
          \@@_trim_auxiii:w
          ##1
        }
      \cs_new:Npn \@@_trim_auxiii:w ##1 #1 \s_@@_nil ##2
        {
          ##2
          ##1 \s_@@_nil
          \@@_trim_auxiii:w
        }
%    \end{macrocode}
%   This is the one macro which differs from the original definition.
%    \begin{macrocode}
      \cs_new:Npn \@@_trim_auxiv:w
          \s_@@_mark ##1 \s_@@_nil
          \@@_trim_auxiii:w \s_@@_nil \@@_trim_auxiii:w
          ##2
        { ##2 { ##1 } }
    }
  \@@_tmp:w { ~ }
\group_end:
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Constants and variables}
%
%    \begin{macrocode}
%<@@=keys>
%    \end{macrocode}
%
% \begin{variable}
%   {
%     \c_@@_code_root_str    ,
%     \c_@@_default_root_str ,
%     \c_@@_groups_root_str  ,
%     \c_@@_inherit_root_str  ,
%     \c_@@_type_root_str    ,
%     \c_@@_validate_root_str
%   }
%   Various storage areas for the different data which make up keys.
%    \begin{macrocode}
\str_const:Nn \c_@@_code_root_str     { key~code~>~ }
\str_const:Nn \c_@@_default_root_str  { key~default~>~ }
\str_const:Nn \c_@@_groups_root_str   { key~groups~>~ }
\str_const:Nn \c_@@_inherit_root_str  { key~inherit~>~ }
\str_const:Nn \c_@@_type_root_str     { key~type~>~ }
\str_const:Nn \c_@@_validate_root_str { key~validate~>~ }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\c_@@_props_root_str}
%   The prefix for storing properties.
%    \begin{macrocode}
\str_const:Nn \c_@@_props_root_str { key~prop~>~ }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_choice_int, \l_keys_choice_tl}
%   Publicly accessible data on which choice is being used when several
%   are generated as a set.
%    \begin{macrocode}
\int_new:N \l_keys_choice_int
\tl_new:N \l_keys_choice_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_groups_clist}
%   Used for storing and recovering the list of groups which apply to a key:
%   set as a comma list but at one point we have to use this for a token
%   list recovery.
%    \begin{macrocode}
\clist_new:N \l_@@_groups_clist
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_key_str, \l_keys_key_tl}
%   The name of a key itself: needed when setting keys. The |tl| version
%   is deprecated but has to be handled manually.
%    \begin{macrocode}
\str_new:N \l_keys_key_str
\tl_new:N \l_keys_key_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_module_str}
%   The module for an entire set of keys.
%    \begin{macrocode}
\str_new:N \l_@@_module_str
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_no_value_bool}
%   A marker is needed internally to show if only a key or a key plus a
%   value was seen: this is recorded here.
%    \begin{macrocode}
\bool_new:N \l_@@_no_value_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_only_known_bool}
%   Used to track if only \enquote{known} keys are being set.
%    \begin{macrocode}
\bool_new:N \l_@@_only_known_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_path_str, \l_keys_path_tl}
%   The \enquote{path} of the current key is stored here: this is
%   available to the programmer and so is public. The older version is
%   deprecated but has to be handled manually.
%    \begin{macrocode}
\str_new:N \l_keys_path_str
\tl_new:N \l_keys_path_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_inherit_str}
%    \begin{macrocode}
\str_new:N \l_@@_inherit_str
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_relative_tl}
%   The relative path for passing keys back to the user. As this can
%   be explicitly no-value, it must be a token list.
%    \begin{macrocode}
\tl_new:N \l_@@_relative_tl
\tl_set:Nn \l_@@_relative_tl { \q_@@_no_value }
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_property_str}
%   The \enquote{property} begin set for a key at definition time is
%   stored here.
%    \begin{macrocode}
\str_new:N \l_@@_property_str
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_selective_bool, \l_@@_filtered_bool}
%   Two flags for using key groups: one to indicate that \enquote{selective}
%   setting is active, a second to specify which type (\enquote{opt-in}
%   or \enquote{opt-out}).
%    \begin{macrocode}
\bool_new:N \l_@@_selective_bool
\bool_new:N \l_@@_filtered_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_selective_seq}
%   The list of key groups being filtered in or out during selective setting.
%    \begin{macrocode}
\seq_new:N \l_@@_selective_seq
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_unused_clist}
%   Used when setting only some keys to store those left over.
%    \begin{macrocode}
\tl_new:N \l_@@_unused_clist
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_keys_value_tl}
%   The value given for a key: may be empty if no value was given.
%    \begin{macrocode}
\tl_new:N \l_keys_value_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_tmp_bool, \l_@@_tmpa_tl, \l_@@_tmpb_tl}
%   Scratch space.
%    \begin{macrocode}
\bool_new:N \l_@@_tmp_bool
\tl_new:N \l_@@_tmpa_tl
\tl_new:N \l_@@_tmpb_tl
%    \end{macrocode}
% \end{variable}
%
% \subsubsection{Internal auxiliaries}
%
% \begin{variable}{\s_@@_nil,\s_@@_mark,\s_@@_stop}
%   Internal scan marks.
%    \begin{macrocode}
\scan_new:N \s_@@_nil
\scan_new:N \s_@@_mark
\scan_new:N \s_@@_stop
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\q_@@_no_value}
%   Internal quarks.
%    \begin{macrocode}
\quark_new:N \q_@@_no_value
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}[pTF]{\@@_quark_if_no_value:N}
%   Branching quark conditional.
%    \begin{macrocode}
\__kernel_quark_new_conditional:Nn \@@_quark_if_no_value:N { TF }
%    \end{macrocode}
% \end{macro}
%
% \subsection{The key defining mechanism}
%
% \begin{macro}{\keys_define:nn}
% \begin{macro}{\@@_define:nnn, \@@_define:onn}
%   The public function for definitions is just a wrapper for the lower
%   level mechanism, more or less. The outer function is designed to
%   keep a track of the current module, to allow safe nesting. The module is set
%   removing any leading |/| (which is not needed here).
