This file is indexed.

/usr/share/uim/lib/srfi-1.scm is in libuim-data 1:1.8.6-15.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

   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
;;; SRFI-1 list-processing library 			-*- Scheme -*-
;;; Reference implementation
;;;
;;; Copyright (c) 1998, 1999 by Olin Shivers. You may do as you please with
;;; this code as long as you do not remove this copyright notice or
;;; hold me liable for its use. Please send bug reports to shivers@ai.mit.edu.
;;;     -Olin

;;; Copyright (c) 2007-2008 SigScheme Project <uim-en AT googlegroups.com>

;; ChangeLog
;;
;; 2007-06-15 yamaken   - Imported from
;;                        http://srfi.schemers.org/srfi-1/srfi-1-reference.scm
;;                        and adapted to SigScheme
;;                      - Add for-each
;; 2007-06-30 yamaken   - Fix broken arguments receiving of delete-duplicates!
;;                      - Fix broken lset-difference call of lset-xor and
;;                        lset-xor! (as like as Scheme48)
;; 2007-07-01 yamaken   - Fix broken comparison of list= on 3 or more lists
;; 2007-07-13 yamaken   - Change default value for make-list to #<undef>


;;; This is a library of list- and pair-processing functions. I wrote it after
;;; carefully considering the functions provided by the libraries found in
;;; R4RS/R5RS Scheme, MIT Scheme, Gambit, RScheme, MzScheme, slib, Common
;;; Lisp, Bigloo, guile, T, APL and the SML standard basis. It is a pretty
;;; rich toolkit, providing a superset of the functionality found in any of
;;; the various Schemes I considered.

;;; This implementation is intended as a portable reference implementation
;;; for SRFI-1. See the porting notes below for more information.

;;; Exported:
;;; xcons tree-copy make-list list-tabulate cons* list-copy 
;;; proper-list? circular-list? dotted-list? not-pair? null-list? list=
;;; circular-list length+
;;; iota
;;; first second third fourth fifth sixth seventh eighth ninth tenth
;;; car+cdr
;;; take       drop       
;;; take-right drop-right 
;;; take!      drop-right!
;;; split-at   split-at!
;;; last last-pair
;;; zip unzip1 unzip2 unzip3 unzip4 unzip5
;;; count
;;; append! append-reverse append-reverse! concatenate concatenate! 
;;; unfold       fold       pair-fold       reduce
;;; unfold-right fold-right pair-fold-right reduce-right
;;; append-map append-map! map! pair-for-each filter-map map-in-order
;;; filter  partition  remove
;;; filter! partition! remove! 
;;; find find-tail any every list-index
;;; take-while drop-while take-while!
;;; span break span! break!
;;; delete delete!
;;; alist-cons alist-copy
;;; delete-duplicates delete-duplicates!
;;; alist-delete alist-delete!
;;; reverse! 
;;; lset<= lset= lset-adjoin  
;;; lset-union  lset-intersection  lset-difference  lset-xor  lset-diff+intersection
;;; lset-union! lset-intersection! lset-difference! lset-xor! lset-diff+intersection!
;;; 
;;; In principle, the following R4RS list- and pair-processing procedures
;;; are also part of this package's exports, although they are not defined
;;; in this file:
;;;   Primitives: cons pair? null? car cdr set-car! set-cdr!
;;;   Non-primitives: list length append reverse cadr ... cddddr list-ref
;;;                   memq memv assq assv
;;;   (The non-primitives are defined in this file, but commented out.)
;;;
;;; These R4RS procedures have extended definitions in SRFI-1 and are defined
;;; in this file:
;;;   map for-each member assoc
;;;
;;; The remaining two R4RS list-processing procedures are not included: 
;;;   list-tail (use drop)
;;;   list? (use proper-list?)


;;; A note on recursion and iteration/reversal:
;;; Many iterative list-processing algorithms naturally compute the elements
;;; of the answer list in the wrong order (left-to-right or head-to-tail) from
;;; the order needed to cons them into the proper answer (right-to-left, or
;;; tail-then-head). One style or idiom of programming these algorithms, then,
;;; loops, consing up the elements in reverse order, then destructively 
;;; reverses the list at the end of the loop. I do not do this. The natural
;;; and efficient way to code these algorithms is recursively. This trades off
;;; intermediate temporary list structure for intermediate temporary stack
;;; structure. In a stack-based system, this improves cache locality and
;;; lightens the load on the GC system. Don't stand on your head to iterate!
;;; Recurse, where natural. Multiple-value returns make this even more
;;; convenient, when the recursion/iteration has multiple state values.

;;; Porting:
;;; This is carefully tuned code; do not modify casually.
;;;   - It is careful to share storage when possible;
;;;   - Side-effecting code tries not to perform redundant writes.
;;; 
;;; That said, a port of this library to a specific Scheme system might wish
;;; to tune this code to exploit particulars of the implementation. 
;;; The single most important compiler-specific optimisation you could make
;;; to this library would be to add rewrite rules or transforms to:
;;; - transform applications of n-ary procedures (e.g. LIST=, CONS*, APPEND,
;;;   LSET-UNION) into multiple applications of a primitive two-argument 
;;;   variant.
;;; - transform applications of the mapping functions (MAP, FOR-EACH, FOLD, 
;;;   ANY, EVERY) into open-coded loops. The killer here is that these 
;;;   functions are n-ary. Handling the general case is quite inefficient,
;;;   requiring many intermediate data structures to be allocated and
;;;   discarded.
;;; - transform applications of procedures that take optional arguments
;;;   into calls to variants that do not take optional arguments. This
;;;   eliminates unnecessary consing and parsing of the rest parameter.
;;;
;;; These transforms would provide BIG speedups. In particular, the n-ary
;;; mapping functions are particularly slow and cons-intensive, and are good
;;; candidates for tuning. I have coded fast paths for the single-list cases,
;;; but what you really want to do is exploit the fact that the compiler
;;; usually knows how many arguments are being passed to a particular
;;; application of these functions -- they are usually explicitly called, not
;;; passed around as higher-order values. If you can arrange to have your
;;; compiler produce custom code or custom linkages based on the number of
;;; arguments in the call, you can speed these functions up a *lot*. But this
;;; kind of compiler technology no longer exists in the Scheme world as far as
;;; I can see.
;;;
;;; Note that this code is, of course, dependent upon standard bindings for
;;; the R5RS procedures -- i.e., it assumes that the variable CAR is bound
;;; to the procedure that takes the car of a list. If your Scheme 
;;; implementation allows user code to alter the bindings of these procedures
;;; in a manner that would be visible to these definitions, then there might
;;; be trouble. You could consider horrible kludgery along the lines of
;;;    (define fact 
;;;      (let ((= =) (- -) (* *))
;;;        (letrec ((real-fact (lambda (n) 
;;;                              (if (= n 0) 1 (* n (real-fact (- n 1)))))))
;;;          real-fact)))
;;; Or you could consider shifting to a reasonable Scheme system that, say,
;;; has a module system protecting code from this kind of lossage.
;;;
;;; This code does a fair amount of run-time argument checking. If your
;;; Scheme system has a sophisticated compiler that can eliminate redundant
;;; error checks, this is no problem. However, if not, these checks incur
;;; some performance overhead -- and, in a safe Scheme implementation, they
;;; are in some sense redundant: if we don't check to see that the PROC 
;;; parameter is a procedure, we'll find out anyway three lines later when
;;; we try to call the value. It's pretty easy to rip all this argument 
;;; checking code out if it's inappropriate for your implementation -- just
;;; nuke every call to CHECK-ARG.
;;;
;;; On the other hand, if you *do* have a sophisticated compiler that will
;;; actually perform soft-typing and eliminate redundant checks (Rice's systems
;;; being the only possible candidate of which I'm aware), leaving these checks 
;;; in can *help*, since their presence can be elided in redundant cases,
;;; and in cases where they are needed, performing the checks early, at
;;; procedure entry, can "lift" a check out of a loop. 
;;;
;;; Finally, I have only checked the properties that can portably be checked
;;; with R5RS Scheme -- and this is not complete. You may wish to alter
;;; the CHECK-ARG parameter checks to perform extra, implementation-specific
;;; checks, such as procedure arity for higher-order values.
;;;
;;; The code has only these non-R4RS dependencies:
;;;   A few calls to an ERROR procedure;
;;;   Uses of the R5RS multiple-value procedure VALUES and the m-v binding
;;;     RECEIVE macro (which isn't R5RS, but is a trivial macro).
;;;   Many calls to a parameter-checking procedure check-arg:
;;;    (define (check-arg pred val caller)
;;;      (let lp ((val val))
;;;        (if (pred val) val (lp (error "Bad argument" val pred caller)))))
;;;   A few uses of the LET-OPTIONAL and :OPTIONAL macros for parsing
;;;     optional arguments.
;;;
;;; Most of these procedures use the NULL-LIST? test to trigger the
;;; base case in the inner loop or recursion. The NULL-LIST? function
;;; is defined to be a careful one -- it raises an error if passed a
;;; non-nil, non-pair value. The spec allows an implementation to use
;;; a less-careful implementation that simply defines NULL-LIST? to
;;; be NOT-PAIR?. This would speed up the inner loops of these procedures
;;; at the expense of having them silently accept dotted lists.

