aboutsummaryrefslogblamecommitdiffstats
path: root/lib/stdlib/src/lists.erl
blob: eb527471d59b489a66c19bda878986651d850cc1 (plain) (tree)
1
2
3
4
5
6
7
8
9
10
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

                   
  
                                                        
  




                                                                      
  



                                                                         
  



                 


                                   






                                                                              
                                                      



                                                           






                                                                             


                                                                    
                                             








                                                      
                                               








                                                    
                                               








                                                                 
                                            







                                          
                                             










                                        










                                       




                                        




                                        




                                       






                                                                




                                          


                             
                                                                         
 



                                  

















                                                    




                               




                           




                                   







                                         



                                            







                                                



                                            






                                                       



                             










                                                  



                               













                                                                 




                                     
























                                                                 

                                








                                                




                                     







                                                                  



                           








                                                      



                           









                                                            





                                              



                                                 




                                       













                                                        




                                       








                                                                     





                                     






                                                                       





                                         








                                                                        







                                             






                                                                        







                                                 










                                                                        







                                                  






                                                                      









                                                          







                                                     



                               













































                                    




                                      






                                        







                                               










                                                       
                                              










                                                        





                                       










                                                                   
                                    









                                                                


                                                       












                                                                       


                                      



                                   



                                          










                                                        


                                            

                                                    










                                            
                                      




























                                                              





                                                      








                                                         


                                                                 



                            









                                                                       





                                                                            









                                                      


                                                               



                                 
 









                                                                  




                                               























































                                                                  




                                                            


                       













                                                                           
                                                          










                                                              




                                                































































                                                                   




                                                             


                       













                                                                             
                                                  










                                                              


                                                       


                            







                                                                               




                                                












                                              






                                            







                                                                     
                                                              





                                                       




                                      

























                                                   






                                             







                                                                       
                                                               








                                                        



                                





















































                                                                        




                                       







                                                             







                                                











                                                                     
                                               










                                                                          





                                        











                                                                     
                                     































                                                                               



                                            







                                                 



                                            







                                                  





                                   




                                        





                                       




                                            







                                               




                                                  







                                               




                                                  




                                            






                                                                      





                                                               
























                                                                



                                        





                                            









                                                      






                                                          









                                                      






                                                          




                                            







                                                    




                                            







                                                    





                                                  











                                                       





                                            
























































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































                                                                                  
%%
%% %CopyrightBegin%
%%
%% Copyright Ericsson AB 1996-2012. All Rights Reserved.
%%
%% The contents of this file are subject to the Erlang Public License,
%% Version 1.1, (the "License"); you may not use this file except in
%% compliance with the License. You should have received a copy of the
%% Erlang Public License along with this software. If not, it can be
%% retrieved online at http://www.erlang.org/.
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and limitations
%% under the License.
%%
%% %CopyrightEnd%
%%
-module(lists).

-compile({no_auto_import,[max/2]}).
-compile({no_auto_import,[min/2]}).

-export([append/2, append/1, subtract/2, reverse/1,
	 nth/2, nthtail/2, prefix/2, suffix/2, last/1, 
	 seq/2, seq/3, sum/1, duplicate/2, min/1, max/1, sublist/2, sublist/3,
	 delete/2,
	 unzip/1, unzip3/1, zip/2, zip3/3, zipwith/3, zipwith3/4,
	 sort/1, merge/1, merge/2, rmerge/2, merge3/3, rmerge3/3,
	 usort/1, umerge/1, umerge3/3, umerge/2, rumerge3/3, rumerge/2,
	 concat/1, flatten/1, flatten/2, flatlength/1,
	 keydelete/3, keyreplace/4, keytake/3, keystore/4,
	 keysort/2, keymerge/3, rkeymerge/3, rukeymerge/3, 
	 ukeysort/2, ukeymerge/3, keymap/3]).

-export([merge/3, rmerge/3, sort/2, umerge/3, rumerge/3, usort/2]).

-export([all/2,any/2,map/2,flatmap/2,foldl/3,foldr/3,filter/2,
	 partition/2,zf/2,
	 mapfoldl/3,mapfoldr/3,foreach/2,takewhile/2,dropwhile/2,splitwith/2,
	 split/2]).

%%% BIFs
-export([keyfind/3, keymember/3, keysearch/3, member/2, reverse/2]).

%% Shadowed by erl_bif_types: lists:keyfind/3
-spec keyfind(Key, N, TupleList) -> Tuple | false when
      Key :: term(),
      N :: pos_integer(),
      TupleList :: [Tuple],
      Tuple :: tuple().

keyfind(_, _, _) ->
    erlang:nif_error(undef).

%% Shadowed by erl_bif_types: lists:keymember/3
-spec keymember(Key, N, TupleList) -> boolean() when
      Key :: term(),
      N :: pos_integer(),
      TupleList :: [Tuple],
      Tuple :: tuple().

keymember(_, _, _) ->
    erlang:nif_error(undef).

%% Shadowed by erl_bif_types: lists:keysearch/3
-spec keysearch(Key, N, TupleList) -> {value, Tuple} | false when
      Key :: term(),
      N :: pos_integer(),
      TupleList :: [Tuple],
      Tuple :: tuple().

keysearch(_, _, _) ->
    erlang:nif_error(undef).

%% Shadowed by erl_bif_types: lists:member/2
-spec member(Elem, List) -> boolean() when
      Elem :: T,
      List :: [T],
      T :: term().

member(_, _) ->
    erlang:nif_error(undef).

%% Shadowed by erl_bif_types: lists:reverse/2
-spec reverse(List1, Tail) -> List2 when
      List1 :: [T],
      Tail :: term(),
      List2 :: [T],
      T :: term().

reverse(_, _) ->
    erlang:nif_error(undef).

%%% End of BIFs

%% member(X, L) -> (true | false)
%%  test if X is a member of the list L
%%  Now a BIF!

%member(X, [X|_]) -> true;
%member(X, [_|Y]) ->
%	member(X, Y);
%member(X, []) -> false.

%% append(X, Y) appends lists X and Y

-spec append(List1, List2) -> List3 when
      List1 :: [T],
      List2 :: [T],
      List3 :: [T],
      T :: term().

append(L1, L2) -> L1 ++ L2.

%% append(L) appends the list of lists L

-spec append(ListOfLists) -> List1 when
      ListOfLists :: [List],
      List :: [T],
      List1 :: [T],
      T :: term().

append([E]) -> E;
append([H|T]) -> H ++ append(T);
append([]) -> [].

%% subtract(List1, List2) subtract elements in List2 form List1.

-spec subtract(List1, List2) -> List3 when
      List1 :: [T],
      List2 :: [T],
      List3 :: [T],
      T :: term().

subtract(L1, L2) -> L1 -- L2.

%% reverse(L) reverse all elements in the list L. reverse/2 is now a BIF!

-spec reverse(List1) -> List2 when
      List1 :: [T],
      List2 :: [T],
      T :: term().

reverse([] = L) ->
    L;
reverse([_] = L) ->
    L;
reverse([A, B]) ->
    [B, A];
reverse([A, B | L]) ->
    lists:reverse(L, [B, A]).

%reverse([H|T], Y) ->
%    reverse(T, [H|Y]);
%reverse([], X) -> X.


%% nth(N, L) returns the N`th element of the list L
%% nthtail(N, L) returns the N`th tail of the list L

-spec nth(N, List) -> Elem when
      N :: pos_integer(),
      List :: [T,...],
      Elem :: T,
      T :: term().

nth(1, [H|_]) -> H;
nth(N, [_|T]) when N > 1 ->
    nth(N - 1, T).

-spec nthtail(N, List) -> Tail when
      N :: non_neg_integer(),
      List :: [T,...],
      Tail :: [T],
      T :: term().

nthtail(1, [_|T]) -> T;
nthtail(N, [_|T]) when N > 1 ->
    nthtail(N - 1, T);
nthtail(0, L) when is_list(L) -> L.

%% prefix(Prefix, List) -> (true | false)

-spec prefix(List1, List2) -> boolean() when
      List1 :: [T],
      List2 :: [T],
      T :: term().

prefix([X|PreTail], [X|Tail]) ->
    prefix(PreTail, Tail);
prefix([], List) when is_list(List) -> true;
prefix([_|_], List) when is_list(List) -> false.

%% suffix(Suffix, List) -> (true | false)

-spec suffix(List1, List2) -> boolean() when
      List1 :: [T],
      List2 :: [T],
      T :: term().

suffix(Suffix, List) ->
    Delta = length(List) - length(Suffix),
    Delta >= 0 andalso nthtail(Delta, List) =:= Suffix.

%% last(List) returns the last element in a list.

-spec last(List) -> Last when
      List :: [T,...],
      Last :: T,
      T :: term().

last([E|Es]) -> last(E, Es).

last(_, [E|Es]) -> last(E, Es);
last(E, []) -> E.

%% seq(Min, Max) -> [Min,Min+1, ..., Max]
%% seq(Min, Max, Incr) -> [Min,Min+Incr, ..., Max]
%%  returns the sequence Min..Max
%%  Min <= Max and Min and Max must be integers

-spec seq(From, To) -> Seq when
      From :: integer(),
      To :: integer(),
      Seq :: [integer()].

seq(First, Last)
    when is_integer(First), is_integer(Last), First-1 =< Last -> 
    seq_loop(Last-First+1, Last, []).

seq_loop(N, X, L) when N >= 4 ->
     seq_loop(N-4, X-4, [X-3,X-2,X-1,X|L]);
seq_loop(N, X, L) when N >= 2 ->
     seq_loop(N-2, X-2, [X-1,X|L]);
seq_loop(1, X, L) ->
     [X|L];
seq_loop(0, _, L) ->
     L.

-spec seq(From, To, Incr) -> Seq when
      From :: integer(),
      To :: integer(),
      Incr :: integer(),
      Seq :: [integer()].

seq(First, Last, Inc) 
    when is_integer(First), is_integer(Last), is_integer(Inc) -> 
    if
        Inc > 0, First - Inc =< Last;
        Inc < 0, First - Inc >= Last ->
            N = (Last - First + Inc) div Inc,
            seq_loop(N, Inc*(N-1)+First, Inc, []);
        Inc =:= 0, First =:= Last ->
            seq_loop(1, First, Inc, [])
    end.

seq_loop(N, X, D, L) when N >= 4 ->
     Y = X-D, Z = Y-D, W = Z-D,
     seq_loop(N-4, W-D, D, [W,Z,Y,X|L]);
seq_loop(N, X, D, L) when N >= 2 ->
     Y = X-D,
     seq_loop(N-2, Y-D, D, [Y,X|L]);
seq_loop(1, X, _, L) ->
     [X|L];
seq_loop(0, _, _, L) ->
     L.

%% sum(L) returns the sum of the elements in L

-spec sum(List) -> number() when
      List :: [number()].

sum(L)          -> sum(L, 0).

sum([H|T], Sum) -> sum(T, Sum + H);
sum([], Sum)    -> Sum.

%% duplicate(N, X) -> [X,X,X,.....,X]  (N times)
%%   return N copies of X

-spec duplicate(N, Elem) -> List when
      N :: non_neg_integer(),
      Elem :: T,
      List :: [T],
      T :: term().

duplicate(N, X) when is_integer(N), N >= 0 -> duplicate(N, X, []).

duplicate(0, _, L) -> L;
duplicate(N, X, L) -> duplicate(N-1, X, [X|L]).