%    \begin{macrocode}
\cs_new_protected:Npn \keys_define:nn
  { \@@_define:onn \l_@@_module_str }
\cs_new_protected:Npn \@@_define:nnn #1#2#3
  {
    \str_set:Nx \l_@@_module_str { \@@_trim_spaces:n {#2} }
    \keyval_parse:NNn \@@_define:n \@@_define:nn {#3}
    \str_set:Nn \l_@@_module_str {#1}
  }
\cs_generate_variant:Nn \@@_define:nnn { o }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_define:n}
% \begin{macro}{\@@_define:nn}
% \begin{macro}{\@@_define_aux:nn}
%   The outer functions here record whether a value was given and then
%   converge on a common internal mechanism. There is first a search for
%   a property in the current key name, then a check to make sure it is
%   known before the code hands off to the next step.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_define:n #1
  {
    \bool_set_true:N \l_@@_no_value_bool
    \@@_define_aux:nn {#1} { }
  }
\cs_new_protected:Npn \@@_define:nn #1#2
  {
    \bool_set_false:N \l_@@_no_value_bool
    \@@_define_aux:nn {#1} {#2}
  }
\cs_new_protected:Npn \@@_define_aux:nn #1#2
  {
    \@@_property_find:n {#1}
    \cs_if_exist:cTF { \c_@@_props_root_str \l_@@_property_str }
      { \@@_define_code:n {#2} }
      {
        \str_if_empty:NF \l_@@_property_str
          {
            \msg_error:nnxx { keys } { property-unknown }
              \l_@@_property_str \l_keys_path_str
          }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_property_find:n}
% \begin{macro}
%   {
%     \@@_property_find_auxi:w   ,
%     \@@_property_find_auxii:w  ,
%     \@@_property_find_auxiii:w ,
%     \@@_property_find_auxiv:w
%   }
%   Searching for a property means finding the last |.| in the input,
%   and storing the text before and after it. Everything is turned into
%   strings, so there is no problem using an \texttt{x}-type expansion. Since
%   \cs{@@_trim_spaces:n} will turn its argument into a string anyway, this
%   function uses \cs{cs_set_nopar:Npx} instead of \cs{tl_set:Nx} to gain some
%   speed.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_property_find:n #1
  {
    \cs_set_nopar:Npx \l_@@_property_str { \@@_trim_spaces:n { #1 } }
    \exp_after:wN \@@_property_find_auxi:w \l_@@_property_str
      \s_@@_nil \@@_property_find_auxii:w
      . \s_@@_nil \@@_property_find_err:w
  }
\cs_new_protected:Npn \@@_property_find_auxi:w #1 . #2 \s_@@_nil #3
  {
    #3 #1 \s_@@_mark #2 \s_@@_nil #3
  }
\cs_new_protected:Npn \@@_property_find_auxii:w
    #1 \s_@@_mark #2 \s_@@_nil \@@_property_find_auxii:w . \s_@@_nil
    \@@_property_find_err:w
  {
    \cs_set_nopar:Npx \l_keys_path_str
      { \str_if_empty:NF \l_@@_module_str { \l_@@_module_str / } #1 }
    \@@_property_find_auxi:w #2 \s_@@_nil \@@_property_find_auxiii:w . \s_@@_nil
      \@@_property_find_auxiv:w
  }
\cs_new_protected:Npn \@@_property_find_auxiii:w #1 \s_@@_mark
  {
    \cs_set_nopar:Npx \l_keys_path_str { \l_keys_path_str . #1 }
    \@@_property_find_auxi:w
  }
\cs_new_protected:Npn \@@_property_find_auxiv:w
    #1 \s_@@_nil \@@_property_find_auxiii:w
    \s_@@_mark \s_@@_nil \@@_property_find_auxiv:w
  {
    \cs_set_nopar:Npx \l_@@_property_str { . #1 }
    \cs_set_nopar:Npx \l_keys_path_str
      { \exp_after:wN \@@_trim_spaces:n \exp_after:wN { \l_keys_path_str } }
    \tl_set_eq:NN \l_keys_path_tl \l_keys_path_str
  }
\cs_new_protected:Npn \@@_property_find_err:w
    #1 \s_@@_nil #2 \@@_property_find_err:w
  {
    \str_clear:N \l_@@_property_str
    \msg_error:nnn { keys } { no-property } {#1}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_define_code:n}
% \begin{macro}[EXP]{\@@_define_code:w}
%   Two possible cases. If there is a value for the key, then just use
%   the function. If not, then a check to make sure there is no need for
%   a value with the property. If there should be one then complain,
%   otherwise execute it. There is no need to check for a |:| as if it
%   was missing the earlier tests would have failed.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_define_code:n #1
  {
    \bool_if:NTF \l_@@_no_value_bool
      {
        \exp_after:wN \@@_define_code:w
          \l_@@_property_str \s_@@_stop
          { \use:c { \c_@@_props_root_str \l_@@_property_str } }
          {
            \msg_error:nnxx { keys } { property-requires-value }
              \l_@@_property_str \l_keys_path_str
          }
      }
      { \use:c { \c_@@_props_root_str \l_@@_property_str } {#1} }
  }
\exp_last_unbraced:NNNNo
  \cs_new:Npn \@@_define_code:w #1 \c_colon_str #2 \s_@@_stop
    { \tl_if_empty:nTF {#2} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Turning properties into actions}
%
% \begin{macro}{\@@_bool_set:Nn, \@@_bool_set:cn}
%   Boolean keys are really just choices, but all done by hand. The
%   second argument here is the scope: either empty or \texttt{ g } for
%   global.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_bool_set:Nn #1#2
  {
    \bool_if_exist:NF #1 { \bool_new:N #1 }
    \@@_choice_make:
    \@@_cmd_set:nx { \l_keys_path_str / true }
      { \exp_not:c { bool_ #2 set_true:N } \exp_not:N #1 }
    \@@_cmd_set:nx { \l_keys_path_str / false }
      { \exp_not:c { bool_ #2 set_false:N } \exp_not:N #1 }
    \@@_cmd_set:nn { \l_keys_path_str / unknown }
      {
        \msg_error:nnx { keys } { boolean-values-only }
          \l_keys_key_str
      }
    \@@_default_set:n { true }
  }
\cs_generate_variant:Nn \@@_bool_set:Nn { c }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_bool_set_inverse:Nn, \@@_bool_set_inverse:cn}
%   Inverse boolean setting is much the same.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_bool_set_inverse:Nn #1#2
  {
    \bool_if_exist:NF #1 { \bool_new:N #1 }
    \@@_choice_make:
    \@@_cmd_set:nx { \l_keys_path_str / true }
      { \exp_not:c { bool_ #2 set_false:N } \exp_not:N #1 }
    \@@_cmd_set:nx { \l_keys_path_str / false }
      { \exp_not:c { bool_ #2 set_true:N } \exp_not:N #1 }
    \@@_cmd_set:nn { \l_keys_path_str / unknown }
      {
        \msg_error:nnx { keys } { boolean-values-only }
          \l_keys_key_str
      }
    \@@_default_set:n { true }
  }
\cs_generate_variant:Nn \@@_bool_set_inverse:Nn { c }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_choice_make:, \@@_multichoice_make:}
% \begin{macro}{\@@_choice_make:N}
% \begin{macro}{\@@_choice_make_aux:N}
%   To make a choice from a key, two steps: set the code, and set the
%   unknown key. As multichoices and choices are essentially the same bar one
%   function, the code is given together.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_choice_make:
  { \@@_choice_make:N \@@_choice_find:n }
\cs_new_protected:Npn \@@_multichoice_make:
  { \@@_choice_make:N \@@_multichoice_find:n }
\cs_new_protected:Npn \@@_choice_make:N #1
  {
    \cs_if_exist:cTF
      { \c_@@_type_root_str \@@_parent:o \l_keys_path_str }
      {
        \str_if_eq:vnTF
          { \c_@@_type_root_str \@@_parent:o \l_keys_path_str }
          { choice }
          {
            \msg_error:nnxx { keys } { nested-choice-key }
              \l_keys_path_tl { \@@_parent:o \l_keys_path_str }
          }
          { \@@_choice_make_aux:N #1 }
      }
      { \@@_choice_make_aux:N #1 }
  }
\cs_new_protected:Npn \@@_choice_make_aux:N #1
  {
    \cs_set_nopar:cpn { \c_@@_type_root_str \l_keys_path_str }
      { choice }
    \@@_cmd_set:nn \l_keys_path_str { #1 {##1} }
    \@@_cmd_set:nn { \l_keys_path_str / unknown }
      {
        \msg_error:nnxx { keys } { choice-unknown }
          \l_keys_path_str {##1}
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_choices_make:nn, \@@_multichoices_make:nn}
% \begin{macro}{\@@_choices_make:Nnn}
%   Auto-generating choices means setting up the root key as a choice, then
%   defining each choice in turn.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_choices_make:nn
  { \@@_choices_make:Nnn \@@_choice_make: }
\cs_new_protected:Npn \@@_multichoices_make:nn
  { \@@_choices_make:Nnn \@@_multichoice_make: }
\cs_new_protected:Npn \@@_choices_make:Nnn #1#2#3
  {
    #1
    \int_zero:N \l_keys_choice_int
    \clist_map_inline:nn {#2}
      {
        \int_incr:N \l_keys_choice_int
        \@@_cmd_set:nx
          { \l_keys_path_str / \@@_trim_spaces:n {##1} }
          {
            \tl_set:Nn \exp_not:N \l_keys_choice_tl {##1}
            \int_set:Nn \exp_not:N \l_keys_choice_int
              { \int_use:N \l_keys_choice_int }
            \exp_not:n {#3}
          }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\@@_cmd_set:nn, \@@_cmd_set:nx, \@@_cmd_set:Vn, \@@_cmd_set:Vo}
%   Setting the code for a key first logs if appropriate that we are
%   defining a new key, then saves the code.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_cmd_set:nn #1#2
  { \cs_set_protected:cpn { \c_@@_code_root_str #1 } ##1 {#2} }
\cs_generate_variant:Nn \@@_cmd_set:nn { nx , Vn , Vo }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_cs_set:NNpn, \@@_cs_set:Ncpn}