;;; A note on dotted lists:
;;; I, personally, take the view that the only consistent view of lists
;;; in Scheme is the view that *everything* is a list -- values such as
;;; 3 or "foo" or 'bar are simply empty dotted lists. This is due to the
;;; fact that Scheme actually has no true list type. It has a pair type,
;;; and there is an *interpretation* of the trees built using this type
;;; as lists.
;;;
;;; I lobbied to have these list-processing procedures hew to this
;;; view, and accept any value as a list argument. I was overwhelmingly
;;; overruled during the SRFI discussion phase. So I am inserting this
;;; text in the reference lib and the SRFI spec as a sort of "minority
;;; opinion" dissent.
;;;
;;; Many of the procedures in this library can be trivially redefined
;;; to handle dotted lists, just by changing the NULL-LIST? base-case
;;; check to NOT-PAIR?, meaning that any non-pair value is taken to be
;;; an empty list. For most of these procedures, that's all that is
;;; required.
;;;
;;; However, we have to do a little more work for some procedures that
;;; *produce* lists from other lists.  Were we to extend these procedures to
;;; accept dotted lists, we would have to define how they terminate the lists
;;; produced as results when passed a dotted list. I designed a coherent set
;;; of termination rules for these cases; this was posted to the SRFI-1
;;; discussion list. I additionally wrote an earlier version of this library
;;; that implemented that spec. It has been discarded during later phases of
;;; the definition and implementation of this library.
;;;
;;; The argument *against* defining these procedures to work on dotted
;;; lists is that dotted lists are the rare, odd case, and that by 
;;; arranging for the procedures to handle them, we lose error checking
;;; in the cases where a dotted list is passed by accident -- e.g., when
;;; the programmer swaps a two arguments to a list-processing function,
;;; one being a scalar and one being a list. For example,
;;;     (member '(1 3 5 7 9) 7)
;;; This would quietly return #f if we extended MEMBER to accept dotted
;;; lists.
;;;
;;; The SRFI discussion record contains more discussion on this topic.

;;; SigScheme adaptation
;;;;;;;;;;;;;;;;;;;;;;;;

(require-extension (srfi 8 23))

(define %srfi-1:undefined (for-each values '()))

(define (check-arg pred val caller)
  (let lp ((val val))
    (if (pred val) val (lp (error "Bad argument" val pred caller)))))
;; If you need efficiency, define this once SRFI-1 has been enabled.
;;(define (check-arg . args) #f)

(define :optional
  (lambda (opt default)
    (case (length opt)
     ((0)  default)
     ((1)  (car opt))
     (else (error "superfluous arguments")))))


;;; Constructors
;;;;;;;;;;;;;;;;

;;; Occasionally useful as a value to be passed to a fold or other
;;; higher-order procedure.
(define (xcons d a) (cons a d))

;;;; Recursively copy every cons.
;(define (tree-copy x)
;  (let recur ((x x))
;    (if (not (pair? x)) x
;	(cons (recur (car x)) (recur (cdr x))))))

;;; Make a list of length LEN.

(define (make-list len . maybe-elt)
  (check-arg (lambda (n) (and (integer? n) (>= n 0))) len make-list)
  (let ((elt (cond ((null? maybe-elt) %srfi-1:undefined) ; Default value
		   ((null? (cdr maybe-elt)) (car maybe-elt))
		   (else (error "Too many arguments to MAKE-LIST"
				(cons len maybe-elt))))))
    (do ((i len (- i 1))
	 (ans '() (cons elt ans)))
	((<= i 0) ans))))


;(define (list . ans) ans)	; R4RS


;;; Make a list of length LEN. Elt i is (PROC i) for 0 <= i < LEN.

(define (list-tabulate len proc)
  (check-arg (lambda (n) (and (integer? n) (>= n 0))) len list-tabulate)
  (check-arg procedure? proc list-tabulate)
  (do ((i (- len 1) (- i 1))
       (ans '() (cons (proc i) ans)))
      ((< i 0) ans)))

;;; (cons* a1 a2 ... an) = (cons a1 (cons a2 (cons ... an)))
;;; (cons* a1) = a1	(cons* a1 a2 ...) = (cons a1 (cons* a2 ...))
;;;
;;; (cons first (unfold not-pair? car cdr rest values))

(define (cons* first . rest)
  (let recur ((x first) (rest rest))
    (if (pair? rest)
	(cons x (recur (car rest) (cdr rest)))
	x)))

;;; (unfold not-pair? car cdr lis values)

(define (list-copy lis)				
  (let recur ((lis lis))			
    (if (pair? lis)				
	(cons (car lis) (recur (cdr lis)))	
	lis)))					

;;; IOTA count [start step]	(start start+step ... start+(count-1)*step)

(define (iota count . maybe-start+step)
  (check-arg integer? count iota)
  (if (< count 0) (error "Negative step count" iota count))
  (let-optionals* maybe-start+step ((start 0) (step 1) . must-be-null)
    (check-arg number? start iota)
    (check-arg number? step iota)
    (if (not (null? must-be-null)) (error "superfluous arguments"))
    (let ((last-val (+ start (* (- count 1) step))))
      (do ((count count (- count 1))
	   (val last-val (- val step))
	   (ans '() (cons val ans)))
	  ((<= count 0)  ans)))))
	  
;;; I thought these were lovely, but the public at large did not share my
;;; enthusiasm...
;;; :IOTA to		(0 ... to-1)
;;; :IOTA from to	(from ... to-1)
;;; :IOTA from to step  (from from+step ...)

;;; IOTA: to		(1 ... to)
;;; IOTA: from to	(from+1 ... to)
;;; IOTA: from to step	(from+step from+2step ...)

;(define (%parse-iota-args arg1 rest-args proc)
;  (let ((check (lambda (n) (check-arg integer? n proc))))
;    (check arg1)
;    (if (pair? rest-args)
;	(let ((arg2 (check (car rest-args)))
;	      (rest (cdr rest-args)))
;	  (if (pair? rest)
;	      (let ((arg3 (check (car rest)))
;		    (rest (cdr rest)))
;		(if (pair? rest) (error "Too many parameters" proc arg1 rest-args)
;		    (values arg1 arg2 arg3)))
;	      (values arg1 arg2 1)))
;	(values 0 arg1 1))))
;
;(define (iota: arg1 . rest-args)
;  (receive (from to step) (%parse-iota-args arg1 rest-args iota:)
;    (let* ((numsteps (floor (/ (- to from) step)))
;	   (last-val (+ from (* step numsteps))))
;      (if (< numsteps 0) (error "Negative step count" iota: from to step))
;      (do ((steps-left numsteps (- steps-left 1))
;	   (val last-val (- val step))
;	   (ans '() (cons val ans)))
;	  ((<= steps-left 0) ans)))))
;
;
;(define (:iota arg1 . rest-args)
;  (receive (from to step) (%parse-iota-args arg1 rest-args :iota)
;    (let* ((numsteps (ceiling (/ (- to from) step)))
;	   (last-val (+ from (* step (- numsteps 1)))))
;      (if (< numsteps 0) (error "Negative step count" :iota from to step))
;      (do ((steps-left numsteps (- steps-left 1))
;	   (val last-val (- val step))
;	   (ans '() (cons val ans)))
;	  ((<= steps-left 0) ans)))))



(define (circular-list val1 . vals)
  (let ((ans (cons val1 vals)))
    (set-cdr! (last-pair ans) ans)
    ans))

;;; <proper-list> ::= ()			; Empty proper list
;;;		  |   (cons <x> <proper-list>)	; Proper-list pair
;;; Note that this definition rules out circular lists -- and this
;;; function is required to detect this case and return false.