%% min(L) -> returns the minimum element of the list L

-spec min(List) -> Min when
      List :: [T,...],
      Min :: T,
      T :: term().

min([H|T]) -> min(T, H).

min([H|T], Min) when H < Min -> min(T, H);
min([_|T], Min)              -> min(T, Min);
min([],    Min)              -> Min. 

%% max(L) -> returns the maximum element of the list L

-spec max(List) -> Max when
      List :: [T,...],
      Max :: T,
      T :: term().

max([H|T]) -> max(T, H).

max([H|T], Max) when H > Max -> max(T, H);
max([_|T], Max)              -> max(T, Max);
max([],    Max)              -> Max.

%% sublist(List, Start, Length)
%%  Returns the sub-list starting at Start of length Length.

-spec sublist(List1, Start, Len) -> List2 when
      List1 :: [T],
      List2 :: [T],
      Start :: pos_integer(),
      Len :: non_neg_integer(),
      T :: term().

sublist(List, S, L) when is_integer(L), L >= 0 ->
    sublist(nthtail(S-1, List), L).

-spec sublist(List1, Len) -> List2 when
      List1 :: [T],
      List2 :: [T],
      Len :: non_neg_integer(),
      T :: term().

sublist(List, L) when is_integer(L), is_list(List) ->
    sublist_2(List, L).

sublist_2([H|T], L) when L > 0 ->
    [H|sublist_2(T, L-1)];
sublist_2(_, 0) ->
    [];
sublist_2(List, L) when is_list(List), L > 0 ->
    [].

%% delete(Item, List) -> List'
%%  Delete the first occurrence of Item from the list L.

-spec delete(Elem, List1) -> List2 when
      Elem :: T,
      List1 :: [T],
      List2 :: [T],
      T :: term().

delete(Item, [Item|Rest]) -> Rest;
delete(Item, [H|Rest]) -> 
    [H|delete(Item, Rest)];
delete(_, []) -> [].

%% Return [{X0, Y0}, {X1, Y1}, ..., {Xn, Yn}] for lists [X0, X1, ...,
%% Xn] and [Y0, Y1, ..., Yn].

-spec zip(List1, List2) -> List3 when
      List1 :: [A],
      List2 :: [B],
      List3 :: [{A, B}],
      A :: term(),
      B :: term().

zip([X | Xs], [Y | Ys]) -> [{X, Y} | zip(Xs, Ys)];
zip([], []) -> [].

%% Return {[X0, X1, ..., Xn], [Y0, Y1, ..., Yn]}, for a list [{X0, Y0},
%% {X1, Y1}, ..., {Xn, Yn}].

-spec unzip(List1) -> {List2, List3} when
      List1 :: [{A, B}],
      List2 :: [A],
      List3 :: [B],
      A :: term(),
      B :: term().

unzip(Ts) -> unzip(Ts, [], []).

unzip([{X, Y} | Ts], Xs, Ys) -> unzip(Ts, [X | Xs], [Y | Ys]);
unzip([], Xs, Ys) -> {reverse(Xs), reverse(Ys)}.

%% Return [{X0, Y0, Z0}, {X1, Y1, Z1}, ..., {Xn, Yn, Zn}] for lists [X0,
%% X1, ..., Xn], [Y0, Y1, ..., Yn] and [Z0, Z1, ..., Zn].

-spec zip3(List1, List2, List3) -> List4 when
      List1 :: [A],
      List2 :: [B],
      List3 :: [C],
      List4 :: [{A, B, C}],
      A :: term(),
      B :: term(),
      C :: term().

zip3([X | Xs], [Y | Ys], [Z | Zs]) -> [{X, Y, Z} | zip3(Xs, Ys, Zs)];
zip3([], [], []) -> [].

%% Return {[X0, X1, ..., Xn], [Y0, Y1, ..., Yn], [Z0, Z1, ..., Zn]}, for
%% a list [{X0, Y0, Z0}, {X1, Y1, Z1}, ..., {Xn, Yn, Zn}].

-spec unzip3(List1) -> {List2, List3, List4} when
      List1 :: [{A, B, C}],
      List2 :: [A],
      List3 :: [B],
      List4 :: [C],
      A :: term(),
      B :: term(),
      C :: term().

unzip3(Ts) -> unzip3(Ts, [], [], []).

unzip3([{X, Y, Z} | Ts], Xs, Ys, Zs) ->
    unzip3(Ts, [X | Xs], [Y | Ys], [Z | Zs]);
unzip3([], Xs, Ys, Zs) ->
    {reverse(Xs), reverse(Ys), reverse(Zs)}.

%% Return [F(X0, Y0), F(X1, Y1), ..., F(Xn, Yn)] for lists [X0, X1, ...,
%% Xn] and [Y0, Y1, ..., Yn].

-spec zipwith(Combine, List1, List2) -> List3 when
      Combine :: fun((X, Y) -> T),
      List1 :: [X],
      List2 :: [Y],
      List3 :: [T],
      X :: term(),
      Y :: term(),
      T :: term().

zipwith(F, [X | Xs], [Y | Ys]) -> [F(X, Y) | zipwith(F, Xs, Ys)];
zipwith(F, [], []) when is_function(F, 2) -> [].

%% Return [F(X0, Y0, Z0), F(X1, Y1, Z1), ..., F(Xn, Yn, Zn)] for lists
%% [X0, X1, ..., Xn], [Y0, Y1, ..., Yn] and [Z0, Z1, ..., Zn].

-spec zipwith3(Combine, List1, List2, List3) -> List4 when
      Combine :: fun((X, Y, Z) -> T),
      List1 :: [X],
      List2 :: [Y],
      List3 :: [Z],
      List4 :: [T],
      X :: term(),
      Y :: term(),
      Z :: term(),
      T :: term().

zipwith3(F, [X | Xs], [Y | Ys], [Z | Zs]) ->
    [F(X, Y, Z) | zipwith3(F, Xs, Ys, Zs)];
zipwith3(F, [], [], []) when is_function(F, 3) -> [].

%% sort(List) -> L
%%  sorts the list L

-spec sort(List1) -> List2 when
      List1 :: [T],
      List2 :: [T],
      T :: term().

sort([X, Y | L] = L0) when X =< Y ->
    case L of
	[] -> 
	    L0;
	[Z] when Y =< Z ->
	    L0;
	[Z] when X =< Z ->
	    [X, Z, Y];
	[Z] ->
	    [Z, X, Y];
	_ when X == Y ->
	    sort_1(Y, L, [X]);
	_ ->
	    split_1(X, Y, L, [], [])
    end;
sort([X, Y | L]) ->
    case L of
	[] ->
	    [Y, X];
	[Z] when X =< Z ->
	    [Y, X | L];
	[Z] when Y =< Z ->
	    [Y, Z, X];
	[Z] ->
	    [Z, Y, X];
	_ ->
	    split_2(X, Y, L, [], [])
    end;
sort([_] = L) ->
    L;
sort([] = L) ->
    L.

sort_1(X, [Y | L], R) when X == Y ->
    sort_1(Y, L, [X | R]);
sort_1(X, [Y | L], R) when X < Y ->
    split_1(X, Y, L, R, []);
sort_1(X, [Y | L], R) ->
    split_2(X, Y, L, R, []);
sort_1(X, [], R) ->
    lists:reverse(R, [X]).

%% merge(List) -> L
%%  merges a list of sorted lists

-spec merge(ListOfLists) -> List1 when
      ListOfLists :: [List],
      List :: [T],
      List1 :: [T],
      T :: term().

merge(L) ->
    mergel(L, []).

%% merge3(X, Y, Z) -> L
%%  merges three sorted lists X, Y and Z

-spec merge3(List1, List2, List3) -> List4 when
      List1 :: [X],
      List2 :: [Y],
      List3 :: [Z],
      List4 :: [(X | Y | Z)],
      X :: term(),
      Y :: term(),
      Z :: term().

merge3(L1, [], L3) ->
   merge(L1, L3);
merge3(L1, L2, []) ->
   merge(L1, L2);
merge3(L1, [H2 | T2], [H3 | T3]) ->
   lists:reverse(merge3_1(L1, [], H2, T2, H3, T3), []).

%% rmerge3(X, Y, Z) -> L
%%  merges three reversed sorted lists X, Y and Z

-spec rmerge3([X], [Y], [Z]) -> [(X | Y | Z)].

rmerge3(L1, [], L3) ->
   rmerge(L1, L3);
rmerge3(L1, L2, []) ->
   rmerge(L1, L2);
rmerge3(L1, [H2 | T2], [H3 | T3]) ->
   lists:reverse(rmerge3_1(L1, [], H2, T2, H3, T3), []).

%% merge(X, Y) -> L
%%  merges two sorted lists X and Y

-spec merge(List1, List2) -> List3 when
      List1 :: [X],
      List2 :: [Y],
      List3 :: [(X | Y)],
      X :: term(),
      Y :: term().

merge(T1, []) ->
    T1;
merge(T1, [H2 | T2]) ->
    lists:reverse(merge2_1(T1, H2, T2, []), []).

%% rmerge(X, Y) -> L
%%  merges two reversed sorted lists X and Y

%% reverse(rmerge(reverse(A),reverse(B))) is equal to merge(I,A,B).

-spec rmerge([X], [Y]) -> [(X | Y)].

rmerge(T1, []) ->
    T1;
rmerge(T1, [H2 | T2]) ->
    lists:reverse(rmerge2_1(T1, H2, T2, []), []).

%% concat(L) concatenate the list representation of the elements
%%  in L - the elements in L can be atoms, numbers of strings.
%%  Returns a list of characters.

-spec concat(Things) -> string() when
      Things :: [Thing],
      Thing :: atom() | integer() | float() | string().

concat(List) ->
    flatmap(fun thing_to_list/1, List).

thing_to_list(X) when is_integer(X) -> integer_to_list(X);
thing_to_list(X) when is_float(X)   -> float_to_list(X);
thing_to_list(X) when is_atom(X)    -> atom_to_list(X);
thing_to_list(X) when is_list(X)    -> X.	%Assumed to be a string

%% flatten(List)
%% flatten(List, Tail)
%%  Flatten a list, adding optional tail.

-spec flatten(DeepList) -> List when
      DeepList :: [term() | DeepList],
      List :: [term()].

flatten(List) when is_list(List) ->
    do_flatten(List, []).

-spec flatten(DeepList, Tail) -> List when
      DeepList :: [term() | DeepList],
      Tail :: [term()],
      List :: [term()].

flatten(List, Tail) when is_list(List), is_list(Tail) ->
    do_flatten(List, Tail).

do_flatten([H|T], Tail) when is_list(H) ->
    do_flatten(H, do_flatten(T, Tail));
do_flatten([H|T], Tail) ->
    [H|do_flatten(T, Tail)];
do_flatten([], Tail) ->
    Tail.

%% flatlength(List)
%%  Calculate the length of a list of lists.

-spec flatlength(DeepList) -> non_neg_integer() when
      DeepList :: [term() | DeepList].

flatlength(List) ->
    flatlength(List, 0).

flatlength([H|T], L) when is_list(H) ->
    flatlength(H, flatlength(T, L));
flatlength([_|T], L) ->
    flatlength(T, L + 1);
flatlength([], L) -> L.