%   Creating control sequences is a bit more tricky than other cases as
%   we need to pick up the |p| argument. To make the internals look clearer,
%   the trailing |n| argument here is just for appearance.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_cs_set:NNpn #1#2#3#
  {
    \cs_set_protected:cpx { \c_@@_code_root_str \l_keys_path_str } ##1
      { #1 \exp_not:N #2 \exp_not:n {#3} {##1} }
    \use_none:n
  }
\cs_generate_variant:Nn \@@_cs_set:NNpn { Nc }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_default_set:n}
%   Setting a default value is easy. These are stored using \cs{cs_set:cpx} as this
%   avoids any worries about whether a token list exists.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_default_set:n #1
  {
    \tl_if_empty:nTF {#1}
      {
        \cs_set_eq:cN
          { \c_@@_default_root_str \l_keys_path_str }
          \tex_undefined:D
      }
      {
        \cs_set_nopar:cpx
          { \c_@@_default_root_str \l_keys_path_str }
          { \exp_not:n {#1} }
        \@@_value_requirement:nn { required } { false }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_groups_set:n}
%   Assigning a key to one or more groups uses comma lists. As the list of
%   groups only exists if there is anything to do, the setting is done using
%   a scratch list. For the usual grouping reasons we use the low-level
%   approach to undefining a list.  We also use the low-level approach for
%   the other case to avoid tripping up the |check-declarations| code.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_groups_set:n #1
  {
    \clist_set:Nn \l_@@_groups_clist {#1}
    \clist_if_empty:NTF \l_@@_groups_clist
      {
        \cs_set_eq:cN { \c_@@_groups_root_str \l_keys_path_str }
          \tex_undefined:D
      }
      {
        \cs_set_eq:cN { \c_@@_groups_root_str \l_keys_path_str }
          \l_@@_groups_clist
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_inherit:n}
%  Inheritance means ignoring anything already said about the key:
%  zap the lot and set up.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_inherit:n #1
  {
    \@@_undefine:
    \cs_set_nopar:cpn { \c_@@_inherit_root_str \l_keys_path_str } {#1}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_initialise:n}
%   A set up for initialisation: just run the code if it exists.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_initialise:n #1
  {
    \cs_if_exist:cTF
      { \c_@@_inherit_root_str \@@_parent:o \l_keys_path_str }
      { \@@_execute_inherit: }
      {
        \str_clear:N \l_@@_inherit_str
        \cs_if_exist:cT { \c_@@_code_root_str \l_keys_path_str }
          { \@@_execute:nn \l_keys_path_str {#1} }
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_meta_make:n}
% \begin{macro}{\@@_meta_make:nn}
%   To create a meta-key, simply set up to pass data through.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_meta_make:n #1
  {
    \@@_cmd_set:Vo \l_keys_path_str
      {
        \exp_after:wN \keys_set:nn \exp_after:wN { \l_@@_module_str } {#1}
      }
  }
\cs_new_protected:Npn \@@_meta_make:nn #1#2
  { \@@_cmd_set:Vn \l_keys_path_str { \keys_set:nn {#1} {#2} } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_prop_put:Nn, \@@_prop_put:cn}
%   Much the same as other variables, but needs a dedicated auxiliary.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_prop_put:Nn #1#2
  {
    \prop_if_exist:NF #1 { \prop_new:N #1 }
    \exp_after:wN \@@_find_key_module:wNN \l_keys_path_str \s_@@_stop
      \l_@@_tmpa_tl \l_@@_tmpb_tl
    \@@_cmd_set:nx \l_keys_path_str
      {
        \exp_not:c { prop_ #2 put:Nnn }
        \exp_not:N #1
        { \l_@@_tmpb_tl }
        \exp_not:n { {##1} }
      }
  }
\cs_generate_variant:Nn \@@_prop_put:Nn { c }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_undefine:}
%   Undefining a key has to be done without \cs{cs_undefine:c} as that
%   function acts globally.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_undefine:
  {
    \clist_map_inline:nn
      { code , default , groups , inherit , type , validate }
      {
        \cs_set_eq:cN
          { \tl_use:c { c_@@_ ##1 _root_str } \l_keys_path_str }
          \tex_undefined:D
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_value_requirement:nn}
% \begin{macro}{\@@_validate_forbidden:, \@@_validate_required:}
%   Validating key input is done using a second function which runs before
%   the main key code. Setting that up means setting it equal to a generic
%   stub which does the check. This approach makes the lookup very fast at
%   the cost of one additional csname per key that needs it. The cleanup here
%   has to know the structure of the following code.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_value_requirement:nn #1#2
  {
    \str_case:nnF {#2}
      {
        { true }
          {
            \cs_set_eq:cc
              { \c_@@_validate_root_str \l_keys_path_str }
              { @@_validate_ #1 : }
          }
        { false }
          {
            \cs_if_eq:ccT
              { \c_@@_validate_root_str \l_keys_path_str }
              { @@_validate_ #1 : }
              {
                \cs_set_eq:cN
                  { \c_@@_validate_root_str \l_keys_path_str }
                  \tex_undefined:D
              }
          }
      }
      {
        \msg_error:nnx { keys }
          { property-boolean-values-only }
          { .value_ #1 :n }
      }
  }
\cs_new_protected:Npn \@@_validate_forbidden:
  {
    \bool_if:NF \l_@@_no_value_bool
      {
        \msg_error:nnxx { keys } { value-forbidden }
          \l_keys_path_str \l_keys_value_tl
        \use_none:nnn
      }
  }
\cs_new_protected:Npn \@@_validate_required:
  {
    \bool_if:NT \l_@@_no_value_bool
      {
        \msg_error:nnx { keys } { value-required }
          \l_keys_path_str
        \use_none:nnn
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_variable_set:NnnN, \@@_variable_set:cnnN}
% \begin{macro}{\@@_variable_set_required:NnnN, \@@_variable_set_required:cnnN}
%   Setting a variable takes the type and scope separately so that
%   it is easy to make a new variable if needed.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_variable_set:NnnN #1#2#3#4
  {
    \use:c { #2_if_exist:NF } #1 { \use:c { #2 _new:N } #1 }
    \@@_cmd_set:nx \l_keys_path_str
      {
        \exp_not:c { #2 _ #3 set:N #4 }
        \exp_not:N #1
        \exp_not:n  { {##1} }
      }
  }
\cs_generate_variant:Nn \@@_variable_set:NnnN { c }
\cs_new_protected:Npn \@@_variable_set_required:NnnN #1#2#3#4
  {
    \@@_variable_set:NnnN #1 {#2} {#3} #4
    \@@_value_requirement:nn { required } { true }
  }
\cs_generate_variant:Nn \@@_variable_set_required:NnnN { c }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Creating key properties}
%
% The key property functions are all wrappers for internal functions,
% meaning that things stay readable and can also be altered later on.
%
% Importantly, while key properties have \enquote{normal} argument specs, the
% underlying code always supplies one braced argument to these. As such, argument
% expansion is handled by hand rather than using the standard tools. This shows
% up particularly for the two-argument properties, where things would otherwise
% go badly wrong.
%
% \begin{macro}{.bool_set:N, .bool_set:c}
% \begin{macro}{.bool_gset:N, .bool_gset:c}
%   One function for this.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .bool_set:N } #1
  { \@@_bool_set:Nn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .bool_set:c } #1
  { \@@_bool_set:cn {#1} { } }
\cs_new_protected:cpn { \c_@@_props_root_str .bool_gset:N } #1
  { \@@_bool_set:Nn #1 { g } }
\cs_new_protected:cpn { \c_@@_props_root_str .bool_gset:c } #1
  { \@@_bool_set:cn {#1} { g } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.bool_set_inverse:N, .bool_set_inverse:c}
% \begin{macro}{.bool_gset_inverse:N, .bool_gset_inverse:c}
%   One function for this.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .bool_set_inverse:N } #1
  { \@@_bool_set_inverse:Nn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .bool_set_inverse:c } #1
  { \@@_bool_set_inverse:cn {#1} { } }
\cs_new_protected:cpn { \c_@@_props_root_str .bool_gset_inverse:N } #1
  { \@@_bool_set_inverse:Nn #1 { g } }
\cs_new_protected:cpn { \c_@@_props_root_str .bool_gset_inverse:c } #1
  { \@@_bool_set_inverse:cn {#1} { g } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.choice:}
%   Making a choice is handled internally, as it is also needed by
%   \texttt{.generate_choices:n}.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .choice: }
  { \@@_choice_make: }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {.choices:nn, .choices:Vn, .choices:on, .choices:xn}
%   For auto-generation of a series of mutually-exclusive choices.