(define (proper-list? x)
  (let lp ((x x) (lag x))
    (if (pair? x)
	(let ((x (cdr x)))
	  (if (pair? x)
	      (let ((x   (cdr x))
		    (lag (cdr lag)))
		(and (not (eq? x lag)) (lp x lag)))
	      (null? x)))
	(null? x))))


;;; A dotted list is a finite list (possibly of length 0) terminated
;;; by a non-nil value. Any non-cons, non-nil value (e.g., "foo" or 5)
;;; is a dotted list of length 0.
;;;
;;; <dotted-list> ::= <non-nil,non-pair>	; Empty dotted list
;;;               |   (cons <x> <dotted-list>)	; Proper-list pair

(define (dotted-list? x)
  (let lp ((x x) (lag x))
    (if (pair? x)
	(let ((x (cdr x)))
	  (if (pair? x)
	      (let ((x   (cdr x))
		    (lag (cdr lag)))
		(and (not (eq? x lag)) (lp x lag)))
	      (not (null? x))))
	(not (null? x)))))

(define (circular-list? x)
  (let lp ((x x) (lag x))
    (and (pair? x)
	 (let ((x (cdr x)))
	   (and (pair? x)
		(let ((x   (cdr x))
		      (lag (cdr lag)))
		  (or (eq? x lag) (lp x lag))))))))

(define (not-pair? x) (not (pair? x)))	; Inline me.

;;; This is a legal definition which is fast and sloppy:
;;;     (define null-list? not-pair?)
;;; but we'll provide a more careful one:
(define (null-list? l)
  (cond ((pair? l) #f)
	((null? l) #t)
	(else (error "null-list?: argument out of domain" l))))
           

(define (list= = . lists)
  (or (null? lists) ; special case

      (let lp1 ((list-a (car lists)) (others (cdr lists)))
	(or (null? others)
	    (let ((list-b (car others))
		  (others (cdr others)))
	      (if (eq? list-a list-b)	; EQ? => LIST=
		  (lp1 list-b others)
		  (let lp2 ((tail-a list-a) (tail-b list-b))
		    (if (null-list? tail-a)
			(and (null-list? tail-b)
			     (lp1 list-b others))
			(and (not (null-list? tail-b))
			     (= (car tail-a) (car tail-b))
			     (lp2 (cdr tail-a) (cdr tail-b)))))))))))
			


;;; R4RS, so commented out.
;(define (length x)			; LENGTH may diverge or
;  (let lp ((x x) (len 0))		; raise an error if X is
;    (if (pair? x)			; a circular list. This version
;        (lp (cdr x) (+ len 1))		; diverges.
;        len)))

(define (length+ x)			; Returns #f if X is circular.
  (let lp ((x x) (lag x) (len 0))
    (if (pair? x)
	(let ((x (cdr x))
	      (len (+ len 1)))
	  (if (pair? x)
	      (let ((x   (cdr x))
		    (lag (cdr lag))
		    (len (+ len 1)))
		(and (not (eq? x lag)) (lp x lag len)))
	      len))
	len)))

(define (zip list1 . more-lists) (apply map list list1 more-lists))


;;; Selectors
;;;;;;;;;;;;;

;;; R4RS non-primitives:
;(define (caar   x) (car (car x)))
;(define (cadr   x) (car (cdr x)))
;(define (cdar   x) (cdr (car x)))
;(define (cddr   x) (cdr (cdr x)))
;
;(define (caaar  x) (caar (car x)))
;(define (caadr  x) (caar (cdr x)))
;(define (cadar  x) (cadr (car x)))
;(define (caddr  x) (cadr (cdr x)))
;(define (cdaar  x) (cdar (car x)))
;(define (cdadr  x) (cdar (cdr x)))
;(define (cddar  x) (cddr (car x)))
;(define (cdddr  x) (cddr (cdr x)))
;
;(define (caaaar x) (caaar (car x)))
;(define (caaadr x) (caaar (cdr x)))
;(define (caadar x) (caadr (car x)))
;(define (caaddr x) (caadr (cdr x)))
;(define (cadaar x) (cadar (car x)))
;(define (cadadr x) (cadar (cdr x)))
;(define (caddar x) (caddr (car x)))
;(define (cadddr x) (caddr (cdr x)))
;(define (cdaaar x) (cdaar (car x)))
;(define (cdaadr x) (cdaar (cdr x)))
;(define (cdadar x) (cdadr (car x)))
;(define (cdaddr x) (cdadr (cdr x)))
;(define (cddaar x) (cddar (car x)))
;(define (cddadr x) (cddar (cdr x)))
;(define (cdddar x) (cdddr (car x)))
;(define (cddddr x) (cdddr (cdr x)))


(define first  car)
(define second cadr)
(define third  caddr)
(define fourth cadddr)
(define (fifth   x) (car    (cddddr x)))
(define (sixth   x) (cadr   (cddddr x)))
(define (seventh x) (caddr  (cddddr x)))
(define (eighth  x) (cadddr (cddddr x)))
(define (ninth   x) (car  (cddddr (cddddr x))))
(define (tenth   x) (cadr (cddddr (cddddr x))))

(define (car+cdr pair) (values (car pair) (cdr pair)))

;;; take & drop

(define (take lis k)
  (check-arg integer? k take)
  (let recur ((lis lis) (k k))
    (if (zero? k) '()
	(cons (car lis)
	      (recur (cdr lis) (- k 1))))))

(define (drop lis k)
  (check-arg integer? k drop)
  (let iter ((lis lis) (k k))
    (if (zero? k) lis (iter (cdr lis) (- k 1)))))

(define (take! lis k)
  (check-arg integer? k take!)
  (if (zero? k) '()
      (begin (set-cdr! (drop lis (- k 1)) '())
	     lis)))

;;; TAKE-RIGHT and DROP-RIGHT work by getting two pointers into the list, 
;;; off by K, then chasing down the list until the lead pointer falls off
;;; the end.

(define (take-right lis k)
  (check-arg integer? k take-right)
  (let lp ((lag lis)  (lead (drop lis k)))
    (if (pair? lead)
	(lp (cdr lag) (cdr lead))
	lag)))

(define (drop-right lis k)
  (check-arg integer? k drop-right)
  (let recur ((lag lis) (lead (drop lis k)))
    (if (pair? lead)
	(cons (car lag) (recur (cdr lag) (cdr lead)))
	'())))

;;; In this function, LEAD is actually K+1 ahead of LAG. This lets
;;; us stop LAG one step early, in time to smash its cdr to ().
(define (drop-right! lis k)
  (check-arg integer? k drop-right!)
  (let ((lead (drop lis k)))
    (if (pair? lead)

	(let lp ((lag lis)  (lead (cdr lead)))	; Standard case
	  (if (pair? lead)
	      (lp (cdr lag) (cdr lead))
	      (begin (set-cdr! lag '())
		     lis)))

	'())))	; Special case dropping everything -- no cons to side-effect.

;(define (list-ref lis i) (car (drop lis i)))	; R4RS

;;; These use the APL convention, whereby negative indices mean 
;;; "from the right." I liked them, but they didn't win over the
;;; SRFI reviewers.
;;; K >= 0: Take and drop  K elts from the front of the list.
;;; K <= 0: Take and drop -K elts from the end   of the list.