%% keymember(Key, Index, [Tuple]) Now a BIF!
%% keyfind(Key, Index, [Tuple]) A BIF!
%% keysearch(Key, Index, [Tuple]) Now a BIF!
%% keydelete(Key, Index, [Tuple])
%% keyreplace(Key, Index, [Tuple], NewTuple)
%% keytake(Key, Index, [Tuple])
%% keystore(Key, Index, [Tuple], NewTuple)
%% keysort(Index, [Tuple])
%% keymerge(Index, [Tuple], [Tuple])
%% ukeysort(Index, [Tuple])
%% ukeymerge(Index, [Tuple], [Tuple])
%% keymap(Function, Index, [Tuple])
%% keymap(Function, ExtraArgs, Index, [Tuple])

%keymember(K,N,L) when is_integer(N), N > 0 ->
%    keymember3(K,N,L).

%keymember3(Key, N, [T|Ts]) when element(N, T) == Key -> true;
%keymember3(Key, N, [T|Ts]) ->
%    keymember3(Key, N, Ts);
%keymember3(Key, N, []) -> false.

%keysearch(K, N, L) when is_integer(N), N > 0 ->
%    keysearch3(K, N, L).

%keysearch3(Key, N, [H|T]) when element(N, H) == Key ->
%    {value, H};
%keysearch3(Key, N, [H|T]) ->
%    keysearch3(Key, N, T);
%keysearch3(Key, N, []) -> false.

-spec keydelete(Key, N, TupleList1) -> TupleList2 when
      Key :: term(),
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple],
      Tuple :: tuple().

keydelete(K, N, L) when is_integer(N), N > 0 ->
    keydelete3(K, N, L).

keydelete3(Key, N, [H|T]) when element(N, H) == Key -> T;
keydelete3(Key, N, [H|T]) ->
    [H|keydelete3(Key, N, T)];
keydelete3(_, _, []) -> [].

-spec keyreplace(Key, N, TupleList1, NewTuple) -> TupleList2 when
      Key :: term(),
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple],
      NewTuple :: Tuple,
      Tuple :: tuple().

keyreplace(K, N, L, New) when is_integer(N), N > 0, is_tuple(New) ->
    keyreplace3(K, N, L, New).

keyreplace3(Key, Pos, [Tup|Tail], New) when element(Pos, Tup) == Key ->
    [New|Tail];
keyreplace3(Key, Pos, [H|T], New) ->
    [H|keyreplace3(Key, Pos, T, New)];
keyreplace3(_, _, [], _) -> [].

-spec keytake(Key, N, TupleList1) -> {value, Tuple, TupleList2} | false when
      Key :: term(),
      N :: pos_integer(),
      TupleList1 :: [tuple()],
      TupleList2 :: [tuple()],
      Tuple :: tuple().

keytake(Key, N, L) when is_integer(N), N > 0 ->
    keytake(Key, N, L, []).

keytake(Key, N, [H|T], L) when element(N, H) == Key ->
    {value, H, lists:reverse(L, T)};
keytake(Key, N, [H|T], L) ->
    keytake(Key, N, T, [H|L]);
keytake(_K, _N, [], _L) -> false.

-spec keystore(Key, N, TupleList1, NewTuple) -> TupleList2 when
      Key :: term(),
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple, ...],
      NewTuple :: Tuple,
      Tuple :: tuple().

keystore(K, N, L, New) when is_integer(N), N > 0, is_tuple(New) ->
    keystore2(K, N, L, New).

keystore2(Key, N, [H|T], New) when element(N, H) == Key ->
    [New|T];
keystore2(Key, N, [H|T], New) ->
    [H|keystore2(Key, N, T, New)];
keystore2(_Key, _N, [], New) ->
    [New].

-spec keysort(N, TupleList1) -> TupleList2 when
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple],
      Tuple :: tuple().

keysort(I, L) when is_integer(I), I > 0 ->
    case L of
	[] -> L;
	[_] -> L;
	[X, Y | T] ->
	    case {element(I, X), element(I, Y)} of
		{EX, EY} when EX =< EY ->
		    case T of
			[] ->
			    L;
			[Z] ->
			    case element(I, Z) of
				EZ when EY =< EZ ->
				    L;
				EZ when EX =< EZ ->
				    [X, Z, Y];
				_EZ ->
				    [Z, X, Y]
			    end;
			_ when X == Y ->
			    keysort_1(I, Y, EY, T, [X]);
			_ ->
			    keysplit_1(I, X, EX, Y, EY, T, [], [])
		    end;
		{EX, EY} ->
		    case T of
			[] ->
			    [Y, X];
			[Z] ->
			    case element(I, Z) of
				EZ when EX =< EZ ->
				    [Y, X | T];
				EZ when EY =< EZ ->
				    [Y, Z, X];
				_EZ ->
				    [Z, Y, X]
			    end;
			_ ->
			    keysplit_2(I, X, EX, Y, EY, T, [], [])
		    end
	    end
    end.

keysort_1(I, X, EX, [Y | L], R) when X == Y ->
    keysort_1(I, Y, EX, L, [X | R]);
keysort_1(I, X, EX, [Y | L], R) ->
    case element(I, Y) of
	EY when EX =< EY ->
	    keysplit_1(I, X, EX, Y, EY, L, R, []);
	EY ->
	    keysplit_2(I, X, EX, Y, EY, L, R, [])
    end;
keysort_1(_I, X, _EX, [], R) ->
    lists:reverse(R, [X]).

-spec keymerge(N, TupleList1, TupleList2) -> TupleList3 when
      N :: pos_integer(),
      TupleList1 :: [T1],
      TupleList2 :: [T2],
      TupleList3 :: [(T1 | T2)],
      T1 :: Tuple,
      T2 :: Tuple,
      Tuple :: tuple().

keymerge(Index, T1, L2) when is_integer(Index), Index > 0 -> 
    case L2 of
	[] ->
	    T1;
	[H2 | T2] ->
	    E2 = element(Index, H2),
	    M = keymerge2_1(Index, T1, E2, H2, T2, []),
	    lists:reverse(M, [])
    end.

%% reverse(rkeymerge(I,reverse(A),reverse(B))) is equal to keymerge(I,A,B).

-spec rkeymerge(pos_integer(), [X], [Y]) ->
	[R] when X :: tuple(), Y :: tuple(), R :: tuple().

rkeymerge(Index, T1, L2) when is_integer(Index), Index > 0 -> 
    case L2 of
	[] ->
	    T1;
	[H2 | T2] ->
	    E2 = element(Index, H2),
	    M = rkeymerge2_1(Index, T1, E2, H2, T2, []),
	    lists:reverse(M, [])
    end.

-spec ukeysort(N, TupleList1) -> TupleList2 when
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple],
      Tuple :: tuple().

ukeysort(I, L) when is_integer(I), I > 0 ->
    case L of
	[] -> L;
	[_] -> L;
	[X, Y | T] ->
            case {element(I, X), element(I, Y)} of
                {EX, EY} when EX == EY ->
                    ukeysort_1(I, X, EX, T);
                {EX, EY} when EX < EY ->
                    case T of
                        [] ->
                            L;
                        [Z] ->
                            case element(I, Z) of
                                EZ when EY == EZ ->
                                    [X, Y];
                                EZ when EY < EZ ->
                                    [X, Y, Z];
                                EZ when EZ == EX ->
                                    [X, Y];
                                EZ when EX =< EZ ->
                                    [X, Z, Y];
                                _EZ ->
                                    [Z, X, Y]
                            end;
                        _ ->
                            ukeysplit_1(I, X, EX, Y, EY, T, [], [])
                    end;
                {EX, EY} ->
                    case T of
                        [] ->
                            [Y, X];
                        [Z] ->
                            case element(I, Z) of
                                EZ when EX == EZ ->
                                    [Y, X];
                                EZ when EX < EZ ->
                                    [Y, X, Z];
                                EZ when EY == EZ ->
                                    [Y, X];
                                EZ when EY =< EZ ->
                                    [Y, Z, X];
                                _EZ ->
                                    [Z, Y, X]
                            end;
                        _ ->
			    ukeysplit_2(I, Y, EY, T, [X])
                    end
	    end
    end.

ukeysort_1(I, X, EX, [Y | L]) ->
    case element(I, Y) of
        EY when EX == EY ->
            ukeysort_1(I, X, EX, L);
	EY when EX < EY ->
	    ukeysplit_1(I, X, EX, Y, EY, L, [], []);
	EY ->
	    ukeysplit_2(I, Y, EY, L, [X])
    end;
ukeysort_1(_I, X, _EX, []) ->
    [X].

-spec ukeymerge(N, TupleList1, TupleList2) -> TupleList3 when
      N :: pos_integer(),
      TupleList1 :: [T1],
      TupleList2 :: [T2],
      TupleList3 :: [(T1 | T2)],
      T1 :: Tuple,
      T2 :: Tuple,
      Tuple :: tuple().

ukeymerge(Index, L1, T2) when is_integer(Index), Index > 0 ->
    case L1 of
	[] ->
	    T2;
	[H1 | T1] ->
	    E1 = element(Index, H1),
	    M = ukeymerge2_2(Index, T1, E1, H1, T2, []),
	    lists:reverse(M, [])
    end.

%% reverse(rukeymerge(I,reverse(A),reverse(B))) is equal to ukeymerge(I,A,B).

-spec rukeymerge(pos_integer(), [X], [Y]) ->
	[(X | Y)] when X :: tuple(), Y :: tuple().

rukeymerge(Index, T1, L2) when is_integer(Index), Index > 0 ->
    case L2 of
	[] ->
	    T1;
	[H2 | T2] ->
	    E2 = element(Index, H2),
	    M = rukeymerge2_1(Index, T1, E2, T2, [], H2),
	    lists:reverse(M, [])
    end.

-spec keymap(Fun, N, TupleList1) -> TupleList2 when
      Fun :: fun((Term1 :: term()) -> Term2 :: term()),
      N :: pos_integer(),
      TupleList1 :: [Tuple],
      TupleList2 :: [Tuple],
      Tuple :: tuple().

keymap(Fun, Index, [Tup|Tail]) ->
   [setelement(Index, Tup, Fun(element(Index, Tup)))|keymap(Fun, Index, Tail)];
keymap(Fun, Index, []) when is_integer(Index), Index >= 1, 
                            is_function(Fun, 1) -> [].

%%% Suggestion from OTP-2948: sort and merge with Fun.

-spec sort(Fun, List1) -> List2 when
      Fun :: fun((A :: T, B :: T) -> boolean()),
      List1 :: [T],
      List2 :: [T],
      T :: term().

sort(Fun, []) when is_function(Fun, 2) ->
    [];
sort(Fun, [_] = L) when is_function(Fun, 2) ->
    L;
sort(Fun, [X, Y | T]) ->
    case Fun(X, Y) of
	true ->
	    fsplit_1(Y, X, Fun, T, [], []);
	false ->
	    fsplit_2(Y, X, Fun, T, [], [])
    end.

-spec merge(Fun, List1, List2) -> List3 when
      Fun :: fun((A, B) -> boolean()),
      List1 :: [A],
      List2 :: [B],
      List3 :: [(A | B)],
      A :: term(),
      B :: term().

merge(Fun, T1, [H2 | T2]) when is_function(Fun, 2) ->
    lists:reverse(fmerge2_1(T1, H2, Fun, T2, []), []);
merge(Fun, T1, []) when is_function(Fun, 2) ->
    T1.