%   Here, |#1| consists of two separate
%   arguments, hence the slightly odd-looking implementation.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .choices:nn } #1
  { \@@_choices_make:nn #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .choices:Vn } #1
  { \exp_args:NV \@@_choices_make:nn #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .choices:on } #1
  { \exp_args:No \@@_choices_make:nn #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .choices:xn } #1
  { \exp_args:Nx \@@_choices_make:nn #1 }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.code:n}
%   Creating code is simply a case of passing through to the underlying
%   \texttt{set} function.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .code:n } #1
  { \@@_cmd_set:nn \l_keys_path_str {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.clist_set:N, .clist_set:c}
% \begin{macro}{.clist_gset:N, .clist_gset:c}
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .clist_set:N } #1
  { \@@_variable_set:NnnN #1 { clist } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .clist_set:c } #1
  { \@@_variable_set:cnnN {#1} { clist } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .clist_gset:N } #1
  { \@@_variable_set:NnnN #1 { clist } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .clist_gset:c } #1
  { \@@_variable_set:cnnN {#1} { clist } { g } n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {.cs_set:Np, .cs_set:cp, .cs_set_protected:Np, .cs_set_protected:cp}
% \begin{macro}
%   {.cs_gset:Np, .cs_gset:cp, .cs_gset_protected:Np, .cs_gset_protected:cp}
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .cs_set:Np } #1
  { \@@_cs_set:NNpn \cs_set:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_set:cp } #1
  { \@@_cs_set:Ncpn \cs_set:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_set_protected:Np } #1
  { \@@_cs_set:NNpn \cs_set_protected:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_set_protected:cp } #1
  { \@@_cs_set:Ncpn \cs_set_protected:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_gset:Np } #1
  { \@@_cs_set:NNpn \cs_gset:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_gset:cp } #1
  { \@@_cs_set:Ncpn \cs_gset:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_gset_protected:Np } #1
  { \@@_cs_set:NNpn \cs_gset_protected:Npn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .cs_gset_protected:cp } #1
  { \@@_cs_set:Ncpn \cs_gset_protected:Npn #1 { } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.default:n, .default:V, .default:o, .default:x}
%   Expansion is left to the internal functions.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .default:n } #1
  { \@@_default_set:n {#1} }
\cs_new_protected:cpn { \c_@@_props_root_str .default:V } #1
  { \exp_args:NV \@@_default_set:n #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .default:o } #1
  { \exp_args:No \@@_default_set:n {#1} }
\cs_new_protected:cpn { \c_@@_props_root_str .default:x } #1
  { \exp_args:Nx \@@_default_set:n {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.dim_set:N, .dim_set:c}
% \begin{macro}{.dim_gset:N, .dim_gset:c}
% Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .dim_set:N } #1
  { \@@_variable_set_required:NnnN #1 { dim } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .dim_set:c } #1
  { \@@_variable_set_required:cnnN {#1} { dim } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .dim_gset:N } #1
  { \@@_variable_set_required:NnnN #1 { dim } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .dim_gset:c } #1
  { \@@_variable_set_required:cnnN {#1} { dim } { g } n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.fp_set:N, .fp_set:c}
% \begin{macro}{.fp_gset:N, .fp_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .fp_set:N } #1
  { \@@_variable_set_required:NnnN #1 { fp } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .fp_set:c } #1
  { \@@_variable_set_required:cnnN {#1} { fp } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .fp_gset:N } #1
  { \@@_variable_set_required:NnnN #1 { fp } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .fp_gset:c } #1
  { \@@_variable_set_required:cnnN {#1} { fp } { g } n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.groups:n}
%   A single property to create groups of keys.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .groups:n } #1
  { \@@_groups_set:n {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.inherit:n}
%   Nothing complex: only one variant at the moment!
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .inherit:n } #1
  { \@@_inherit:n {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.initial:n, .initial:V, .initial:o, .initial:x}
%   The standard hand-off approach.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .initial:n } #1
  { \@@_initialise:n {#1} }
\cs_new_protected:cpn { \c_@@_props_root_str .initial:V } #1
  { \exp_args:NV \@@_initialise:n #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .initial:o } #1
  { \exp_args:No \@@_initialise:n {#1} }
\cs_new_protected:cpn { \c_@@_props_root_str .initial:x } #1
  { \exp_args:Nx \@@_initialise:n {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.int_set:N, .int_set:c}
% \begin{macro}{.int_gset:N, .int_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .int_set:N } #1
  { \@@_variable_set_required:NnnN #1 { int } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .int_set:c } #1
  { \@@_variable_set_required:cnnN {#1} { int } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .int_gset:N } #1
  { \@@_variable_set_required:NnnN #1 { int } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .int_gset:c } #1
  { \@@_variable_set_required:cnnN {#1} { int } { g } n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.meta:n}
%   Making a meta is handled internally.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .meta:n } #1
  { \@@_meta_make:n {#1} }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.meta:nn}
%   Meta with path: potentially lots of variants, but for the moment
%   no so many defined.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .meta:nn } #1
  { \@@_meta_make:nn #1 }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.multichoice:}
% \begin{macro}
%   {
%     .multichoices:nn, .multichoices:Vn, .multichoices:on,
%       .multichoices:xn,
%   }
%   The same idea as \texttt{.choice:} and \texttt{.choices:nn}, but
%   where more than one choice is allowed.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .multichoice: }
  { \@@_multichoice_make: }
\cs_new_protected:cpn { \c_@@_props_root_str .multichoices:nn } #1
  { \@@_multichoices_make:nn #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .multichoices:Vn } #1
  { \exp_args:NV \@@_multichoices_make:nn #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .multichoices:on } #1
  { \exp_args:No \@@_multichoices_make:nn #1 }
\cs_new_protected:cpn { \c_@@_props_root_str .multichoices:xn } #1
  { \exp_args:Nx \@@_multichoices_make:nn #1 }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.muskip_set:N, .muskip_set:c, .muskip_gset:N, .muskip_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .muskip_set:N } #1
  { \@@_variable_set_required:NnnN #1 { muskip } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .muskip_set:c } #1
  { \@@_variable_set_required:cnnN {#1} { muskip } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .muskip_gset:N } #1
  { \@@_variable_set_required:NnnN #1 { muskip } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .muskip_gset:c } #1
  { \@@_variable_set_required:cnnN {#1} { muskip } { g } n }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.prop_put:N, .prop_put:c, .prop_gput:N, .prop_gput:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .prop_put:N } #1
  { \@@_prop_put:Nn #1 { } }
\cs_new_protected:cpn { \c_@@_props_root_str .prop_put:c } #1
  { \@@_prop_put:cn {#1} { } }
\cs_new_protected:cpn { \c_@@_props_root_str .prop_gput:N } #1
  { \@@_prop_put:Nn #1 { g } }
\cs_new_protected:cpn { \c_@@_props_root_str .prop_gput:c } #1
  { \@@_prop_put:cn {#1} { g } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.skip_set:N, .skip_set:c}
% \begin{macro}{.skip_gset:N, .skip_gset:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .skip_set:N } #1
  { \@@_variable_set_required:NnnN #1 { skip } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .skip_set:c } #1
  { \@@_variable_set_required:cnnN {#1} { skip } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .skip_gset:N } #1
  { \@@_variable_set_required:NnnN #1 { skip } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .skip_gset:c } #1
  { \@@_variable_set_required:cnnN {#1} { skip } { g } n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.tl_set:N, .tl_set:c}
% \begin{macro}{.tl_gset:N, .tl_gset:c}
% \begin{macro}{.tl_set_x:N, .tl_set_x:c}
% \begin{macro}{.tl_gset_x:N, .tl_gset_x:c}
%   Setting a variable is very easy: just pass the data along.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .tl_set:N } #1
  { \@@_variable_set:NnnN #1 { tl } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_set:c } #1
  { \@@_variable_set:cnnN {#1} { tl } { } n }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_set_x:N } #1
  { \@@_variable_set:NnnN #1 { tl } { } x }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_set_x:c } #1
  { \@@_variable_set:cnnN {#1} { tl } { } x }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_gset:N } #1
  { \@@_variable_set:NnnN #1 { tl } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_gset:c } #1
  { \@@_variable_set:cnnN {#1} { tl } { g } n }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_gset_x:N } #1
  { \@@_variable_set:NnnN #1 { tl } { g } x }
\cs_new_protected:cpn { \c_@@_props_root_str .tl_gset_x:c } #1
  { \@@_variable_set:cnnN {#1} { tl } { g } x }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{.undefine:}
%   Another simple wrapper.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .undefine: }
  { \@@_undefine: }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{.value_forbidden:n}
% \begin{macro}{.value_required:n}
%   These are very similar, so both call the same function.