;(define (take lis k)
;  (check-arg integer? k take)
;  (if (negative? k)
;      (list-tail lis (+ k (length lis)))
;      (let recur ((lis lis) (k k))
;	(if (zero? k) '()
;	    (cons (car lis)
;		  (recur (cdr lis) (- k 1)))))))
;
;(define (drop lis k)
;  (check-arg integer? k drop)
;  (if (negative? k)
;      (let recur ((lis lis) (nelts (+ k (length lis))))
;	(if (zero? nelts) '()
;	    (cons (car lis)
;		  (recur (cdr lis) (- nelts 1)))))
;      (list-tail lis k)))
;
;
;(define (take! lis k)
;  (check-arg integer? k take!)
;  (cond ((zero? k) '())
;	((positive? k)
;	 (set-cdr! (list-tail lis (- k 1)) '())
;	 lis)
;	(else (list-tail lis (+ k (length lis))))))
;
;(define (drop! lis k)
;  (check-arg integer? k drop!)
;  (if (negative? k)
;      (let ((nelts (+ k (length lis))))
;	(if (zero? nelts) '()
;	    (begin (set-cdr! (list-tail lis (- nelts 1)) '())
;		   lis)))
;      (list-tail lis k)))

(define (split-at x k)
  (check-arg integer? k split-at)
  (let recur ((lis x) (k k))
    (if (zero? k) (values '() lis)
	(receive (prefix suffix) (recur (cdr lis) (- k 1))
	  (values (cons (car lis) prefix) suffix)))))

(define (split-at! x k)
  (check-arg integer? k split-at!)
  (if (zero? k) (values '() x)
      (let* ((prev (drop x (- k 1)))
	     (suffix (cdr prev)))
	(set-cdr! prev '())
	(values x suffix))))


(define (last lis) (car (last-pair lis)))

(define (last-pair lis)
  (check-arg pair? lis last-pair)
  (let lp ((lis lis))
    (let ((tail (cdr lis)))
      (if (pair? tail) (lp tail) lis))))


;;; Unzippers -- 1 through 5
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (unzip1 lis) (map car lis))

(define (unzip2 lis)
  (let recur ((lis lis))
    (if (null-list? lis) (values lis lis)	; Use NOT-PAIR? to handle
	(let ((elt (car lis)))			; dotted lists.
	  (receive (a b) (recur (cdr lis))
	    (values (cons (car  elt) a)
		    (cons (cadr elt) b)))))))

(define (unzip3 lis)
  (let recur ((lis lis))
    (if (null-list? lis) (values lis lis lis)
	(let ((elt (car lis)))
	  (receive (a b c) (recur (cdr lis))
	    (values (cons (car   elt) a)
		    (cons (cadr  elt) b)
		    (cons (caddr elt) c)))))))

(define (unzip4 lis)
  (let recur ((lis lis))
    (if (null-list? lis) (values lis lis lis lis)
	(let ((elt (car lis)))
	  (receive (a b c d) (recur (cdr lis))
	    (values (cons (car    elt) a)
		    (cons (cadr   elt) b)
		    (cons (caddr  elt) c)
		    (cons (cadddr elt) d)))))))

(define (unzip5 lis)
  (let recur ((lis lis))
    (if (null-list? lis) (values lis lis lis lis lis)
	(let ((elt (car lis)))
	  (receive (a b c d e) (recur (cdr lis))
	    (values (cons (car     elt) a)
		    (cons (cadr    elt) b)
		    (cons (caddr   elt) c)
		    (cons (cadddr  elt) d)
		    (cons (car (cddddr  elt)) e)))))))


;;; append! append-reverse append-reverse! concatenate concatenate!
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (append! . lists)
  ;; First, scan through lists looking for a non-empty one.
  (let lp ((lists lists) (prev '()))
    (if (not (pair? lists)) prev
	(let ((first (car lists))
	      (rest (cdr lists)))
	  (if (not (pair? first)) (lp rest first)

	      ;; Now, do the splicing.
	      (let lp2 ((tail-cons (last-pair first))
			(rest rest))
		(if (pair? rest)
		    (let ((next (car rest))
			  (rest (cdr rest)))
		      (set-cdr! tail-cons next)
		      (lp2 (if (pair? next) (last-pair next) tail-cons)
			   rest))
		    first)))))))

;;; APPEND is R4RS.
;(define (append . lists)
;  (if (pair? lists)
;      (let recur ((list1 (car lists)) (lists (cdr lists)))
;        (if (pair? lists)
;            (let ((tail (recur (car lists) (cdr lists))))
;              (fold-right cons tail list1)) ; Append LIST1 & TAIL.
;            list1))
;      '()))

;(define (append-reverse rev-head tail) (fold cons tail rev-head))

;(define (append-reverse! rev-head tail)
;  (pair-fold (lambda (pair tail) (set-cdr! pair tail) pair)
;             tail
;             rev-head))

;;; Hand-inline the FOLD and PAIR-FOLD ops for speed.

(define (append-reverse rev-head tail)
  (let lp ((rev-head rev-head) (tail tail))
    (if (null-list? rev-head) tail
	(lp (cdr rev-head) (cons (car rev-head) tail)))))

(define (append-reverse! rev-head tail)
  (let lp ((rev-head rev-head) (tail tail))
    (if (null-list? rev-head) tail
	(let ((next-rev (cdr rev-head)))
	  (set-cdr! rev-head tail)
	  (lp next-rev rev-head)))))


(define (concatenate  lists) (reduce-right append  '() lists))
(define (concatenate! lists) (reduce-right append! '() lists))

;;; Fold/map internal utilities
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; These little internal utilities are used by the general
;;; fold & mapper funs for the n-ary cases . It'd be nice if they got inlined.
;;; One the other hand, the n-ary cases are painfully inefficient as it is.
;;; An aggressive implementation should simply re-write these functions 
;;; for raw efficiency; I have written them for as much clarity, portability,
;;; and simplicity as can be achieved.
;;;
;;; I use the dreaded call/cc to do local aborts. A good compiler could
;;; handle this with extreme efficiency. An implementation that provides
;;; a one-shot, non-persistent continuation grabber could help the compiler
;;; out by using that in place of the call/cc's in these routines.
;;;
;;; These functions have funky definitions that are precisely tuned to
;;; the needs of the fold/map procs -- for example, to minimize the number
;;; of times the argument lists need to be examined.

;;; Return (map cdr lists). 
;;; However, if any element of LISTS is empty, just abort and return '().
(define (%cdrs lists)
  (call-with-current-continuation
    (lambda (abort)
      (let recur ((lists lists))
	(if (pair? lists)
	    (let ((lis (car lists)))
	      (if (null-list? lis) (abort '())
		  (cons (cdr lis) (recur (cdr lists)))))
	    '())))))

(define (%cars+ lists last-elt)	; (append! (map car lists) (list last-elt))
  (let recur ((lists lists))
    (if (pair? lists) (cons (caar lists) (recur (cdr lists))) (list last-elt))))

;;; LISTS is a (not very long) non-empty list of lists.
;;; Return two lists: the cars & the cdrs of the lists.
;;; However, if any of the lists is empty, just abort and return [() ()].