%% reverse(rmerge(F,reverse(A),reverse(B))) is equal to merge(F,A,B).

-spec rmerge(fun((X, Y) -> boolean()), [X], [Y]) -> [(X | Y)].

rmerge(Fun, T1, [H2 | T2]) when is_function(Fun, 2) ->
    lists:reverse(rfmerge2_1(T1, H2, Fun, T2, []), []);
rmerge(Fun, T1, []) when is_function(Fun, 2) ->
    T1.

-spec usort(Fun, List1) -> List2 when
      Fun :: fun((T, T) -> boolean()),
      List1 :: [T],
      List2 :: [T],
      T :: term().

usort(Fun, [_] = L) when is_function(Fun, 2) ->
    L;
usort(Fun, [] = L) when is_function(Fun, 2) ->
    L;
usort(Fun, [X | L]) when is_function(Fun, 2) ->
    usort_1(Fun, X, L).

usort_1(Fun, X, [Y | L]) ->
    case Fun(X, Y) of
        true ->
            case Fun(Y, X) of
                true -> % X equal to Y
                    case L of
                        [] ->
                            [X];
                        _ ->
                            usort_1(Fun, X, L)
                    end;
                false ->
                    ufsplit_1(Y, X, Fun, L, [], [])
            end;
        false  ->
	    ufsplit_2(Y, L, Fun, [X])
    end.
                    
-spec umerge(Fun, List1, List2) -> List3 when
      Fun :: fun((A, B) -> boolean()),
      List1 :: [A],
      List2 :: [B],
      List3 :: [(A | B)],
      A :: term(),
      B :: term().

umerge(Fun, [], T2) when is_function(Fun, 2) ->
    T2;
umerge(Fun, [H1 | T1], T2) when is_function(Fun, 2) ->
    lists:reverse(ufmerge2_2(H1, T1, Fun, T2, []), []).

%% reverse(rumerge(F,reverse(A),reverse(B))) is equal to umerge(F,A,B).

-spec rumerge(fun((X, Y) -> boolean()), [X], [Y]) -> [(X | Y)].

rumerge(Fun, T1, []) when is_function(Fun, 2) ->
    T1;
rumerge(Fun, T1, [H2 | T2]) when is_function(Fun, 2) ->
    lists:reverse(rufmerge2_1(T1, H2, Fun, T2, []), []).

%% usort(List) -> L
%%  sorts the list L, removes duplicates

-spec usort(List1) -> List2 when
      List1 :: [T],
      List2 :: [T],
      T :: term().

usort([X, Y | L] = L0) when X < Y ->
    case L of
	[] ->
	    L0;
	[Z] when Y < Z ->
	    L0;
	[Z] when Y == Z ->
	    [X, Y];
	[Z] when Z < X ->
	    [Z, X, Y];
	[Z] when Z == X ->
	    [X, Y];
	[Z] ->
	    [X, Z, Y];
	_ ->
	    usplit_1(X, Y, L, [], [])
    end;
usort([X, Y | L]) when X > Y ->
    case L of
	[] ->
	    [Y, X];
	[Z] when X < Z ->
	    [Y, X | L];
	[Z] when X == Z ->
	    [Y, X];
	[Z] when Z < Y ->
	    [Z, Y, X];
	[Z] when Z == Y ->
	    [Y, X];
	[Z] ->
	    [Y, Z, X];
        _ ->
            usplit_2(X, Y, L, [], [])
    end;
usort([X, _Y | L]) ->
    usort_1(X, L);
usort([_] = L) ->
    L;
usort([]) ->
    [].

usort_1(X, [Y | L]) when X == Y ->
    usort_1(X, L);
usort_1(X, [Y | L]) when X < Y ->
    usplit_1(X, Y, L, [], []);
usort_1(X, [Y | L]) ->
    usplit_2(X, Y, L, [], []);
usort_1(X, []) ->
    [X].

%% umerge(List) -> L
%%  merges a list of sorted lists without duplicates, removes duplicates

-spec umerge(ListOfLists) -> List1 when
      ListOfLists :: [List],
      List :: [T],
      List1 :: [T],
      T :: term().

umerge(L) ->
    umergel(L).

%% umerge3(X, Y, Z) -> L
%%  merges three sorted lists X, Y and Z without duplicates, 
%%  removes duplicates

-spec umerge3(List1, List2, List3) -> List4 when
      List1 :: [X],
      List2 :: [Y],
      List3 :: [Z],
      List4 :: [(X | Y | Z)],
      X :: term(),
      Y :: term(),
      Z :: term().

umerge3(L1, [], L3) ->
   umerge(L1, L3);
umerge3(L1, L2, []) ->
   umerge(L1, L2);
umerge3(L1, [H2 | T2], [H3 | T3]) ->
   lists:reverse(umerge3_1(L1, [H2 | H3], T2, H2, [], T3, H3), []).

%% rumerge3(X, Y, Z) -> L
%%  merges three reversed sorted lists X, Y and Z without duplicates,
%%  removes duplicates

-spec rumerge3([X], [Y], [Z]) -> [(X | Y | Z)].

rumerge3(L1, [], L3) ->
   rumerge(L1, L3);
rumerge3(L1, L2, []) ->
   rumerge(L1, L2);
rumerge3(L1, [H2 | T2], [H3 | T3]) ->
   lists:reverse(rumerge3_1(L1, T2, H2, [], T3, H3),[]).

%% umerge(X, Y) -> L
%%  merges two sorted lists X and Y without duplicates, removes duplicates

-spec umerge(List1, List2) -> List3 when
      List1 :: [X],
      List2 :: [Y],
      List3 :: [(X | Y)],
      X :: term(),
      Y :: term().

umerge([], T2) ->
    T2;
umerge([H1 | T1], T2) ->
    lists:reverse(umerge2_2(T1, T2, [], H1), []).

%% rumerge(X, Y) -> L
%%  merges two reversed sorted lists X and Y without duplicates,
%%  removes duplicates

%% reverse(rumerge(reverse(A),reverse(B))) is equal to umerge(I,A,B).

-spec rumerge([X], [Y]) -> [(X | Y)].

rumerge(T1, []) ->
    T1;
rumerge(T1, [H2 | T2]) ->
    lists:reverse(rumerge2_1(T1, T2, [], H2), []).

%% all(Predicate, List)
%% any(Predicate, List)
%% map(Function, List)
%% flatmap(Function, List)
%% foldl(Function, First, List)
%% foldr(Function, Last, List)
%% filter(Predicate, List)
%% zf(Function, List)
%% mapfoldl(Function, First, List)
%% mapfoldr(Function, Last, List)
%% foreach(Function, List)
%% takewhile(Predicate, List)
%% dropwhile(Predicate, List)
%% splitwith(Predicate, List)
%%  for list programming. Function here is a 'fun'. For backward compatibility,
%%  {Module,Function} is still accepted.
%% 
%%  The name zf is a joke!
%%
%%  N.B. Unless where the functions actually needs it only foreach/2/3,
%%  which is meant to be used for its side effects, has a defined order
%%  of evaluation.
%%
%%  There are also versions with an extra argument, ExtraArgs, which is a
%%  list of extra arguments to each call.

-spec all(Pred, List) -> boolean() when
      Pred :: fun((Elem :: T) -> boolean()),
      List :: [T],
      T :: term().

all(Pred, [Hd|Tail]) ->
    case Pred(Hd) of
	true -> all(Pred, Tail);
	false -> false
    end;
all(Pred, []) when is_function(Pred, 1) -> true. 

-spec any(Pred, List) -> boolean() when
      Pred :: fun((Elem :: T) -> boolean()),
      List :: [T],
      T :: term().

any(Pred, [Hd|Tail]) ->
    case Pred(Hd) of
	true -> true;
	false -> any(Pred, Tail)
    end;
any(Pred, []) when is_function(Pred, 1) -> false. 

-spec map(Fun, List1) -> List2 when
      Fun :: fun((A) -> B),
      List1 :: [A],
      List2 :: [B],
      A :: term(),
      B :: term().

map(F, [H|T]) ->
    [F(H)|map(F, T)];
map(F, []) when is_function(F, 1) -> [].

-spec flatmap(Fun, List1) -> List2 when
      Fun :: fun((A) -> [B]),
      List1 :: [A],
      List2 :: [B],
      A :: term(),
      B :: term().

flatmap(F, [Hd|Tail]) ->
    F(Hd) ++ flatmap(F, Tail);
flatmap(F, []) when is_function(F, 1) -> [].

-spec foldl(Fun, Acc0, List) -> Acc1 when
      Fun :: fun((Elem :: T, AccIn) -> AccOut),
      Acc0 :: term(),
      Acc1 :: term(),
      AccIn :: term(),
      AccOut :: term(),
      List :: [T],
      T :: term().

foldl(F, Accu, [Hd|Tail]) ->
    foldl(F, F(Hd, Accu), Tail);
foldl(F, Accu, []) when is_function(F, 2) -> Accu.

-spec foldr(Fun, Acc0, List) -> Acc1 when
      Fun :: fun((Elem :: T, AccIn) -> AccOut),
      Acc0 :: term(),
      Acc1 :: term(),
      AccIn :: term(),
      AccOut :: term(),
      List :: [T],
      T :: term().

foldr(F, Accu, [Hd|Tail]) ->
    F(Hd, foldr(F, Accu, Tail));
foldr(F, Accu, []) when is_function(F, 2) -> Accu.

-spec filter(Pred, List1) -> List2 when
      Pred :: fun((Elem :: T) -> boolean()),
      List1 :: [T],
      List2 :: [T],
      T :: term().

filter(Pred, List) when is_function(Pred, 1) ->
    [ E || E <- List, Pred(E) ].

%% Equivalent to {filter(F, L), filter(NotF, L)}, if NotF = 'fun(X) ->
%% not F(X) end'.

-spec partition(Pred, List) -> {Satisfying, NotSatisfying} when
      Pred :: fun((Elem :: T) -> boolean()),
      List :: [T],
      Satisfying :: [T],
      NotSatisfying :: [T],
      T :: term().

partition(Pred, L) ->
    partition(Pred, L, [], []).

partition(Pred, [H | T], As, Bs) ->
    case Pred(H) of
	true -> partition(Pred, T, [H | As], Bs);
	false -> partition(Pred, T, As, [H | Bs])
    end;
partition(Pred, [], As, Bs) when is_function(Pred, 1) ->
    {reverse(As), reverse(Bs)}.

-spec zf(fun((T) -> boolean() | {'true', X}), [T]) -> [(T | X)].

zf(F, [Hd|Tail]) ->
    case F(Hd) of
	true ->
	    [Hd|zf(F, Tail)];
	{true,Val} ->
	    [Val|zf(F, Tail)];
	false ->
	    zf(F, Tail)
    end;
zf(F, []) when is_function(F, 1) -> [].

-spec foreach(Fun, List) -> ok when
      Fun :: fun((Elem :: T) -> term()),
      List :: [T],
      T :: term().

foreach(F, [Hd|Tail]) ->
    F(Hd),
    foreach(F, Tail);
foreach(F, []) when is_function(F, 1) -> ok.