%    \begin{macrocode}
\cs_new_protected:cpn { \c_@@_props_root_str .value_forbidden:n } #1
  { \@@_value_requirement:nn { forbidden } {#1} }
\cs_new_protected:cpn { \c_@@_props_root_str .value_required:n } #1
  { \@@_value_requirement:nn { required } {#1} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsection{Setting keys}
%
% \begin{macro}
%   {
%     \keys_set:nn, \keys_set:nV, \keys_set:nv, \keys_set:no,
%     \@@_set:nn
%   }
% \begin{macro}{\@@_set:nnn}
%   A simple wrapper allowing for nesting.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_set:nn #1#2
  {
    \use:x
      {
        \bool_set_false:N \exp_not:N \l_@@_only_known_bool
        \bool_set_false:N \exp_not:N \l_@@_filtered_bool
        \bool_set_false:N \exp_not:N \l_@@_selective_bool
        \tl_set:Nn \exp_not:N \l_@@_relative_tl
          { \exp_not:N \q_@@_no_value }
        \@@_set:nn \exp_not:n { {#1} {#2} }
        \bool_if:NT \l_@@_only_known_bool
          { \bool_set_true:N \exp_not:N \l_@@_only_known_bool }
        \bool_if:NT \l_@@_filtered_bool
          { \bool_set_true:N \exp_not:N \l_@@_filtered_bool }
        \bool_if:NT \l_@@_selective_bool
          { \bool_set_true:N \exp_not:N \l_@@_selective_bool }
        \tl_set:Nn \exp_not:N \l_@@_relative_tl
          { \exp_not:o \l_@@_relative_tl }
      }
  }
\cs_generate_variant:Nn \keys_set:nn { nV , nv , no }
\cs_new_protected:Npn \@@_set:nn #1#2
  { \exp_args:No \@@_set:nnn \l_@@_module_str {#1} {#2} }
\cs_new_protected:Npn \@@_set:nnn #1#2#3
  {
    \str_set:Nx \l_@@_module_str { \@@_trim_spaces:n {#2} }
    \keyval_parse:NNn \@@_set_keyval:n \@@_set_keyval:nn {#3}
    \str_set:Nn \l_@@_module_str {#1}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \keys_set_known:nnN, \keys_set_known:nVN,
%     \keys_set_known:nvN, \keys_set_known:noN
%   }
% \begin{macro}
%   {
%     \keys_set_known:nnnN, \keys_set_known:nVnN,
%     \keys_set_known:nvnN, \keys_set_known:nonN
%   }
% \begin{macro}{\@@_set_known:nnnnN}
% \begin{macro}
%   {
%     \keys_set_known:nn, \keys_set_known:nV,
%     \keys_set_known:nv, \keys_set_known:no
%   }
%  \begin{macro}{\@@_set_known:nnn}
%   Setting known keys simply means setting the appropriate flag, then
%   running the standard code. To allow for nested setting, any existing
%   value of \cs{l_@@_unused_clist} is saved on the stack and reset
%   afterwards. Note that for speed/simplicity reasons we use a \texttt{tl}
%   operation to set the \texttt{clist} here!
%    \begin{macrocode}
\cs_new_protected:Npn \keys_set_known:nnN #1#2#3
  {
    \exp_args:No \@@_set_known:nnnnN
      \l_@@_unused_clist \q_@@_no_value {#1} {#2} #3
  }
\cs_generate_variant:Nn \keys_set_known:nnN { nV , nv , no }
\cs_new_protected:Npn \keys_set_known:nnnN #1#2#3#4
  {
    \exp_args:No \@@_set_known:nnnnN
      \l_@@_unused_clist {#3} {#1} {#2} #4
  }
\cs_generate_variant:Nn \keys_set_known:nnnN { nV , nv , no }
\cs_new_protected:Npn \@@_set_known:nnnnN #1#2#3#4#5
  {
    \clist_clear:N \l_@@_unused_clist
    \@@_set_known:nnn {#2} {#3} {#4}
    \__kernel_tl_set:Nx #5 { \exp_not:o \l_@@_unused_clist }
    \tl_set:Nn \l_@@_unused_clist {#1}
  }
\cs_new_protected:Npn \keys_set_known:nn #1#2
  { \@@_set_known:nnn \q_@@_no_value {#1} {#2} }
\cs_generate_variant:Nn \keys_set_known:nn { nV , nv , no }
\cs_new_protected:Npn \@@_set_known:nnn #1#2#3
  {
    \use:x
      {
        \bool_set_true:N \exp_not:N \l_@@_only_known_bool
        \bool_set_false:N \exp_not:N \l_@@_filtered_bool
        \bool_set_false:N \exp_not:N \l_@@_selective_bool
        \tl_set:Nn \exp_not:N \l_@@_relative_tl { \exp_not:n {#1} }
        \@@_set:nn \exp_not:n { {#2} {#3} }
        \bool_if:NF \l_@@_only_known_bool
          { \bool_set_false:N \exp_not:N \l_@@_only_known_bool }
        \bool_if:NT \l_@@_filtered_bool
          { \bool_set_true:N \exp_not:N \l_@@_filtered_bool }
        \bool_if:NT \l_@@_selective_bool
          { \bool_set_true:N \exp_not:N \l_@@_selective_bool }
        \tl_set:Nn \exp_not:N \l_@@_relative_tl
          { \exp_not:o \l_@@_relative_tl }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \keys_set_filter:nnnN, \keys_set_filter:nnVN, \keys_set_filter:nnvN,
%       \keys_set_filter:nnoN
%   }
% \begin{macro}
%   {
%     \keys_set_filter:nnnnN, \keys_set_filter:nnVnN,
%     \keys_set_filter:nnvnN, \keys_set_filter:nnonN
%   }
% \begin{macro}{\@@_set_filter:nnnnnN}
% \begin{macro}
%   {
%     \keys_set_filter:nnn, \keys_set_filter:nnV, \keys_set_filter:nnv,
%       \keys_set_filter:nno
%   }
% \begin{macro}{\@@_set_filter:nnnn}
% \begin{macro}
%   {
%     \keys_set_groups:nnn, \keys_set_groups:nnV, \keys_set_groups:nnv,
%       \keys_set_groups:nno
%   }
%  \begin{macro}{\@@_set_selective:nnn}
%  \begin{macro}{\@@_set_selective:nnnn}
%   The idea of setting keys in a selective manner again uses flags
%   wrapped around the basic code. The comments on \cs{keys_set_known:nnN}
%   also apply here. We have a bit more shuffling to do to keep everything
%   nestable.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_set_filter:nnnN #1#2#3#4
  {
    \exp_args:No \@@_set_filter:nnnnnN
      \l_@@_unused_clist
        \q_@@_no_value {#1} {#2} {#3} #4
  }
\cs_generate_variant:Nn \keys_set_filter:nnnN { nnV , nnv , nno }
\cs_new_protected:Npn \keys_set_filter:nnnnN #1#2#3#4#5
  {
    \exp_args:No \@@_set_filter:nnnnnN
      \l_@@_unused_clist {#4} {#1} {#2} {#3} #5
  }
\cs_generate_variant:Nn \keys_set_filter:nnnnN { nnV , nnv , nno }
\cs_new_protected:Npn \@@_set_filter:nnnnnN #1#2#3#4#5#6
  {
    \clist_clear:N \l_@@_unused_clist
    \@@_set_filter:nnnn {#2} {#3} {#4} {#5}
    \__kernel_tl_set:Nx #6 { \exp_not:o \l_@@_unused_clist }
    \tl_set:Nn \l_@@_unused_clist {#1}
  }
\cs_new_protected:Npn \keys_set_filter:nnn #1#2#3
  {\@@_set_filter:nnnn \q_@@_no_value {#1} {#2} {#3} }
\cs_generate_variant:Nn \keys_set_filter:nnn { nnV , nnv , nno }
\cs_new_protected:Npn \@@_set_filter:nnnn #1#2#3#4
  {
    \use:x