(define (%cars+cdrs lists)
  (call-with-current-continuation
    (lambda (abort)
      (let recur ((lists lists))
        (if (pair? lists)
	    (receive (list other-lists) (car+cdr lists)
	      (if (null-list? list) (abort '() '()) ; LIST is empty -- bail out
		  (receive (a d) (car+cdr list)
		    (receive (cars cdrs) (recur other-lists)
		      (values (cons a cars) (cons d cdrs))))))
	    (values '() '()))))))

;;; Like %CARS+CDRS, but we pass in a final elt tacked onto the end of the
;;; cars list. What a hack.
(define (%cars+cdrs+ lists cars-final)
  (call-with-current-continuation
    (lambda (abort)
      (let recur ((lists lists))
        (if (pair? lists)
	    (receive (list other-lists) (car+cdr lists)
	      (if (null-list? list) (abort '() '()) ; LIST is empty -- bail out
		  (receive (a d) (car+cdr list)
		    (receive (cars cdrs) (recur other-lists)
		      (values (cons a cars) (cons d cdrs))))))
	    (values (list cars-final) '()))))))

;;; Like %CARS+CDRS, but blow up if any list is empty.
(define (%cars+cdrs/no-test lists)
  (let recur ((lists lists))
    (if (pair? lists)
	(receive (list other-lists) (car+cdr lists)
	  (receive (a d) (car+cdr list)
	    (receive (cars cdrs) (recur other-lists)
	      (values (cons a cars) (cons d cdrs)))))
	(values '() '()))))


;;; count
;;;;;;;;;
(define (count pred list1 . lists)
  (check-arg procedure? pred count)
  (if (pair? lists)

      ;; N-ary case
      (let lp ((list1 list1) (lists lists) (i 0))
	(if (null-list? list1) i
	    (receive (as ds) (%cars+cdrs lists)
	      (if (null? as) i
		  (lp (cdr list1) ds
		      (if (apply pred (car list1) as) (+ i 1) i))))))

      ;; Fast path
      (let lp ((lis list1) (i 0))
	(if (null-list? lis) i
	    (lp (cdr lis) (if (pred (car lis)) (+ i 1) i))))))


;;; fold/unfold
;;;;;;;;;;;;;;;

(define (unfold-right p f g seed . maybe-tail)
  (check-arg procedure? p unfold-right)
  (check-arg procedure? f unfold-right)
  (check-arg procedure? g unfold-right)
  (let lp ((seed seed) (ans (:optional maybe-tail '())))
    (if (p seed) ans
	(lp (g seed)
	    (cons (f seed) ans)))))


(define (unfold p f g seed . maybe-tail-gen)
  (check-arg procedure? p unfold)
  (check-arg procedure? f unfold)
  (check-arg procedure? g unfold)
  (if (pair? maybe-tail-gen)

      (let ((tail-gen (car maybe-tail-gen)))
	(if (pair? (cdr maybe-tail-gen))
	    (apply error "Too many arguments" unfold p f g seed maybe-tail-gen)

	    (let recur ((seed seed))
	      (if (p seed) (tail-gen seed)
		  (cons (f seed) (recur (g seed)))))))

      (let recur ((seed seed))
	(if (p seed) '()
	    (cons (f seed) (recur (g seed)))))))
      

(define (fold kons knil lis1 . lists)
  (check-arg procedure? kons fold)
  (if (pair? lists)
      (let lp ((lists (cons lis1 lists)) (ans knil))	; N-ary case
	(receive (cars+ans cdrs) (%cars+cdrs+ lists ans)
	  (if (null? cars+ans) ans ; Done.
	      (lp cdrs (apply kons cars+ans)))))
	    
      (let lp ((lis lis1) (ans knil))			; Fast path
	(if (null-list? lis) ans
	    (lp (cdr lis) (kons (car lis) ans))))))


(define (fold-right kons knil lis1 . lists)
  (check-arg procedure? kons fold-right)
  (if (pair? lists)
      (let recur ((lists (cons lis1 lists)))		; N-ary case
	(let ((cdrs (%cdrs lists)))
	  (if (null? cdrs) knil
	      (apply kons (%cars+ lists (recur cdrs))))))

      (let recur ((lis lis1))				; Fast path
	(if (null-list? lis) knil
	    (let ((head (car lis)))
	      (kons head (recur (cdr lis))))))))


(define (pair-fold-right f zero lis1 . lists)
  (check-arg procedure? f pair-fold-right)
  (if (pair? lists)
      (let recur ((lists (cons lis1 lists)))		; N-ary case
	(let ((cdrs (%cdrs lists)))
	  (if (null? cdrs) zero
	      (apply f (append! lists (list (recur cdrs)))))))

      (let recur ((lis lis1))				; Fast path
	(if (null-list? lis) zero (f lis (recur (cdr lis)))))))

(define (pair-fold f zero lis1 . lists)
  (check-arg procedure? f pair-fold)
  (if (pair? lists)
      (let lp ((lists (cons lis1 lists)) (ans zero))	; N-ary case
	(let ((tails (%cdrs lists)))
	  (if (null? tails) ans
	      (lp tails (apply f (append! lists (list ans)))))))

      (let lp ((lis lis1) (ans zero))
	(if (null-list? lis) ans
	    (let ((tail (cdr lis)))		; Grab the cdr now,
	      (lp tail (f lis ans)))))))	; in case F SET-CDR!s LIS.
      

;;; REDUCE and REDUCE-RIGHT only use RIDENTITY in the empty-list case.
;;; These cannot meaningfully be n-ary.

(define (reduce f ridentity lis)
  (check-arg procedure? f reduce)
  (if (null-list? lis) ridentity
      (fold f (car lis) (cdr lis))))

(define (reduce-right f ridentity lis)
  (check-arg procedure? f reduce-right)
  (if (null-list? lis) ridentity
      (let recur ((head (car lis)) (lis (cdr lis)))
	(if (pair? lis)
	    (f head (recur (car lis) (cdr lis)))
	    head))))



;;; Mappers: append-map append-map! pair-for-each map! filter-map map-in-order
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (append-map f lis1 . lists)
  (really-append-map append-map  append  f lis1 lists))
(define (append-map! f lis1 . lists) 
  (really-append-map append-map! append! f lis1 lists))

(define (really-append-map who appender f lis1 lists)
  (check-arg procedure? f who)
  (if (pair? lists)
      (receive (cars cdrs) (%cars+cdrs (cons lis1 lists))
	(if (null? cars) '()
	    (let recur ((cars cars) (cdrs cdrs))
	      (let ((vals (apply f cars)))
		(receive (cars2 cdrs2) (%cars+cdrs cdrs)
		  (if (null? cars2) vals
		      (appender vals (recur cars2 cdrs2))))))))

      ;; Fast path
      (if (null-list? lis1) '()
	  (let recur ((elt (car lis1)) (rest (cdr lis1)))
	    (let ((vals (f elt)))
	      (if (null-list? rest) vals
		  (appender vals (recur (car rest) (cdr rest)))))))))


(define (pair-for-each proc lis1 . lists)
  (check-arg procedure? proc pair-for-each)
  (if (pair? lists)

      (let lp ((lists (cons lis1 lists)))
	(let ((tails (%cdrs lists)))
	  (if (pair? tails)
	      (begin (apply proc lists)
		     (lp tails)))))

      ;; Fast path.
      (let lp ((lis lis1))
	(if (not (null-list? lis))
	    (let ((tail (cdr lis)))	; Grab the cdr now,
	      (proc lis)		; in case PROC SET-CDR!s LIS.
	      (lp tail))))))

;;; We stop when LIS1 runs out, not when any list runs out.
(define (map! f lis1 . lists)
  (check-arg procedure? f map!)
  (if (pair? lists)
      (let lp ((lis1 lis1) (lists lists))
	(if (not (null-list? lis1))
	    (receive (heads tails) (%cars+cdrs/no-test lists)
	      (set-car! lis1 (apply f (car lis1) heads))
	      (lp (cdr lis1) tails))))

      ;; Fast path.
      (pair-for-each (lambda (pair) (set-car! pair (f (car pair)))) lis1))
  lis1)


;;; Map F across L, and save up all the non-false results.
(define (filter-map f lis1 . lists)
  (check-arg procedure? f filter-map)
  (if (pair? lists)
      (let recur ((lists (cons lis1 lists)))
	(receive (cars cdrs) (%cars+cdrs lists)
	  (if (pair? cars)
	      (cond ((apply f cars) => (lambda (x) (cons x (recur cdrs))))
		    (else (recur cdrs))) ; Tail call in this arm.
	      '())))
	    
      ;; Fast path.
      (let recur ((lis lis1))
	(if (null-list? lis) lis
	    (let ((tail (recur (cdr lis))))
	      (cond ((f (car lis)) => (lambda (x) (cons x tail)))
		    (else tail)))))))


;;; Map F across lists, guaranteeing to go left-to-right.
;;; NOTE: Some implementations of R5RS MAP are compliant with this spec;
;;; in which case this procedure may simply be defined as a synonym for MAP.