-spec mapfoldl(Fun, Acc0, List1) -> {List2, Acc1} when
      Fun :: fun((A, AccIn) -> {B, AccOut}),
      Acc0 :: term(),
      Acc1 :: term(),
      AccIn :: term(),
      AccOut :: term(),
      List1 :: [A],
      List2 :: [B],
      A :: term(),
      B :: term().

mapfoldl(F, Accu0, [Hd|Tail]) ->
    {R,Accu1} = F(Hd, Accu0),
    {Rs,Accu2} = mapfoldl(F, Accu1, Tail),
    {[R|Rs],Accu2};
mapfoldl(F, Accu, []) when is_function(F, 2) -> {[],Accu}.

-spec mapfoldr(Fun, Acc0, List1) -> {List2, Acc1} when
      Fun :: fun((A, AccIn) -> {B, AccOut}),
      Acc0 :: term(),
      Acc1 :: term(),
      AccIn :: term(),
      AccOut :: term(),
      List1 :: [A],
      List2 :: [B],
      A :: term(),
      B :: term().

mapfoldr(F, Accu0, [Hd|Tail]) ->
    {Rs,Accu1} = mapfoldr(F, Accu0, Tail),
    {R,Accu2} = F(Hd, Accu1),
    {[R|Rs],Accu2};
mapfoldr(F, Accu, []) when is_function(F, 2) -> {[],Accu}.

-spec takewhile(Pred, List1) -> List2 when
      Pred :: fun((Elem :: T) -> boolean()),
      List1 :: [T],
      List2 :: [T],
      T :: term().

takewhile(Pred, [Hd|Tail]) ->
    case Pred(Hd) of
	true -> [Hd|takewhile(Pred, Tail)];
	false -> []
    end;
takewhile(Pred, []) when is_function(Pred, 1) -> [].

-spec dropwhile(Pred, List1) -> List2 when
      Pred :: fun((Elem :: T) -> boolean()),
      List1 :: [T],
      List2 :: [T],
      T :: term().

dropwhile(Pred, [Hd|Tail]=Rest) ->
    case Pred(Hd) of
	true -> dropwhile(Pred, Tail);
	false -> Rest
    end;
dropwhile(Pred, []) when is_function(Pred, 1) -> [].

-spec splitwith(Pred, List) -> {List1, List2} when
      Pred :: fun((T) -> boolean()),
      List :: [T],
      List1 :: [T],
      List2 :: [T],
      T :: term().

splitwith(Pred, List) when is_function(Pred, 1) ->
    splitwith(Pred, List, []).

splitwith(Pred, [Hd|Tail], Taken) ->
    case Pred(Hd) of
	true -> splitwith(Pred, Tail, [Hd|Taken]);
	false -> {reverse(Taken), [Hd|Tail]}
    end;
splitwith(Pred, [], Taken) when is_function(Pred, 1) ->
    {reverse(Taken),[]}.

-spec split(N, List1) -> {List2, List3} when
      N :: non_neg_integer(),
      List1 :: [T],
      List2 :: [T],
      List3 :: [T],
      T :: term().

split(N, List) when is_integer(N), N >= 0, is_list(List) ->
    case split(N, List, []) of
	{_, _} = Result -> Result;
	Fault when is_atom(Fault) ->
	    erlang:error(Fault, [N,List])
    end;
split(N, List) ->
    erlang:error(badarg, [N,List]).

split(0, L, R) ->
    {lists:reverse(R, []), L};
split(N, [H|T], R) ->
    split(N-1, T, [H|R]);
split(_, [], _) ->
    badarg.

%%% =================================================================
%%% Here follows the implementation of the sort functions.
%%%
%%% These functions used to be in their own module (lists_sort),
%%% but have now been placed here to allow Dialyzer to produce better
%%% type information.
%%% =================================================================

-compile({inline, 
          [{merge3_12,7}, {merge3_21,7}, {rmerge3_12,7}, {rmerge3_21,7}]}).

-compile({inline, 
          [{umerge3_12,8}, {umerge3_21,8},
	   {rumerge3_12a,7}, {rumerge3_12b,8}]}).

-compile({inline, 
          [{keymerge3_12,12}, {keymerge3_21,12}, 
           {rkeymerge3_12,12}, {rkeymerge3_21,12}]}).

-compile({inline,
          [{ukeymerge3_12,13}, {ukeymerge3_21,13},
	   {rukeymerge3_12a,11}, {rukeymerge3_21a,13},
	   {rukeymerge3_12b,12}, {rukeymerge3_21b,12}]}).

%% sort/1

%% Ascending.
split_1(X, Y, [Z | L], R, Rs) when Z >= Y ->
    split_1(Y, Z, L, [X | R], Rs);
split_1(X, Y, [Z | L], R, Rs) when Z >= X ->
    split_1(Z, Y, L, [X | R], Rs);
split_1(X, Y, [Z | L], [], Rs) ->
    split_1(X, Y, L, [Z], Rs);
split_1(X, Y, [Z | L], R, Rs) ->
    split_1_1(X, Y, L, R, Rs, Z);
split_1(X, Y, [], R, Rs) ->
    rmergel([[Y, X | R] | Rs], []).

split_1_1(X, Y, [Z | L], R, Rs, S) when Z >= Y ->
    split_1_1(Y, Z, L, [X | R], Rs, S);
split_1_1(X, Y, [Z | L], R, Rs, S) when Z >= X ->
    split_1_1(Z, Y, L, [X | R], Rs, S);
split_1_1(X, Y, [Z | L], R, Rs, S) when S =< Z ->
    split_1(S, Z, L, [], [[Y, X | R] | Rs]);
split_1_1(X, Y, [Z | L], R, Rs, S) ->
    split_1(Z, S, L, [], [[Y, X | R] | Rs]);
split_1_1(X, Y, [], R, Rs, S) ->
    rmergel([[S], [Y, X | R] | Rs], []).

%% Descending.
split_2(X, Y, [Z | L], R, Rs) when Z =< Y ->
    split_2(Y, Z, L, [X | R], Rs);
split_2(X, Y, [Z | L], R, Rs) when Z =< X ->
    split_2(Z, Y, L, [X | R], Rs);
split_2(X, Y, [Z | L], [], Rs) ->
    split_2(X, Y, L, [Z], Rs);
split_2(X, Y, [Z | L], R, Rs) ->
    split_2_1(X, Y, L, R, Rs, Z);
split_2(X, Y, [], R, Rs) ->
    mergel([[Y, X | R] | Rs], []).

split_2_1(X, Y, [Z | L], R, Rs, S) when Z =< Y ->
    split_2_1(Y, Z, L, [X | R], Rs, S);
split_2_1(X, Y, [Z | L], R, Rs, S) when Z =< X ->
    split_2_1(Z, Y, L, [X | R], Rs, S);
split_2_1(X, Y, [Z | L], R, Rs, S) when S > Z ->
    split_2(S, Z, L, [], [[Y, X | R] | Rs]);
split_2_1(X, Y, [Z | L], R, Rs, S) ->
    split_2(Z, S, L, [], [[Y, X | R] | Rs]);
split_2_1(X, Y, [], R, Rs, S) ->
    mergel([[S], [Y, X | R] | Rs], []).

%% merge/1

mergel([[] | L], Acc) ->
    mergel(L, Acc);
mergel([T1, [H2 | T2], [H3 | T3] | L], Acc) ->
    mergel(L, [merge3_1(T1, [], H2, T2, H3, T3) | Acc]);
mergel([T1, [H2 | T2]], Acc) ->
    rmergel([merge2_1(T1, H2, T2, []) | Acc], []);
mergel([L], []) ->
    L;
mergel([L], Acc) ->
    rmergel([lists:reverse(L, []) | Acc], []);
mergel([], []) ->
    [];
mergel([], Acc) ->
    rmergel(Acc, []);
mergel([A, [] | L], Acc) ->
    mergel([A | L], Acc);
mergel([A, B, [] | L], Acc) ->
    mergel([A, B | L], Acc).

rmergel([[H3 | T3], [H2 | T2], T1 | L], Acc) ->
    rmergel(L, [rmerge3_1(T1, [], H2, T2, H3, T3) | Acc]);
rmergel([[H2 | T2], T1], Acc) ->
    mergel([rmerge2_1(T1, H2, T2, []) | Acc], []);
rmergel([L], Acc) ->
    mergel([lists:reverse(L, []) | Acc], []);
rmergel([], Acc) ->
    mergel(Acc, []).

%% merge3/3

%% Take L1 apart.
merge3_1([H1 | T1], M, H2, T2, H3, T3) when H1 =< H2 ->
    merge3_12(T1, H1, H2, T2, H3, T3, M);
merge3_1([H1 | T1], M, H2, T2, H3, T3) ->
    merge3_21(T1, H1, H2, T2, H3, T3, M);
merge3_1([], M, H2, T2, H3, T3) when H2 =< H3 ->
    merge2_1(T2, H3, T3, [H2 | M]);
merge3_1([], M, H2, T2, H3, T3) ->
    merge2_2(T2, H3, T3, M, H2).

%% Take L2 apart.
merge3_2(T1, H1, M, [H2 | T2], H3, T3) when H1 =< H2 ->
    merge3_12(T1, H1, H2, T2, H3, T3, M);
merge3_2(T1, H1, M, [H2 | T2], H3, T3) ->
    merge3_21(T1, H1, H2, T2, H3, T3, M);
merge3_2(T1, H1, M, [], H3, T3) when H1 =< H3 ->
    merge2_1(T1, H3, T3, [H1 | M]);
merge3_2(T1, H1, M, [], H3, T3) ->
    merge2_2(T1, H3, T3, M, H1).

% H1 =< H2. Inlined.
merge3_12(T1, H1, H2, T2, H3, T3, M) when H1 =< H3 ->
    merge3_1(T1, [H1 | M], H2, T2, H3, T3);
merge3_12(T1, H1, H2, T2, H3, T3, M) ->
    merge3_12_3(T1, H1, H2, T2, [H3 | M], T3).

% H1 =< H2, take L3 apart.
merge3_12_3(T1, H1, H2, T2, M, [H3 | T3]) when H1 =< H3 ->
    merge3_1(T1, [H1 | M], H2, T2, H3, T3);
merge3_12_3(T1, H1, H2, T2, M, [H3 | T3]) ->
    merge3_12_3(T1, H1, H2, T2, [H3 | M], T3);
merge3_12_3(T1, H1, H2, T2, M, []) ->
    merge2_1(T1, H2, T2, [H1 | M]).

% H1 > H2. Inlined.
merge3_21(T1, H1, H2, T2, H3, T3, M) when H2 =< H3 ->
    merge3_2(T1, H1, [H2 | M], T2, H3, T3);
merge3_21(T1, H1, H2, T2, H3, T3, M) ->
    merge3_21_3(T1, H1, H2, T2, [H3 | M], T3).

% H1 > H2, take L3 apart.
merge3_21_3(T1, H1, H2, T2, M, [H3 | T3]) when H2 =< H3 ->
    merge3_2(T1, H1, [H2 | M], T2, H3, T3);
merge3_21_3(T1, H1, H2, T2, M, [H3 | T3]) ->
    merge3_21_3(T1, H1, H2, T2, [H3 | M], T3);
merge3_21_3(T1, H1, H2, T2, M, []) ->
    merge2_2(T1, H2, T2, M, H1).