      {
        \bool_set_false:N \exp_not:N \l_@@_only_known_bool
        \bool_set_true:N \exp_not:N \l_@@_filtered_bool
        \bool_set_true:N \exp_not:N \l_@@_selective_bool
        \tl_set:Nn \exp_not:N \l_@@_relative_tl { \exp_not:n {#1} }
        \@@_set_selective:nnn \exp_not:n { {#2} {#3} {#4} }
        \bool_if:NT \l_@@_only_known_bool
          { \bool_set_true:N \exp_not:N \l_@@_only_known_bool }
        \bool_if:NF \l_@@_filtered_bool
          { \bool_set_false:N \exp_not:N \l_@@_filtered_bool }
        \bool_if:NF \l_@@_selective_bool
          { \bool_set_false:N \exp_not:N \l_@@_selective_bool }
        \tl_set:Nn \exp_not:N \l_@@_relative_tl
          { \exp_not:o \l_@@_relative_tl }
      }
  }
\cs_new_protected:Npn \keys_set_groups:nnn #1#2#3
  {
    \use:x
      {
        \bool_set_false:N \exp_not:N \l_@@_only_known_bool
        \bool_set_false:N \exp_not:N \l_@@_filtered_bool
        \bool_set_true:N \exp_not:N \l_@@_selective_bool
        \tl_set:Nn \exp_not:N \l_@@_relative_tl
          { \exp_not:N \q_@@_no_value }
        \@@_set_selective:nnn \exp_not:n { {#1} {#2} {#3} }
        \bool_if:NT \l_@@_only_known_bool
          { \bool_set_true:N \exp_not:N \l_@@_only_known_bool }
        \bool_if:NF \l_@@_filtered_bool
          { \bool_set_true:N \exp_not:N \l_@@_filtered_bool }
        \bool_if:NF \l_@@_selective_bool
          { \bool_set_false:N \exp_not:N \l_@@_selective_bool }
        \tl_set:Nn \exp_not:N \l_@@_relative_tl
          { \exp_not:o \l_@@_relative_tl }
      }
  }
\cs_generate_variant:Nn \keys_set_groups:nnn { nnV , nnv , nno }
\cs_new_protected:Npn \@@_set_selective:nnn
  { \exp_args:No \@@_set_selective:nnnn \l_@@_selective_seq }
\cs_new_protected:Npn \@@_set_selective:nnnn #1#2#3#4
  {
    \seq_set_from_clist:Nn \l_@@_selective_seq {#3}
    \@@_set:nn {#2} {#4}
    \tl_set:Nn \l_@@_selective_seq {#1}
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_set_keyval:n, \@@_set_keyval:nn}
% \begin{macro}{\@@_set_keyval:nnn, \@@_set_keyval:onn}
% \begin{macro}{\@@_find_key_module:wNN}
% \begin{macro}
%   {
%     \@@_find_key_module_auxi:Nw   ,
%     \@@_find_key_module_auxii:Nw  ,
%     \@@_find_key_module_auxiii:Nn ,
%     \@@_find_key_module_auxiv:Nw
%   }
% \begin{macro}{\@@_set_selective:}
%   A shared system once again. First, set the current path and add a
%   default if needed. There are then checks to see if the a value is
%   required or forbidden. If everything passes, move on to execute the
%   code.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_set_keyval:n #1
  {
    \bool_set_true:N \l_@@_no_value_bool
    \@@_set_keyval:onn \l_@@_module_str {#1} { }
  }
\cs_new_protected:Npn \@@_set_keyval:nn #1#2
  {
    \bool_set_false:N \l_@@_no_value_bool
    \@@_set_keyval:onn \l_@@_module_str {#1} {#2}
  }
%    \end{macrocode}
%   The key path here can be fully defined, after which there is a search
%   for the key and module names: the user may have passed them with part
%   of what is actually the module (for our purposes) in the key name. As
%   that happens on a per-key basis, we use the stack approach to restore
%   the module name without a group.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_set_keyval:nnn #1#2#3
  {
    \__kernel_tl_set:Nx \l_keys_path_str
      {
        \tl_if_blank:nF {#1}
          { #1 / }
        \@@_trim_spaces:n {#2}
      }
    \str_clear:N \l_@@_module_str
    \str_clear:N \l_@@_inherit_str
    \exp_after:wN \@@_find_key_module:wNN \l_keys_path_str \s_@@_stop
      \l_@@_module_str \l_keys_key_str
    \tl_set_eq:NN \l_keys_key_tl \l_keys_key_str
    \@@_value_or_default:n {#3}
    \bool_if:NTF \l_@@_selective_bool
      \@@_set_selective:
      \@@_execute:
    \str_set:Nn \l_@@_module_str {#1}
  }
\cs_generate_variant:Nn \@@_set_keyval:nnn { o }
%    \end{macrocode}
%   This function uses \cs{cs_set_nopar:Npx} internally for performance reasons,
%   the argument |#1| is already a string in every usage, so turning it into a
%   string again seems unnecessary.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_find_key_module:wNN #1 \s_@@_stop #2 #3
  {
    \@@_find_key_module_auxi:Nw #2 #1 \s_@@_nil \@@_find_key_module_auxii:Nw
      / \s_@@_nil \@@_find_key_module_auxiv:Nw #3
  }
\cs_new_protected:Npn \@@_find_key_module_auxi:Nw #1 #2 / #3 \s_@@_nil #4
  {
    #4 #1 #2 \s_@@_mark #3 \s_@@_nil #4
  }
\cs_new_protected:Npn \@@_find_key_module_auxii:Nw
    #1 #2 \s_@@_mark #3 \s_@@_nil \@@_find_key_module_auxii:Nw
  {
    \cs_set_nopar:Npx #1 { \tl_if_empty:NF #1 { #1 / } #2 }
    \@@_find_key_module_auxi:Nw #1 #3 \s_@@_nil \@@_find_key_module_auxiii:Nw
  }
\cs_new_protected:Npn \@@_find_key_module_auxiii:Nw #1 #2 \s_@@_mark
  {
    \cs_set_nopar:Npx #1 { \tl_if_empty:NF #1 { #1 / } #2 }
    \@@_find_key_module_auxi:Nw #1
  }
\cs_new_protected:Npn \@@_find_key_module_auxiv:Nw
    #1 #2 \s_@@_nil #3 \s_@@_mark
    \s_@@_nil \@@_find_key_module_auxiv:Nw #4
  {
    \cs_set_nopar:Npn #4 { #2 }
  }
%    \end{macrocode}
%  If selective setting is active, there are a number of possible sub-cases
%  to consider. The key name may not be known at all or if it is, it may not
%  have any groups assigned. There is then the question of whether the
%  selection is opt-in or opt-out.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_set_selective:
  {
    \cs_if_exist:cTF { \c_@@_groups_root_str \l_keys_path_str }
      {
        \clist_set_eq:Nc \l_@@_groups_clist
          { \c_@@_groups_root_str \l_keys_path_str }
        \@@_check_groups:
      }
      {
        \bool_if:NTF \l_@@_filtered_bool
          \@@_execute:
          \@@_store_unused:
      }
  }
%    \end{macrocode}
%    In the case where selective setting requires a comparison of the list
%    of groups which apply to a key with the list of those which have been
%    set active. That requires two mappings, and again a different outcome
%    depending on whether opt-in or opt-out is set.