(define (map-in-order f lis1 . lists)
  (check-arg procedure? f map-in-order)
  (if (pair? lists)
      (let recur ((lists (cons lis1 lists)))
	(receive (cars cdrs) (%cars+cdrs lists)
	  (if (pair? cars)
	      (let ((x (apply f cars)))		; Do head first,
		(cons x (recur cdrs)))		; then tail.
	      '())))
	    
      ;; Fast path.
      (let recur ((lis lis1))
	(if (null-list? lis) lis
	    (let ((tail (cdr lis))
		  (x (f (car lis))))		; Do head first,
	      (cons x (recur tail)))))))	; then tail.


;;; We extend MAP to handle arguments of unequal length.
(define map map-in-order)	

;; Added by yamaken 2007-06-15
(define for-each
  (lambda args
    (apply map-in-order args)
    %srfi-1:undefined))

;;; filter, remove, partition
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; FILTER, REMOVE, PARTITION and their destructive counterparts do not
;;; disorder the elements of their argument.

;; This FILTER shares the longest tail of L that has no deleted elements.
;; If Scheme had multi-continuation calls, they could be made more efficient.

(define (filter pred lis)			; Sleazing with EQ? makes this
  (check-arg procedure? pred filter)		; one faster.
  (let recur ((lis lis))		
    (if (null-list? lis) lis			; Use NOT-PAIR? to handle dotted lists.
	(let ((head (car lis))
	      (tail (cdr lis)))
	  (if (pred head)
	      (let ((new-tail (recur tail)))	; Replicate the RECUR call so
		(if (eq? tail new-tail) lis
		    (cons head new-tail)))
	      (recur tail))))))			; this one can be a tail call.


;;; Another version that shares longest tail.
;(define (filter pred lis)
;  (receive (ans no-del?)
;      ;; (recur l) returns L with (pred x) values filtered.
;      ;; It also returns a flag NO-DEL? if the returned value
;      ;; is EQ? to L, i.e. if it didn't have to delete anything.
;      (let recur ((l l))
;	(if (null-list? l) (values l #t)
;	    (let ((x  (car l))
;		  (tl (cdr l)))
;	      (if (pred x)
;		  (receive (ans no-del?) (recur tl)
;		    (if no-del?
;			(values l #t)
;			(values (cons x ans) #f)))
;		  (receive (ans no-del?) (recur tl) ; Delete X.
;		    (values ans #f))))))
;    ans))



;(define (filter! pred lis)			; Things are much simpler
;  (let recur ((lis lis))			; if you are willing to
;    (if (pair? lis)				; push N stack frames & do N
;        (cond ((pred (car lis))		; SET-CDR! writes, where N is
;               (set-cdr! lis (recur (cdr lis))); the length of the answer.
;               lis)				
;              (else (recur (cdr lis))))
;        lis)))


;;; This implementation of FILTER!
;;; - doesn't cons, and uses no stack;
;;; - is careful not to do redundant SET-CDR! writes, as writes to memory are 
;;;   usually expensive on modern machines, and can be extremely expensive on 
;;;   modern Schemes (e.g., ones that have generational GC's).
;;; It just zips down contiguous runs of in and out elts in LIS doing the 
;;; minimal number of SET-CDR!s to splice the tail of one run of ins to the 
;;; beginning of the next.

(define (filter! pred lis)
  (check-arg procedure? pred filter!)
  (let lp ((ans lis))
    (cond ((null-list? ans)       ans)			; Scan looking for
	  ((not (pred (car ans))) (lp (cdr ans)))	; first cons of result.

	  ;; ANS is the eventual answer.
	  ;; SCAN-IN: (CDR PREV) = LIS and (CAR PREV) satisfies PRED.
	  ;;          Scan over a contiguous segment of the list that
	  ;;          satisfies PRED.
	  ;; SCAN-OUT: (CAR PREV) satisfies PRED. Scan over a contiguous
	  ;;           segment of the list that *doesn't* satisfy PRED.
	  ;;           When the segment ends, patch in a link from PREV
	  ;;           to the start of the next good segment, and jump to
	  ;;           SCAN-IN.
	  (else (letrec ((scan-in (lambda (prev lis)
				    (if (pair? lis)
					(if (pred (car lis))
					    (scan-in lis (cdr lis))
					    (scan-out prev (cdr lis))))))
			 (scan-out (lambda (prev lis)
				     (let lp ((lis lis))
				       (if (pair? lis)
					   (if (pred (car lis))
					       (begin (set-cdr! prev lis)
						      (scan-in lis (cdr lis)))
					       (lp (cdr lis)))
					   (set-cdr! prev lis))))))
		  (scan-in ans (cdr ans))
		  ans)))))



;;; Answers share common tail with LIS where possible; 
;;; the technique is slightly subtle.

(define (partition pred lis)
  (check-arg procedure? pred partition)
  (let recur ((lis lis))
    (if (null-list? lis) (values lis lis)	; Use NOT-PAIR? to handle dotted lists.
	(let ((elt (car lis))
	      (tail (cdr lis)))
	  (receive (in out) (recur tail)
	    (if (pred elt)
		(values (if (pair? out) (cons elt in) lis) out)
		(values in (if (pair? in) (cons elt out) lis))))))))



;(define (partition! pred lis)			; Things are much simpler
;  (let recur ((lis lis))			; if you are willing to
;    (if (null-list? lis) (values lis lis)	; push N stack frames & do N
;        (let ((elt (car lis)))			; SET-CDR! writes, where N is
;          (receive (in out) (recur (cdr lis))	; the length of LIS.
;            (cond ((pred elt)
;                   (set-cdr! lis in)
;                   (values lis out))
;                  (else (set-cdr! lis out)
;                        (values in lis))))))))


;;; This implementation of PARTITION!
;;; - doesn't cons, and uses no stack;
;;; - is careful not to do redundant SET-CDR! writes, as writes to memory are
;;;   usually expensive on modern machines, and can be extremely expensive on 
;;;   modern Schemes (e.g., ones that have generational GC's).
;;; It just zips down contiguous runs of in and out elts in LIS doing the
;;; minimal number of SET-CDR!s to splice these runs together into the result 
;;; lists.

(define (partition! pred lis)
  (check-arg procedure? pred partition!)
  (if (null-list? lis) (values lis lis)

      ;; This pair of loops zips down contiguous in & out runs of the
      ;; list, splicing the runs together. The invariants are
      ;;   SCAN-IN:  (cdr in-prev)  = LIS.
      ;;   SCAN-OUT: (cdr out-prev) = LIS.
      (letrec ((scan-in (lambda (in-prev out-prev lis)
			  (let lp ((in-prev in-prev) (lis lis))
			    (if (pair? lis)
				(if (pred (car lis))
				    (lp lis (cdr lis))
				    (begin (set-cdr! out-prev lis)
					   (scan-out in-prev lis (cdr lis))))
				(set-cdr! out-prev lis))))) ; Done.

	       (scan-out (lambda (in-prev out-prev lis)
			   (let lp ((out-prev out-prev) (lis lis))
			     (if (pair? lis)
				 (if (pred (car lis))
				     (begin (set-cdr! in-prev lis)
					    (scan-in lis out-prev (cdr lis)))
				     (lp lis (cdr lis)))
				 (set-cdr! in-prev lis)))))) ; Done.

	;; Crank up the scan&splice loops.
	(if (pred (car lis))
	    ;; LIS begins in-list. Search for out-list's first pair.
	    (let lp ((prev-l lis) (l (cdr lis)))
	      (cond ((not (pair? l)) (values lis l))
		    ((pred (car l)) (lp l (cdr l)))
		    (else (scan-out prev-l l (cdr l))
			  (values lis l))))	; Done.