%% rmerge/3

%% Take L1 apart.
rmerge3_1([H1 | T1], M, H2, T2, H3, T3) when H1 =< H2 ->
    rmerge3_12(T1, H1, H2, T2, H3, T3, M);
rmerge3_1([H1 | T1], M, H2, T2, H3, T3) ->
    rmerge3_21(T1, H1, H2, T2, H3, T3, M);
rmerge3_1([], M, H2, T2, H3, T3) when H2 =< H3 ->
    rmerge2_2(T2, H3, T3, M, H2);
rmerge3_1([], M, H2, T2, H3, T3) ->
    rmerge2_1(T2, H3, T3, [H2 | M]).

%% Take L2 apart.
rmerge3_2(T1, H1, M, [H2 | T2], H3, T3) when H1 =< H2 ->
    rmerge3_12(T1, H1, H2, T2, H3, T3, M);
rmerge3_2(T1, H1, M, [H2 | T2], H3, T3) ->
    rmerge3_21(T1, H1, H2, T2, H3, T3, M);
rmerge3_2(T1, H1, M, [], H3, T3) when H1 =< H3 ->
    rmerge2_2(T1, H3, T3, M, H1);
rmerge3_2(T1, H1, M, [], H3, T3) ->
    rmerge2_1(T1, H3, T3, [H1 | M]).

% H1 =< H2. Inlined.
rmerge3_12(T1, H1, H2, T2, H3, T3, M) when H2 =< H3 ->
    rmerge3_12_3(T1, H1, H2, T2, [H3 | M], T3);
rmerge3_12(T1, H1, H2, T2, H3, T3, M) ->
    rmerge3_2(T1, H1, [H2 | M], T2, H3, T3).

% H1 =< H2, take L3 apart.
rmerge3_12_3(T1, H1, H2, T2, M, [H3 | T3]) when H2 =< H3 ->
    rmerge3_12_3(T1, H1, H2, T2, [H3 | M], T3);
rmerge3_12_3(T1, H1, H2, T2, M, [H3 | T3]) ->
    rmerge3_2(T1, H1, [H2 | M], T2, H3, T3);
rmerge3_12_3(T1, H1, H2, T2, M, []) ->
    rmerge2_2(T1, H2, T2, M, H1).

% H1 > H2. Inlined.
rmerge3_21(T1, H1, H2, T2, H3, T3, M) when H1 =< H3 ->
    rmerge3_21_3(T1, H1, H2, T2, [H3 | M], T3);
rmerge3_21(T1, H1, H2, T2, H3, T3, M) ->
    rmerge3_1(T1, [H1 | M], H2, T2, H3, T3).

% H1 > H2, take L3 apart.
rmerge3_21_3(T1, H1, H2, T2, M, [H3 | T3]) when H1 =< H3 ->
    rmerge3_21_3(T1, H1, H2, T2, [H3 | M], T3);
rmerge3_21_3(T1, H1, H2, T2, M, [H3 | T3]) ->
    rmerge3_1(T1, [H1 | M], H2, T2, H3, T3);
rmerge3_21_3(T1, H1, H2, T2, M, []) ->
    rmerge2_1(T1, H2, T2, [H1 | M]).

%% merge/2

merge2_1([H1 | T1], H2, T2, M) when H1 =< H2 ->
    merge2_1(T1, H2, T2, [H1 | M]);
merge2_1([H1 | T1], H2, T2, M) ->
    merge2_2(T1, H2, T2, M, H1);
merge2_1([], H2, T2, M) ->
    lists:reverse(T2, [H2 | M]).

merge2_2(T1, HdM, [H2 | T2], M, H1) when H1 =< H2 ->
    merge2_1(T1, H2, T2, [H1, HdM | M]);
merge2_2(T1, HdM, [H2 | T2], M, H1) ->
    merge2_2(T1, H2, T2, [HdM | M], H1);
merge2_2(T1, HdM, [], M, H1) ->
    lists:reverse(T1, [H1, HdM | M]).

%% rmerge/2

rmerge2_1([H1 | T1], H2, T2, M) when H1 =< H2 ->
    rmerge2_2(T1, H2, T2, M, H1);
rmerge2_1([H1 | T1], H2, T2, M) ->
    rmerge2_1(T1, H2, T2, [H1 | M]);
rmerge2_1([], H2, T2, M) ->
    lists:reverse(T2, [H2 | M]).

rmerge2_2(T1, HdM, [H2 | T2], M, H1) when H1 =< H2 ->
    rmerge2_2(T1, H2, T2, [HdM | M], H1);
rmerge2_2(T1, HdM, [H2 | T2], M, H1) ->
    rmerge2_1(T1, H2, T2, [H1, HdM | M]);
rmerge2_2(T1, HdM, [], M, H1) ->
    lists:reverse(T1, [H1, HdM | M]).

%% usort/1

%% Ascending.
usplit_1(X, Y, [Z | L], R, Rs) when Z > Y ->
    usplit_1(Y, Z, L, [X | R], Rs);
usplit_1(X, Y, [Z | L], R, Rs) when Z == Y ->
    usplit_1(X, Y, L, R, Rs);
usplit_1(X, Y, [Z | L], R, Rs) when Z > X ->
    usplit_1(Z, Y, L, [X | R], Rs);
usplit_1(X, Y, [Z | L], R, Rs) when Z == X ->
    usplit_1(X, Y, L, R, Rs);
usplit_1(X, Y, [Z | L], [], Rs) ->
    usplit_1(X, Y, L, [Z], Rs);
usplit_1(X, Y, [Z | L], R, Rs) ->
    usplit_1_1(X, Y, L, R, Rs, Z);
usplit_1(X, Y, [], R, Rs) ->
    rumergel([[Y, X | R] | Rs], [], asc).

usplit_1_1(X, Y, [Z | L], R, Rs, S) when Z > Y ->
    usplit_1_1(Y, Z, L, [X | R], Rs, S);
usplit_1_1(X, Y, [Z | L], R, Rs, S) when Z == Y ->
    usplit_1_1(X, Y, L, R, Rs, S);
usplit_1_1(X, Y, [Z | L], R, Rs, S) when Z > X ->
    usplit_1_1(Z, Y, L, [X | R], Rs, S);
usplit_1_1(X, Y, [Z | L], R, Rs, S) when Z == X ->
    usplit_1_1(X, Y, L, R, Rs, S);
usplit_1_1(X, Y, [Z | L], R, Rs, S) when Z > S ->
    usplit_1(S, Z, L, [], [[Y, X | R] | Rs]);
usplit_1_1(X, Y, [Z | L], R, Rs, S) when Z == S ->
    usplit_1_1(X, Y, L, R, Rs, S);
usplit_1_1(X, Y, [Z | L], R, Rs, S) ->
    usplit_1(Z, S, L, [], [[Y, X | R] | Rs]);
usplit_1_1(X, Y, [], R, Rs, S) ->
    rumergel([[S], [Y, X | R] | Rs], [], asc).

%% Descending.
usplit_2(X, Y, [Z | L], R, Rs) when Z < Y ->
    usplit_2(Y, Z, L, [X | R], Rs);
usplit_2(X, Y, [Z | L], R, Rs) when Z == Y ->
    usplit_2(X, Y, L, R, Rs);
usplit_2(X, Y, [Z | L], R, Rs) when Z < X ->
    usplit_2(Z, Y, L, [X | R], Rs);
usplit_2(X, Y, [Z | L], R, Rs) when Z == X ->
    usplit_2(X, Y, L, R, Rs);
usplit_2(X, Y, [Z | L], [], Rs) ->
    usplit_2(X, Y, L, [Z], Rs);
usplit_2(X, Y, [Z | L], R, Rs) ->
    usplit_2_1(X, Y, L, R, Rs, Z);
usplit_2(X, Y, [], R, Rs) ->
    umergel([[Y, X | R] | Rs], [], desc).

usplit_2_1(X, Y, [Z | L], R, Rs, S) when Z < Y ->
    usplit_2_1(Y, Z, L, [X | R], Rs, S);
usplit_2_1(X, Y, [Z | L], R, Rs, S) when Z == Y ->
    usplit_2_1(X, Y, L, R, Rs, S);
usplit_2_1(X, Y, [Z | L], R, Rs, S) when Z < X ->
    usplit_2_1(Z, Y, L, [X | R], Rs, S);
usplit_2_1(X, Y, [Z | L], R, Rs, S) when Z == X ->
    usplit_2_1(X, Y, L, R, Rs, S);
usplit_2_1(X, Y, [Z | L], R, Rs, S) when Z < S ->
    usplit_2(S, Z, L, [], [[Y, X | R] | Rs]);
usplit_2_1(X, Y, [Z | L], R, Rs, S) when Z == S ->
    usplit_2_1(X, Y, L, R, Rs, S);
usplit_2_1(X, Y, [Z | L], R, Rs, S) ->
    usplit_2(Z, S, L, [], [[Y, X | R] | Rs]);
usplit_2_1(X, Y, [], R, Rs, S) ->
    umergel([[S], [Y, X | R] | Rs], [], desc).

%% umerge/1

umergel(L) ->
    umergel(L, [], asc).

umergel([[] | L], Acc, O) ->
    umergel(L, Acc, O);
umergel([T1, [H2 | T2], [H3 | T3] | L], Acc, asc) ->
    umergel(L, [umerge3_1(T1, [H2 | H3], T2, H2, [], T3, H3) | Acc], asc);
umergel([[H3 | T3], [H2 | T2], T1 | L], Acc, desc) ->
    umergel(L, [umerge3_1(T1, [H2 | H3], T2, H2, [], T3, H3) | Acc], desc);
umergel([A, [] | L], Acc, O) ->
    umergel([A | L], Acc, O);
umergel([A, B, [] | L], Acc, O) ->
    umergel([A, B | L], Acc, O);
umergel([[H1 | T1], T2 | L], Acc, asc) ->
    umergel(L, [umerge2_2(T1, T2, [], H1) | Acc], asc);
umergel([T2, [H1 | T1] | L], Acc, desc) ->
    umergel(L, [umerge2_2(T1, T2, [], H1) | Acc], desc);
umergel([L], [], _O) ->
    L;
umergel([L], Acc, O) ->
    rumergel([lists:reverse(L, []) | Acc], [], O);
umergel([], [], _O) ->
    [];
umergel([], Acc, O) ->
    rumergel(Acc, [], O).

rumergel([[H3 | T3], [H2 | T2], T1 | L], Acc, asc) ->
    rumergel(L, [rumerge3_1(T1, T2, H2, [], T3, H3) | Acc], asc);
rumergel([T1, [H2 | T2], [H3 | T3] | L], Acc, desc) ->
    rumergel(L, [rumerge3_1(T1, T2, H2, [], T3, H3) | Acc], desc);
rumergel([[H2 | T2], T1 | L], Acc, asc) ->
    rumergel(L, [rumerge2_1(T1, T2, [], H2) | Acc], asc);
rumergel([T1, [H2 | T2] | L], Acc, desc) ->
    rumergel(L, [rumerge2_1(T1, T2, [], H2) | Acc], desc);
rumergel([L], Acc, O) ->
    umergel([lists:reverse(L, []) | Acc], [], O);
rumergel([], Acc, O) ->
    umergel(Acc, [], O).