%    We cannot replace the clist mapping by \cs{clist_if_in:NnTF} because
%    catcodes may not be the same; they cannot be normalized easily in the
%    clist because of the remote possibility that some items need braces
%    if they involve commas or leading/trailing spaces.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_check_groups:
  {
    \bool_set_false:N \l_@@_tmp_bool
    \seq_map_inline:Nn \l_@@_selective_seq
      {
        \clist_map_inline:Nn \l_@@_groups_clist
          {
            \str_if_eq:nnT {##1} {####1}
              {
                \bool_set_true:N \l_@@_tmp_bool
                \clist_map_break:n \seq_map_break:
              }
          }
      }
    \bool_if:NTF \l_@@_tmp_bool
      {
        \bool_if:NTF \l_@@_filtered_bool
          \@@_store_unused:
          \@@_execute:
      }
      {
        \bool_if:NTF \l_@@_filtered_bool
          \@@_execute:
          \@@_store_unused:
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_value_or_default:n}
% \begin{macro}{\@@_default_inherit:}
%   If a value is given, return it as |#1|, otherwise send a default if
%   available.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_value_or_default:n #1
  {
    \bool_if:NTF \l_@@_no_value_bool
      {
        \cs_if_exist:cTF { \c_@@_default_root_str \l_keys_path_str }
          {
            \tl_set_eq:Nc
              \l_keys_value_tl
              { \c_@@_default_root_str \l_keys_path_str }
          }
          {
            \tl_clear:N \l_keys_value_tl
            \cs_if_exist:cT
              { \c_@@_inherit_root_str \@@_parent:o \l_keys_path_str }
              { \@@_default_inherit: }
          }
      }
      { \tl_set:Nn \l_keys_value_tl {#1} }
  }
\cs_new_protected:Npn \@@_default_inherit:
  {
    \clist_map_inline:cn
      { \c_@@_inherit_root_str \@@_parent:o \l_keys_path_str }
      {
        \cs_if_exist:cT
          { \c_@@_default_root_str ##1 / \l_keys_key_str }
          {
            \tl_set_eq:Nc
              \l_keys_value_tl
              { \c_@@_default_root_str ##1 / \l_keys_key_str }
            \clist_map_break:
          }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_execute:, \@@_execute_inherit:, \@@_execute_unknown:}
% \begin{macro}[EXP]{\@@_execute:nn, \@@_execute:no}
% \begin{macro}{\@@_store_unused:,\@@_store_unused_aux:}
%   Actually executing a key is done in two parts. First, look for the
%   key itself, then look for the \texttt{unknown} key with the same
%   path. If both of these fail, complain. What exactly happens if a key
%   is unknown depends on whether unknown keys are being skipped or if
%   an error should be raised.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_execute:
  {
    \cs_if_exist:cTF { \c_@@_code_root_str \l_keys_path_str }
      {
        \cs_if_exist_use:c { \c_@@_validate_root_str \l_keys_path_str }
        \@@_execute:no \l_keys_path_str \l_keys_value_tl
      }
      {
        \cs_if_exist:cTF
          { \c_@@_inherit_root_str \@@_parent:o \l_keys_path_str }
          { \@@_execute_inherit: }
          { \@@_execute_unknown: }
      }
  }
%    \end{macrocode}
%   To deal with the case where there is no hit, we leave
%   \cs{@@_execute_unknown:} in the input stream and clean it up using the
%    break function: that avoids needing a boolean.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_execute_inherit:
  {
    \clist_map_inline:cn
      { \c_@@_inherit_root_str \@@_parent:o \l_keys_path_str }
      {
        \cs_if_exist:cT
          { \c_@@_code_root_str ##1 / \l_keys_key_str }
          {
            \str_set:Nn \l_@@_inherit_str {##1}
            \cs_if_exist_use:c { \c_@@_validate_root_str ##1 / \l_keys_key_str }
            \@@_execute:no { ##1 / \l_keys_key_str } \l_keys_value_tl
            \clist_map_break:n \use_none:n
          }
      }
    \@@_execute_unknown:
  }
\cs_new_protected:Npn \@@_execute_unknown:
  {
    \bool_if:NTF \l_@@_only_known_bool
      { \@@_store_unused: }
      {
        \cs_if_exist:cTF
          { \c_@@_code_root_str \l_@@_module_str / unknown }
          { \@@_execute:no { \l_@@_module_str / unknown } \l_keys_value_tl }
          {
            \msg_error:nnxx { keys } { unknown }
              \l_keys_path_str \l_@@_module_str
          }
      }
  }
%    \end{macrocode}
%   A key's code is in the control sequence with csname
%   \cs{c_@@_code_root_str} |#1|.  We expand it once to get the
%   replacement text (with argument |#2|) and call \cs{use:n}
%   with this replacement as its argument.  This ensures that any
%   undefined control sequence error in the key's code will lead to an
%   error message of the form |<argument>|\ldots{}\meta{control
%   sequence} in which one can read the (undefined) \meta{control
%   sequence} in full, rather than an error message that starts with the
%   potentially very long key name, which would make the (undefined)
%   \meta{control sequence} be truncated or sometimes completely hidden.
%   See \url{https://github.com/latex3/latex2e/issues/351}.
%    \begin{macrocode}
\cs_new:Npn \@@_execute:nn #1#2
  { \@@_execute:no {#1} { \prg_do_nothing: #2 } }
\cs_new:Npn \@@_execute:no #1#2
  {
    \exp_args:NNo \exp_args:No \use:n
      {
        \cs:w \c_@@_code_root_str #1 \exp_after:wN \cs_end:
        \exp_after:wN {#2}
      }
  }
%    \end{macrocode}
%   When there is no relative path, things here are easy: just save the key
%   name and value. When we are working with a relative path, first we
%   need to turn it into a string: that can't happen earlier as we need
%   to store \cs{q_@@_no_value}. Then, use a standard delimited approach to fish
%   out the partial path.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_store_unused:
  {
    \@@_quark_if_no_value:NTF \l_@@_relative_tl
      {
        \clist_put_right:Nx \l_@@_unused_clist
          {
            \l_keys_key_str
            \bool_if:NF \l_@@_no_value_bool
              { = { \exp_not:o \l_keys_value_tl } }
          }
      }
      {
        \tl_if_empty:NTF \l_@@_relative_tl
          {
            \clist_put_right:Nx \l_@@_unused_clist
              {
                \l_keys_path_str
                \bool_if:NF \l_@@_no_value_bool
                  { = { \exp_not:o \l_keys_value_tl } }
              }
          }
          { \@@_store_unused_aux: }
      }
  }
\cs_new_protected:Npn \@@_store_unused_aux:
  {
    \__kernel_tl_set:Nx \l_@@_relative_tl
      { \exp_args:No \@@_trim_spaces:n \l_@@_relative_tl }
    \use:x
      {
        \cs_set_protected:Npn \@@_store_unused:w
          ####1 \l_@@_relative_tl /
          ####2 \l_@@_relative_tl /
          ####3 \s_@@_stop
      }
        {
          \tl_if_blank:nF {##1}
            {
              \msg_error:nnxx { keys } { bad-relative-key-path }
                \l_keys_path_str
                \l_@@_relative_tl
            }
          \clist_put_right:Nx \l_@@_unused_clist
            {
              \exp_not:n {##2}
              \bool_if:NF \l_@@_no_value_bool
                { = { \exp_not:o \l_keys_value_tl } }
            }
        }
    \use:x
      {
        \@@_store_unused:w \l_keys_path_str
          \l_@@_relative_tl / \l_@@_relative_tl /
          \s_@@_stop
      }
  }
\cs_new_protected:Npn \@@_store_unused:w { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_choice_find:n}
% \begin{macro}[EXP]{\@@_choice_find:nn}
% \begin{macro}[EXP]{\@@_multichoice_find:n}
%   Executing a choice has two parts. First, try the choice given, then
%   if that fails call the unknown key. That always exists, as it is created
%   when a choice is first made. So there is no need for any escape code.