	    ;; LIS begins out-list. Search for in-list's first pair.
	    (let lp ((prev-l lis) (l (cdr lis)))
	      (cond ((not (pair? l)) (values l lis))
		    ((pred (car l))
		     (scan-in l prev-l (cdr l))
		     (values l lis))		; Done.
		    (else (lp l (cdr l)))))))))


;;; Inline us, please.
(define (remove  pred l) (filter  (lambda (x) (not (pred x))) l))
(define (remove! pred l) (filter! (lambda (x) (not (pred x))) l))



;;; Here's the taxonomy for the DELETE/ASSOC/MEMBER functions.
;;; (I don't actually think these are the world's most important
;;; functions -- the procedural FILTER/REMOVE/FIND/FIND-TAIL variants
;;; are far more general.)
;;;
;;; Function			Action
;;; ---------------------------------------------------------------------------
;;; remove pred lis		Delete by general predicate
;;; delete x lis [=]		Delete by element comparison
;;;					     
;;; find pred lis		Search by general predicate
;;; find-tail pred lis		Search by general predicate
;;; member x lis [=]		Search by element comparison
;;;
;;; assoc key lis [=]		Search alist by key comparison
;;; alist-delete key alist [=]	Alist-delete by key comparison

(define (delete x lis . maybe-=) 
  (let ((= (:optional maybe-= equal?)))
    (filter (lambda (y) (not (= x y))) lis)))

(define (delete! x lis . maybe-=)
  (let ((= (:optional maybe-= equal?)))
    (filter! (lambda (y) (not (= x y))) lis)))

;;; Extended from R4RS to take an optional comparison argument.
(define (member x lis . maybe-=)
  (let ((= (:optional maybe-= equal?)))
    (find-tail (lambda (y) (= x y)) lis)))

;;; R4RS, hence we don't bother to define.
;;; The MEMBER and then FIND-TAIL call should definitely
;;; be inlined for MEMQ & MEMV.
;(define (memq    x lis) (member x lis eq?))
;(define (memv    x lis) (member x lis eqv?))


;;; right-duplicate deletion
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; delete-duplicates delete-duplicates!
;;;
;;; Beware -- these are N^2 algorithms. To efficiently remove duplicates
;;; in long lists, sort the list to bring duplicates together, then use a 
;;; linear-time algorithm to kill the dups. Or use an algorithm based on
;;; element-marking. The former gives you O(n lg n), the latter is linear.

(define (delete-duplicates lis . maybe-=)
  (let ((elt= (:optional maybe-= equal?)))
    (check-arg procedure? elt= delete-duplicates)
    (let recur ((lis lis))
      (if (null-list? lis) lis
	  (let* ((x (car lis))
		 (tail (cdr lis))
		 (new-tail (recur (delete x tail elt=))))
	    (if (eq? tail new-tail) lis (cons x new-tail)))))))

(define (delete-duplicates! lis . maybe-=)
  (let ((elt= (:optional maybe-= equal?)))
    (check-arg procedure? elt= delete-duplicates!)
    (let recur ((lis lis))
      (if (null-list? lis) lis
	  (let* ((x (car lis))
		 (tail (cdr lis))
		 (new-tail (recur (delete! x tail elt=))))
	    (if (eq? tail new-tail) lis (cons x new-tail)))))))


;;; alist stuff
;;;;;;;;;;;;;;;

;;; Extended from R4RS to take an optional comparison argument.
(define (assoc x lis . maybe-=)
  (let ((= (:optional maybe-= equal?)))
    (find (lambda (entry) (= x (car entry))) lis)))

(define (alist-cons key datum alist) (cons (cons key datum) alist))

(define (alist-copy alist)
  (map (lambda (elt) (cons (car elt) (cdr elt)))
       alist))

(define (alist-delete key alist . maybe-=)
  (let ((= (:optional maybe-= equal?)))
    (filter (lambda (elt) (not (= key (car elt)))) alist)))

(define (alist-delete! key alist . maybe-=)
  (let ((= (:optional maybe-= equal?)))
    (filter! (lambda (elt) (not (= key (car elt)))) alist)))


;;; find find-tail take-while drop-while span break any every list-index
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (find pred list)
  (cond ((find-tail pred list) => car)
	(else #f)))

(define (find-tail pred list)
  (check-arg procedure? pred find-tail)
  (let lp ((list list))
    (and (not (null-list? list))
	 (if (pred (car list)) list
	     (lp (cdr list))))))

(define (take-while pred lis)
  (check-arg procedure? pred take-while)
  (let recur ((lis lis))
    (if (null-list? lis) '()
	(let ((x (car lis)))
	  (if (pred x)
	      (cons x (recur (cdr lis)))
	      '())))))

(define (drop-while pred lis)
  (check-arg procedure? pred drop-while)
  (let lp ((lis lis))
    (if (null-list? lis) '()
	(if (pred (car lis))
	    (lp (cdr lis))
	    lis))))

(define (take-while! pred lis)
  (check-arg procedure? pred take-while!)
  (if (or (null-list? lis) (not (pred (car lis)))) '()
      (begin (let lp ((prev lis) (rest (cdr lis)))
	       (if (pair? rest)
		   (let ((x (car rest)))
		     (if (pred x) (lp rest (cdr rest))
			 (set-cdr! prev '())))))
	     lis)))

(define (span pred lis)
  (check-arg procedure? pred span)
  (let recur ((lis lis))
    (if (null-list? lis) (values '() '())
	(let ((x (car lis)))
	  (if (pred x)
	      (receive (prefix suffix) (recur (cdr lis))
		(values (cons x prefix) suffix))
	      (values '() lis))))))

(define (span! pred lis)
  (check-arg procedure? pred span!)
  (if (or (null-list? lis) (not (pred (car lis)))) (values '() lis)
      (let ((suffix (let lp ((prev lis) (rest (cdr lis)))
		      (if (null-list? rest) rest
			  (let ((x (car rest)))
			    (if (pred x) (lp rest (cdr rest))
				(begin (set-cdr! prev '())
				       rest)))))))
	(values lis suffix))))
  

(define (break  pred lis) (span  (lambda (x) (not (pred x))) lis))
(define (break! pred lis) (span! (lambda (x) (not (pred x))) lis))

(define (any pred lis1 . lists)
  (check-arg procedure? pred any)
  (if (pair? lists)

      ;; N-ary case
      (receive (heads tails) (%cars+cdrs (cons lis1 lists))
	(and (pair? heads)
	     (let lp ((heads heads) (tails tails))
	       (receive (next-heads next-tails) (%cars+cdrs tails)
		 (if (pair? next-heads)
		     (or (apply pred heads) (lp next-heads next-tails))
		     (apply pred heads)))))) ; Last PRED app is tail call.

      ;; Fast path
      (and (not (null-list? lis1))
	   (let lp ((head (car lis1)) (tail (cdr lis1)))
	     (if (null-list? tail)
		 (pred head)		; Last PRED app is tail call.
		 (or (pred head) (lp (car tail) (cdr tail))))))))


;(define (every pred list)              ; Simple definition.
;  (let lp ((list list))                ; Doesn't return the last PRED value.
;    (or (not (pair? list))
;        (and (pred (car list))
;             (lp (cdr list))))))

(define (every pred lis1 . lists)
  (check-arg procedure? pred every)
  (if (pair? lists)

      ;; N-ary case
      (receive (heads tails) (%cars+cdrs (cons lis1 lists))
	(or (not (pair? heads))
	    (let lp ((heads heads) (tails tails))
	      (receive (next-heads next-tails) (%cars+cdrs tails)
		(if (pair? next-heads)
		    (and (apply pred heads) (lp next-heads next-tails))
		    (apply pred heads)))))) ; Last PRED app is tail call.