%% umerge3/3

%% Take L1 apart.
umerge3_1([H1 | T1], HdM, T2, H2, M, T3, H3) when H1 =< H2 ->
    umerge3_12(T1, H1, T2, H2, M, T3, H3, HdM);
umerge3_1([H1 | T1], HdM, T2, H2, M, T3, H3) when H2 == HdM ->
    umerge3_2(T1, H1, T2, H2, M, T3, H3);
umerge3_1([H1 | T1], HdM, T2, H2, M, T3, H3) ->
    umerge3_21(T1, H1, T2, H2, M, T3, H3, HdM);
umerge3_1([], HdM, T2, H2, M, T3, H3) when H2 == HdM ->
    umerge2_1(T2, T3, M, HdM, H3);
umerge3_1([], _HdM, T2, H2, M, T3, H3) when H2 =< H3 ->
    umerge2_1(T2, T3, [H2 | M], H2, H3);
umerge3_1([], HdM, T2, H2, M, T3, H3) when H3 == HdM ->
    umerge2_2(T2, T3, M, H2);
umerge3_1([], _HdM, T2, H2, M, T3, H3) ->
    umerge2_2(T2, T3, [H3 | M], H2).

%% Take L2 apart.
umerge3_2(T1, H1, [H2 | T2], HdM, M, T3, H3) when H1 =< H2 ->
    umerge3_12(T1, H1, T2, H2, M, T3, H3, HdM);
umerge3_2(T1, H1, [H2 | T2], HdM, M, T3, H3) ->
    umerge3_21(T1, H1, T2, H2, M, T3, H3, HdM);
umerge3_2(T1, H1, [], _HdM, M, T3, H3) when H1 =< H3 ->
    umerge2_1(T1, T3, [H1 | M], H1, H3);
umerge3_2(T1, H1, [], HdM, M, T3, H3) when H3 == HdM ->
    umerge2_2(T1, T3, M, H1);
umerge3_2(T1, H1, [], _HdM, M, T3, H3) ->
    umerge2_2(T1, T3, [H3 | M], H1).

% H1 =< H2. Inlined.
umerge3_12(T1, H1, T2, H2, M, T3, H3, _HdM) when H1 =< H3 ->
    umerge3_1(T1, H1, T2, H2, [H1 | M], T3, H3);
umerge3_12(T1, H1, T2, H2, M, T3, H3, HdM) when H3 == HdM ->
    umerge3_12_3(T1, H1, T2, H2, M, T3);
umerge3_12(T1, H1, T2, H2, M, T3, H3, _HdM) ->
    umerge3_12_3(T1, H1, T2, H2, [H3 | M], T3).

% H1 =< H2, take L3 apart.
umerge3_12_3(T1, H1, T2, H2, M, [H3 | T3]) when H1 =< H3 ->
    umerge3_1(T1, H1, T2, H2, [H1 | M], T3, H3);
umerge3_12_3(T1, H1, T2, H2, M, [H3 | T3]) ->
    umerge3_12_3(T1, H1, T2, H2, [H3 | M], T3);
umerge3_12_3(T1, H1, T2, H2, M, []) ->
    umerge2_1(T1, T2, [H1 | M], H1, H2).

% H1 > H2. Inlined.
umerge3_21(T1, H1, T2, H2, M, T3, H3, _HdM) when H2 =< H3 ->
    umerge3_2(T1, H1, T2, H2, [H2 | M], T3, H3);
umerge3_21(T1, H1, T2, H2, M, T3, H3, HdM) when H3 == HdM ->
    umerge3_21_3(T1, H1, T2, H2, M, T3);
umerge3_21(T1, H1, T2, H2, M, T3, H3, _HdM) ->
    umerge3_21_3(T1, H1, T2, H2, [H3 | M], T3).

% H1 > H2, take L3 apart.
umerge3_21_3(T1, H1, T2, H2, M, [H3 | T3]) when H2 =< H3 ->
    umerge3_2(T1, H1, T2, H2, [H2 | M], T3, H3);
umerge3_21_3(T1, H1, T2, H2, M, [H3 | T3]) ->
    umerge3_21_3(T1, H1, T2, H2, [H3 | M], T3);
umerge3_21_3(T1, H1, T2, H2, M, []) ->
    umerge2_2(T1, T2, [H2 | M], H1).

%% Take L1 apart.
rumerge3_1([H1 | T1], T2, H2, M, T3, H3) when H1 =< H2 ->
    rumerge3_12a(T1, H1, T2, H2, M, T3, H3);
rumerge3_1([H1 | T1], T2, H2, M, T3, H3) when H1 =< H3 ->
    rumerge3_21_3(T1, T2, H2, M, T3, H3, H1);
rumerge3_1([H1 | T1], T2, H2, M, T3, H3) ->
    rumerge3_1(T1, T2, H2, [H1 | M], T3, H3);
rumerge3_1([], T2, H2, M, T3, H3) when H2 =< H3 ->
    rumerge2_2(T2, T3, M, H3, H2);
rumerge3_1([], T2, H2, M, T3, H3) ->
    rumerge2_1(T2, T3, [H2 | M], H3).

% H1 =< H2. Inlined.
rumerge3_12a(T1, H1, T2, H2, M, T3, H3) when H2 =< H3 ->
    rumerge3_12_3(T1, T2, H2, M, T3, H3, H1);
rumerge3_12a(T1, H1, T2, H2, M, T3, H3) ->
    rumerge3_2(T1, T2, H2, M, T3, H3, H1).

%% Take L2 apart. H2M > H3. H2M > H2.
rumerge3_2(T1, [H2 | T2], H2M, M, T3, H3, H1) when H1 =< H2 ->
    % H2M > H1.
    rumerge3_12b(T1, H1, T2, H2, M, T3, H3, H2M);
rumerge3_2(T1, [H2 | T2], H2M, M, T3, H3, H1) when H1 == H2M ->
    rumerge3_1(T1, T2, H2, [H1 | M], T3, H3);
rumerge3_2(T1, [H2 | T2], H2M, M, T3, H3, H1) when H1 =< H3 ->
    % H2M > H1.
    rumerge3_21_3(T1, T2, H2, [H2M | M], T3, H3, H1);
rumerge3_2(T1, [H2 | T2], H2M, M, T3, H3, H1) ->
    % H2M > H1.
    rumerge3_1(T1, T2, H2, [H1, H2M | M], T3, H3);
rumerge3_2(T1, [], H2M, M, T3, H3, H1) when H1 == H2M ->
    rumerge2_1(T1, T3, [H1 | M], H3);
rumerge3_2(T1, [], H2M, M, T3, H3, H1) when H1 =< H3 ->
    rumerge2_2(T1, T3, [H2M | M], H3, H1);
rumerge3_2(T1, [], H2M, M, T3, H3, H1) ->
    rumerge2_1(T1, T3, [H1, H2M | M], H3).

% H1 =< H2. Inlined.
rumerge3_12b(T1, H1, T2, H2, M, T3, H3, H2M) when H2 =< H3 ->
    rumerge3_12_3(T1, T2, H2, [H2M | M], T3, H3, H1);
rumerge3_12b(T1, H1, T2, H2, M, T3, H3, H2M) ->
    rumerge3_2(T1, T2, H2, [H2M | M], T3, H3, H1).

% H1 =< H2, take L3 apart.
rumerge3_12_3(T1, T2, H2, M, [H3 | T3], H3M, H1) when H2 =< H3 ->
    rumerge3_12_3(T1, T2, H2, [H3M | M], T3, H3, H1);
rumerge3_12_3(T1, T2, H2, M, [H3 | T3], H3M, H1) when H2 == H3M ->
    rumerge3_2(T1, T2, H2, M, T3, H3, H1);
rumerge3_12_3(T1, T2, H2, M, [H3 | T3], H3M, H1) ->
    rumerge3_2(T1, T2, H2, [H3M | M], T3, H3, H1);
rumerge3_12_3(T1, T2, H2, M, [], H3M, H1) when H2 == H3M ->
    rumerge2_2(T1, T2, M, H2, H1);
rumerge3_12_3(T1, T2, H2, M, [], H3M, H1) ->
    rumerge2_2(T1, T2, [H3M | M], H2, H1).

% H1 > H2, take L3 apart.
rumerge3_21_3(T1, T2, H2, M, [H3 | T3], H3M, H1) when H1 =< H3 ->
    rumerge3_21_3(T1, T2, H2, [H3M | M], T3, H3, H1);
rumerge3_21_3(T1, T2, H2, M, [H3 | T3], H3M, H1) when H1 == H3M ->
    rumerge3_1(T1, T2, H2, [H1 | M], T3, H3);
rumerge3_21_3(T1, T2, H2, M, [H3 | T3], H3M, H1) ->
    rumerge3_1(T1, T2, H2, [H1, H3M | M], T3, H3);
rumerge3_21_3(T1, T2, H2, M, [], H3M, H1) when H1 == H3M ->
    rumerge2_1(T1, T2, [H1 | M], H2);
rumerge3_21_3(T1, T2, H2, M, [], H3M, H1) ->
    rumerge2_1(T1, T2, [H1, H3M | M], H2).

%% umerge/2

%% Elements from the first list are kept and prioritized.
umerge2_1([H1 | T1], T2, M, _HdM, H2) when H1 =< H2 ->
    umerge2_1(T1, T2, [H1 | M], H1, H2);
umerge2_1([H1 | T1], T2, M, HdM, H2) when H2 == HdM ->
    umerge2_2(T1, T2, M, H1);
umerge2_1([H1 | T1], T2, M, _HdM, H2) ->
    umerge2_2(T1, T2, [H2 | M], H1);
umerge2_1([], T2, M, HdM, H2) when H2 == HdM ->
    lists:reverse(T2, M);
umerge2_1([], T2, M, _HdM, H2) ->
    lists:reverse(T2, [H2 | M]).

umerge2_2(T1, [H2 | T2], M, H1) when H1 =< H2 ->
    umerge2_1(T1, T2, [H1 | M], H1, H2);
umerge2_2(T1, [H2 | T2], M, H1) ->
    umerge2_2(T1, T2, [H2 | M], H1);
umerge2_2(T1, [], M, H1) ->
    lists:reverse(T1, [H1 | M]).

%% rumerge/2

%% Elements from the first list are kept and prioritized.
rumerge2_1([H1 | T1], T2, M, H2) when H1 =< H2 ->
    rumerge2_2(T1, T2, M, H2, H1);
rumerge2_1([H1 | T1], T2, M, H2) ->
    rumerge2_1(T1, T2, [H1 | M], H2);
rumerge2_1([], T2, M, H2) ->
    lists:reverse(T2, [H2 | M]).

% H1 =< H2M.
rumerge2_2(T1, [H2 | T2], M, H2M, H1) when H1 =< H2 ->
    rumerge2_2(T1, T2, [H2M | M], H2, H1);
rumerge2_2(T1, [H2 | T2], M, H2M, H1) when H1 == H2M ->
    rumerge2_1(T1, T2, [H1 | M], H2);
rumerge2_2(T1, [H2 | T2], M, H2M, H1) ->
    rumerge2_1(T1, T2, [H1, H2M | M], H2);
rumerge2_2(T1, [], M, H2M, H1) when H1 == H2M ->
    lists:reverse(T1, [H1 | M]);
rumerge2_2(T1, [], M, H2M, H1) ->
    lists:reverse(T1, [H1, H2M | M]).