%   For multiple choices, the same code ends up used in a mapping.
%    \begin{macrocode}
\cs_new:Npn \@@_choice_find:n #1
  {
    \str_if_empty:NTF \l_@@_inherit_str
      { \@@_choice_find:nn \l_keys_path_str {#1} }
      {
        \@@_choice_find:nn
          { \l_@@_inherit_str / \l_keys_key_str } {#1}
      }
  }
\cs_new:Npn \@@_choice_find:nn #1#2
  {
    \cs_if_exist:cTF { \c_@@_code_root_str #1 / \@@_trim_spaces:n {#2} }
      { \@@_execute:nn { #1 / \@@_trim_spaces:n {#2} } {#2} }
      { \@@_execute:nn { #1 / unknown } {#2} }
  }
\cs_new:Npn \@@_multichoice_find:n #1
  { \clist_map_function:nN {#1} \@@_choice_find:n }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsection{Utilities}
%
% \begin{macro}[EXP]{\@@_parent:o}
% \begin{macro}[EXP]
%   {
%     \@@_parent_auxi:w   ,
%     \@@_parent_auxii:w  ,
%     \@@_parent_auxiii:n ,
%     \@@_parent_auxiv:w
%   }
%   Used to strip off the ending part of the key path after the last~|/|.
%    \begin{macrocode}
\cs_new:Npn \@@_parent:o #1
  {
    \exp_after:wN \@@_parent_auxi:w #1 \q_nil \@@_parent_auxii:w
      / \q_nil \@@_parent_auxiv:w
  }
\cs_new:Npn \@@_parent_auxi:w #1 / #2 \q_nil #3
  {
    #3 { #1 } #2 \q_nil #3
  }
\cs_new:Npn \@@_parent_auxii:w #1 #2 \q_nil \@@_parent_auxii:w
  {
    #1 \@@_parent_auxi:w #2 \q_nil \@@_parent_auxiii:n
  }
\cs_new:Npn \@@_parent_auxiii:n #1
  {
    / #1 \@@_parent_auxi:w
  }
\cs_new:Npn \@@_parent_auxiv:w #1 \q_nil \@@_parent_auxiv:w
  {
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_trim_spaces:n}
% \begin{macro}[EXP]
%   {
%     \@@_trim_spaces_auxi:w   ,
%     \@@_trim_spaces_auxii:w  ,
%     \@@_trim_spaces_auxiii:w
%   }
%   Space stripping has to allow for the fact that the key here might have
%   several parts, and spaces need to be stripped from each part. Since the key
%   name is turned into a string groups can't be stripped accidentally and the
%   precautions of \cs{tl_trim_spaces:n} aren't necessary, in this case it is
%   much faster to just directly strip spaces around |/|.
%    \begin{macrocode}
\group_begin:
  \cs_set:Npn \@@_tmp:w #1
    {
      \cs_new:Npn \@@_trim_spaces:n ##1
        {
          \exp_after:wN \@@_trim_spaces_auxi:w \tl_to_str:n { / ##1 } /
            \s_@@_nil  \@@_trim_spaces_auxi:w
            \s_@@_mark \@@_trim_spaces_auxii:w
            #1 / #1
            \s_@@_nil  \@@_trim_spaces_auxii:w
            \s_@@_mark \@@_trim_spaces_auxiii:w
        }
    }
  \@@_tmp:w { ~ }
\group_end:
\cs_new:Npn \@@_trim_spaces_auxi:w #1 ~ / #2 \s_@@_nil #3
  {
    #3 #1 / #2 \s_@@_nil #3
  }
\cs_new:Npn \@@_trim_spaces_auxii:w #1 / ~ #2 \s_@@_mark #3
  {
    #3 #1 / #2 \s_@@_mark #3
  }
\cs_new:Npn \@@_trim_spaces_auxiii:w
    / #1 /
    \s_@@_nil  \@@_trim_spaces_auxi:w
    \s_@@_mark \@@_trim_spaces_auxii:w
    /
    \s_@@_nil  \@@_trim_spaces_auxii:w
    \s_@@_mark \@@_trim_spaces_auxiii:w
  {
    #1
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}[EXP,pTF]{\keys_if_exist:nn}
%   A utility for others to see if a key exists.
%    \begin{macrocode}
\prg_new_conditional:Npnn \keys_if_exist:nn #1#2 { p , T , F , TF }
  {
    \cs_if_exist:cTF
      { \c_@@_code_root_str \@@_trim_spaces:n { #1 / #2 } }
      { \prg_return_true: }
      { \prg_return_false: }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP,pTF]{\keys_if_choice_exist:nnn}
%   Just an alternative view on \cs{keys_if_exist:nnTF}.
%    \begin{macrocode}
\prg_new_conditional:Npnn \keys_if_choice_exist:nnn #1#2#3
  { p , T , F , TF }
  {
    \cs_if_exist:cTF
      { \c_@@_code_root_str \@@_trim_spaces:n { #1 / #2 / #3 } }
      { \prg_return_true: }
      { \prg_return_false: }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\keys_show:nn, \keys_log:nn, \@@_show:Nnn}
%   To show a key, show its code using a message.
%    \begin{macrocode}
\cs_new_protected:Npn \keys_show:nn
  { \@@_show:Nnn \msg_show:nnxxxx }
\cs_new_protected:Npn \keys_log:nn
  { \@@_show:Nnn \msg_log:nnxxxx }
\cs_new_protected:Npn \@@_show:Nnn #1#2#3
  {
    #1 { keys } { show-key }
      { \@@_trim_spaces:n { #2 / #3 } }
      {
        \keys_if_exist:nnT {#2} {#3}
          {
            \exp_args:Nnf \msg_show_item_unbraced:nn { code }
              {
                \exp_args:Nc \cs_replacement_spec:N
                  {
                    \c_@@_code_root_str
                    \@@_trim_spaces:n { #2 / #3 }
                  }
              }
          }
      }
      { } { }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsection{Messages}
%
% For when there is a need to complain.
%    \begin{macrocode}
\msg_new:nnnn { keys } { bad-relative-key-path }
  { The~key~'#1'~is~not~inside~the~'#2'~path. }
  { The~key~'#1'~cannot~be~expressed~relative~to~path~'#2'. }
\msg_new:nnnn { keys } { boolean-values-only }
  { Key~'#1'~accepts~boolean~values~only. }
  { The~key~'#1'~only~accepts~the~values~'true'~and~'false'. }
\msg_new:nnnn { keys } { choice-unknown }
  { Key~'#1'~accepts~only~a~fixed~set~of~choices. }
  {
    The~key~'#1'~only~accepts~predefined~values,~
    and~'#2'~is~not~one~of~these.
  }
\msg_new:nnnn { keys } { unknown }
  { The~key~'#1'~is~unknown~and~is~being~ignored. }
  {
    The~module~'#2'~does~not~have~a~key~called~'#1'.\\
    Check~that~you~have~spelled~the~key~name~correctly.
  }
\msg_new:nnnn { keys } { nested-choice-key }
  { Attempt~to~define~'#1'~as~a~nested~choice~key. }
  {
    The~key~'#1'~cannot~be~defined~as~a~choice~as~the~parent~key~'#2'~is~
    itself~a~choice.
  }
\msg_new:nnnn { keys } { value-forbidden }
  { The~key~'#1'~does~not~take~a~value. }
  {
    The~key~'#1'~should~be~given~without~a~value.\\
    The~value~'#2'~was~present:~the~key~will~be~ignored.
  }
\msg_new:nnnn { keys } { value-required }
  { The~key~'#1'~requires~a~value. }
  {
    The~key~'#1'~must~have~a~value.\\
    No~value~was~present:~the~key~will~be~ignored.
  }
\msg_new:nnn { keys } { show-key }
  {
    The~key~#1~
    \tl_if_empty:nTF {#2}
      { is~undefined. }
      { has~the~properties: #2 . }
  }
\prop_gput:Nnn \g_msg_module_name_prop { keys } { LaTeX3 }
\prop_gput:Nnn \g_msg_module_type_prop { keys } { }
%    \end{macrocode}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
%\end{implementation}
%
%\PrintIndex