      ;; Fast path
      (or (null-list? lis1)
	  (let lp ((head (car lis1))  (tail (cdr lis1)))
	    (if (null-list? tail)
		(pred head)	; Last PRED app is tail call.
		(and (pred head) (lp (car tail) (cdr tail))))))))

(define (list-index pred lis1 . lists)
  (check-arg procedure? pred list-index)
  (if (pair? lists)

      ;; N-ary case
      (let lp ((lists (cons lis1 lists)) (n 0))
	(receive (heads tails) (%cars+cdrs lists)
	  (and (pair? heads)
	       (if (apply pred heads) n
		   (lp tails (+ n 1))))))

      ;; Fast path
      (let lp ((lis lis1) (n 0))
	(and (not (null-list? lis))
	     (if (pred (car lis)) n (lp (cdr lis) (+ n 1)))))))

;;; Reverse
;;;;;;;;;;;

;R4RS, so not defined here.
;(define (reverse lis) (fold cons '() lis))
				      
;(define (reverse! lis)
;  (pair-fold (lambda (pair tail) (set-cdr! pair tail) pair) '() lis))

(define (reverse! lis)
  (let lp ((lis lis) (ans '()))
    (if (null-list? lis) ans
        (let ((tail (cdr lis)))
          (set-cdr! lis ans)
          (lp tail lis)))))

;;; Lists-as-sets
;;;;;;;;;;;;;;;;;

;;; This is carefully tuned code; do not modify casually.
;;; - It is careful to share storage when possible;
;;; - Side-effecting code tries not to perform redundant writes.
;;; - It tries to avoid linear-time scans in special cases where constant-time
;;;   computations can be performed.
;;; - It relies on similar properties from the other list-lib procs it calls.
;;;   For example, it uses the fact that the implementations of MEMBER and
;;;   FILTER in this source code share longest common tails between args
;;;   and results to get structure sharing in the lset procedures.

(define (%lset2<= = lis1 lis2) (every (lambda (x) (member x lis2 =)) lis1))

(define (lset<= = . lists)
  (check-arg procedure? = lset<=)
  (or (not (pair? lists)) ; 0-ary case
      (let lp ((s1 (car lists)) (rest (cdr lists)))
	(or (not (pair? rest))
	    (let ((s2 (car rest))  (rest (cdr rest)))
	      (and (or (eq? s2 s1)	; Fast path
		       (%lset2<= = s1 s2)) ; Real test
		   (lp s2 rest)))))))

(define (lset= = . lists)
  (check-arg procedure? = lset=)
  (or (not (pair? lists)) ; 0-ary case
      (let lp ((s1 (car lists)) (rest (cdr lists)))
	(or (not (pair? rest))
	    (let ((s2   (car rest))
		  (rest (cdr rest)))
	      (and (or (eq? s1 s2)	; Fast path
		       (and (%lset2<= = s1 s2) (%lset2<= = s2 s1))) ; Real test
		   (lp s2 rest)))))))


(define (lset-adjoin = lis . elts)
  (check-arg procedure? = lset-adjoin)
  (fold (lambda (elt ans) (if (member elt ans =) ans (cons elt ans)))
	lis elts))


(define (lset-union = . lists)
  (check-arg procedure? = lset-union)
  (reduce (lambda (lis ans)		; Compute ANS + LIS.
	    (cond ((null? lis) ans)	; Don't copy any lists
		  ((null? ans) lis) 	; if we don't have to.
		  ((eq? lis ans) ans)
		  (else
		   (fold (lambda (elt ans) (if (any (lambda (x) (= x elt)) ans)
					       ans
					       (cons elt ans)))
			 ans lis))))
	  '() lists))

(define (lset-union! = . lists)
  (check-arg procedure? = lset-union!)
  (reduce (lambda (lis ans)		; Splice new elts of LIS onto the front of ANS.
	    (cond ((null? lis) ans)	; Don't copy any lists
		  ((null? ans) lis) 	; if we don't have to.
		  ((eq? lis ans) ans)
		  (else
		   (pair-fold (lambda (pair ans)
				(let ((elt (car pair)))
				  (if (any (lambda (x) (= x elt)) ans)
				      ans
				      (begin (set-cdr! pair ans) pair))))
			      ans lis))))
	  '() lists))


(define (lset-intersection = lis1 . lists)
  (check-arg procedure? = lset-intersection)
  (let ((lists (delete lis1 lists eq?))) ; Throw out any LIS1 vals.
    (cond ((any null-list? lists) '())		; Short cut
	  ((null? lists)          lis1)		; Short cut
	  (else (filter (lambda (x)
			  (every (lambda (lis) (member x lis =)) lists))
			lis1)))))

(define (lset-intersection! = lis1 . lists)
  (check-arg procedure? = lset-intersection!)
  (let ((lists (delete lis1 lists eq?))) ; Throw out any LIS1 vals.
    (cond ((any null-list? lists) '())		; Short cut
	  ((null? lists)          lis1)		; Short cut
	  (else (filter! (lambda (x)
			   (every (lambda (lis) (member x lis =)) lists))
			 lis1)))))


(define (lset-difference = lis1 . lists)
  (check-arg procedure? = lset-difference)
  (let ((lists (filter pair? lists)))	; Throw out empty lists.
    (cond ((null? lists)     lis1)	; Short cut
	  ((memq lis1 lists) '())	; Short cut
	  (else (filter (lambda (x)
			  (every (lambda (lis) (not (member x lis =)))
				 lists))
			lis1)))))

(define (lset-difference! = lis1 . lists)
  (check-arg procedure? = lset-difference!)
  (let ((lists (filter pair? lists)))	; Throw out empty lists.
    (cond ((null? lists)     lis1)	; Short cut
	  ((memq lis1 lists) '())	; Short cut
	  (else (filter! (lambda (x)
			   (every (lambda (lis) (not (member x lis =)))
				  lists))
			 lis1)))))


(define (lset-xor = . lists)
  (check-arg procedure? = lset-xor)
  (reduce (lambda (b a)			; Compute A xor B:
	    ;; Note that this code relies on the constant-time
	    ;; short-cuts provided by LSET-DIFF+INTERSECTION,
	    ;; LSET-DIFFERENCE & APPEND to provide constant-time short
	    ;; cuts for the cases A = (), B = (), and A eq? B. It takes
	    ;; a careful case analysis to see it, but it's carefully
	    ;; built in.

	    ;; Compute a-b and a^b, then compute b-(a^b) and
	    ;; cons it onto the front of a-b.
	    (receive (a-b a-int-b)   (lset-diff+intersection = a b)
	      (cond ((null? a-b)     (lset-difference = b a))
		    ((null? a-int-b) (append b a))
		    (else (fold (lambda (xb ans)
				  (if (member xb a-int-b =) ans (cons xb ans)))
				a-b
				b)))))
	  '() lists))


(define (lset-xor! = . lists)
  (check-arg procedure? = lset-xor!)
  (reduce (lambda (b a)			; Compute A xor B:
	    ;; Note that this code relies on the constant-time
	    ;; short-cuts provided by LSET-DIFF+INTERSECTION,
	    ;; LSET-DIFFERENCE & APPEND to provide constant-time short
	    ;; cuts for the cases A = (), B = (), and A eq? B. It takes
	    ;; a careful case analysis to see it, but it's carefully
	    ;; built in.

	    ;; Compute a-b and a^b, then compute b-(a^b) and
	    ;; cons it onto the front of a-b.
	    (receive (a-b a-int-b)   (lset-diff+intersection! = a b)
	      (cond ((null? a-b)     (lset-difference! = b a))
		    ((null? a-int-b) (append! b a))
		    (else (pair-fold (lambda (b-pair ans)
				       (if (member (car b-pair) a-int-b =) ans
					   (begin (set-cdr! b-pair ans) b-pair)))
				     a-b
				     b)))))
	  '() lists))


(define (lset-diff+intersection = lis1 . lists)
  (check-arg procedure? = lset-diff+intersection)
  (cond ((every null-list? lists) (values lis1 '()))	; Short cut
	((memq lis1 lists)        (values '() lis1))	; Short cut
	(else (partition (lambda (elt)
			   (not (any (lambda (lis) (member elt lis =))
				     lists)))
			 lis1))))

(define (lset-diff+intersection! = lis1 . lists)
  (check-arg procedure? = lset-diff+intersection!)
  (cond ((every null-list? lists) (values lis1 '()))	; Short cut
	((memq lis1 lists)        (values '() lis1))	; Short cut
	(else (partition! (lambda (elt)
			    (not (any (lambda (lis) (member elt lis =))
				      lists)))
			  lis1))))