%% keysort/2

%% Ascending.
keysplit_1(I, X, EX, Y, EY, [Z | L], R, Rs) ->
    case element(I, Z) of
	EZ when EY =< EZ ->
            keysplit_1(I, Y, EY, Z, EZ, L, [X | R], Rs);
        EZ when EX =< EZ ->
            keysplit_1(I, Z, EZ, Y, EY, L, [X | R], Rs);
        _EZ when R == [] ->
            keysplit_1(I, X, EX, Y, EY, L, [Z], Rs);
        EZ ->
            keysplit_1_1(I, X, EX, Y, EY, EZ, R, Rs, Z, L)
    end;
keysplit_1(I, X, _EX, Y, _EY, [], R, Rs) ->
    rkeymergel(I, [[Y, X | R] | Rs], [], asc).

keysplit_1_1(I, X, EX, Y, EY, ES, R, Rs, S, [Z | L]) ->
    case element(I, Z) of
	EZ when EY =< EZ ->
            keysplit_1_1(I, Y, EY, Z, EZ, ES, [X | R], Rs, S, L);
        EZ when EX =< EZ ->
            keysplit_1_1(I, Z, EZ, Y, EY, ES, [X | R], Rs, S, L);
        EZ when ES =< EZ ->
            keysplit_1(I, S, ES, Z, EZ, L, [], [[Y, X | R] | Rs]);
        EZ ->
            keysplit_1(I, Z, EZ, S, ES, L, [], [[Y, X | R] | Rs])
    end;
keysplit_1_1(I, X, _EX, Y, _EY, _ES, R, Rs, S, []) ->
    rkeymergel(I, [[S], [Y, X | R] | Rs], [], asc).

%% Descending.
keysplit_2(I, X, EX, Y, EY, [Z | L], R, Rs) ->
    case element(I, Z) of
	EZ when EY > EZ ->
            keysplit_2(I, Y, EY, Z, EZ, L, [X | R], Rs);
        EZ when EX > EZ ->
            keysplit_2(I, Z, EZ, Y, EY, L, [X | R], Rs);
        _EZ when R == [] ->
            keysplit_2(I, X, EX, Y, EY, L, [Z], Rs);
        EZ ->
            keysplit_2_1(I, X, EX, Y, EY, EZ, R, Rs, Z, L)
    end;
keysplit_2(I, X, _EX, Y, _EY, [], R, Rs) ->
    keymergel(I, [[Y, X | R] | Rs], [], desc).

keysplit_2_1(I, X, EX, Y, EY, ES, R, Rs, S, [Z | L]) ->
    case element(I, Z) of
        EZ when EY > EZ ->
            keysplit_2_1(I, Y, EY, Z, EZ, ES, [X | R], Rs, S, L);
        EZ when EX > EZ ->
            keysplit_2_1(I, Z, EZ, Y, EY, ES, [X | R], Rs, S, L);
        EZ when ES > EZ ->
            keysplit_2(I, S, ES, Z, EZ, L, [], [[Y, X | R] | Rs]);
        EZ ->
            keysplit_2(I, Z, EZ, S, ES, L, [], [[Y, X | R] | Rs])
    end;
keysplit_2_1(I, X, _EX, Y, _EY, _ES, R, Rs, S, []) ->
    keymergel(I, [[S], [Y, X | R] | Rs], [], desc).

keymergel(I, [T1, [H2 | T2], [H3 | T3] | L], Acc, O) when O == asc ->
    M = keymerge3_1(I, T1, [],O,element(I,H2), H2, T2, element(I,H3), H3, T3),
    keymergel(I, L, [M | Acc], O);
keymergel(I, [[H3 | T3], [H2 | T2], T1 | L], Acc, O) when O == desc ->
    M = keymerge3_1(I, T1, [],O,element(I,H2), H2, T2, element(I,H3), H3, T3),
    keymergel(I, L, [M | Acc], O);
keymergel(I, [T1, [H2 | T2] | L], Acc, asc) ->
    keymergel(I, L, [keymerge2_1(I, T1, element(I,H2),H2,T2,[]) | Acc], asc);
keymergel(I, [[H2 | T2], T1 | L], Acc, desc) ->
    keymergel(I, L, [keymerge2_1(I, T1, element(I,H2),H2,T2,[]) | Acc], desc);
keymergel(_I, [L], [], _O) ->
    L;
keymergel(I, [L], Acc, O) ->
    rkeymergel(I, [lists:reverse(L, []) | Acc], [], O);
keymergel(I, [], Acc, O) ->
    rkeymergel(I, Acc, [], O).

rkeymergel(I, [[H3 | T3], [H2 | T2], T1 | L], Acc, O) when O == asc ->
    M = rkeymerge3_1(I, T1, [],O,element(I,H2), H2, T2, element(I,H3), H3,T3),
    rkeymergel(I, L, [M | Acc], O);
rkeymergel(I, [T1, [H2 | T2], [H3 | T3] | L], Acc, O) when O == desc ->
    M = rkeymerge3_1(I, T1, [],O,element(I,H2), H2, T2, element(I,H3), H3,T3),
    rkeymergel(I, L, [M | Acc], O);
rkeymergel(I, [[H2 | T2], T1 | L], Acc, asc) ->
    rkeymergel(I, L, [rkeymerge2_1(I, T1, element(I,H2),H2,T2,[]) | Acc],asc);
rkeymergel(I, [T1, [H2 | T2] | L], Acc, desc) ->
    rkeymergel(I, L, [rkeymerge2_1(I,T1, element(I,H2),H2,T2,[]) | Acc],desc);
rkeymergel(I, [L], Acc, O) ->
    keymergel(I, [lists:reverse(L, []) | Acc], [], O);
rkeymergel(I, [], Acc, O) ->
    keymergel(I, Acc, [], O).

%%% An extra argument, D, just to avoid some move instructions.

%% Take L1 apart.
keymerge3_1(I, [H1 | T1], M, D, E2, H2, T2, E3, H3, T3) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            keymerge3_12(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D);
        E1 ->
            keymerge3_21(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, T2)
    end;
keymerge3_1(I, [], M, _D, E2, H2, T2, E3, H3, T3) when E2 =< E3 ->
    keymerge2_1(I, T2, E3, H3, T3, [H2 | M]);
keymerge3_1(I, [], M, _D, E2, H2, T2, _E3, H3, T3) ->
    keymerge2_2(I, T2, E2, H3, T3, M, H2).

%% Take L2 apart.
keymerge3_2(I, E1, H1, T1, [H2 | T2], M, D, E3, H3, T3) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            keymerge3_12(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, T1);
        E2 ->
            keymerge3_21(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D)
    end;
keymerge3_2(I, E1, H1, T1, [], M, _D, E3, H3, T3) when E1 =< E3 ->
    keymerge2_1(I, T1, E3, H3, T3, [H1 | M]);
keymerge3_2(I, E1, H1, T1, [], M, _D, _E3, H3, T3) ->
    keymerge2_2(I, T1, E1, H3, T3, M, H1).

% E1 =< E2. Inlined.
keymerge3_12(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D) when E1 =< E3 ->
    keymerge3_1(I, T1, [H1 | M], D, E2, H2, T2, E3, H3, T3);
keymerge3_12(I, E1, H1, T1, E2, H2, T2, _E3, H3, T3, M, _D) ->
    keymerge3_12_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M]).

% E1 =< E2, take L3 apart.
keymerge3_12_3(I, E1, H1, T1, E2, H2, T2, [H3 | T3], M) ->
    case element(I, H3) of
	E3 when E1 =< E3 ->
            keymerge3_1(I, T1, [H1 | M], T1, E2, H2, T2, E3, H3, T3);
        _E3 ->
            keymerge3_12_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M])
    end;
keymerge3_12_3(I, _E1, H1, T1, E2, H2, T2, [], M) ->
    keymerge2_1(I, T1, E2, H2, T2, [H1 | M]).

% E1 > E2. Inlined.
keymerge3_21(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D) when E2 =< E3 ->
    keymerge3_2(I, E1, H1, T1, T2, [H2 | M], D, E3, H3, T3);
keymerge3_21(I, E1, H1, T1, E2, H2, T2, _E3, H3, T3, M, _D) ->
    keymerge3_21_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M]).

% E1 > E2, take L3 apart.
keymerge3_21_3(I, E1, H1, T1, E2, H2, T2, [H3 | T3], M) ->
    case element(I, H3) of
	E3 when E2 =< E3 ->
            keymerge3_2(I, E1, H1, T1, T2, [H2 | M], T2, E3, H3, T3);
        _E3 ->
            keymerge3_21_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M])
    end;
keymerge3_21_3(I, E1, H1, T1, _E2, H2, T2, [], M) ->
    keymerge2_2(I, T1, E1, H2, T2, M, H1).

%% Take L1 apart.
rkeymerge3_1(I, [H1 | T1], M, D, E2, H2, T2, E3, H3, T3) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            rkeymerge3_12(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, T2);
        E1 ->
            rkeymerge3_21(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D)
    end;
rkeymerge3_1(I, [], M, _D, E2, H2, T2, E3, H3, T3) when E2 =< E3 ->
    rkeymerge2_2(I, E2, T2, H3, T3, M, H2);
rkeymerge3_1(I, [], M, _D, _E2, H2, T2, E3, H3, T3) ->
    rkeymerge2_1(I, T2, E3, H3, T3, [H2 | M]).

%% Take L2 apart.
rkeymerge3_2(I, E1, H1, T1, [H2 | T2], M, D, E3, H3, T3) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            rkeymerge3_12(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D);
        E2 ->
            rkeymerge3_21(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, T1)
    end;
rkeymerge3_2(I, E1, H1, T1, [], M, _D, E3, H3, T3) when E1 =< E3 ->
    rkeymerge2_2(I, E1, T1, H3, T3, M, H1);
rkeymerge3_2(I, _E1, H1, T1, [], M, _D, E3, H3, T3) ->
    rkeymerge2_1(I, T1, E3, H3, T3, [H1 | M]).

% E1 =< E2. Inlined.
rkeymerge3_12(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, _D) when E2 =< E3 ->
    rkeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M]);
rkeymerge3_12(I, E1, H1, T1, _E2, H2, T2, E3, H3, T3, M, D) ->
    rkeymerge3_2(I, E1, H1, T1, T2, [H2 | M], D, E3, H3, T3).

% E1 =< E2, take L3 apart.
rkeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, [H3 | T3], M) ->
    case element(I, H3) of
	E3 when E2 =< E3 ->
            rkeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M]);
        E3 ->
            rkeymerge3_2(I, E1, H1, T1, T2, [H2 | M], T2, E3, H3, T3)
    end;
rkeymerge3_12_3(I, E1, H1, T1, _E2, H2, T2, [], M) ->
    rkeymerge2_2(I, E1, T1, H2, T2, M, H1).

% E1 > E2. Inlined.
rkeymerge3_21(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, _D) when E1 =< E3 ->
    rkeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M]);
rkeymerge3_21(I, _E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D) ->
    rkeymerge3_1(I, T1, [H1 | M], D, E2, H2, T2, E3, H3, T3).

% E1 > E2, take L3 apart.
rkeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, [H3 | T3], M) ->
    case element(I, H3) of
	E3 when E1 =< E3 ->
            rkeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, T3, [H3 | M]);
        E3 ->
            rkeymerge3_1(I, T1, [H1 | M], T1, E2, H2, T2, E3, H3, T3)
    end;
rkeymerge3_21_3(I, _E1, H1, T1, E2, H2, T2, [], M) ->
    rkeymerge2_1(I, T1, E2, H2, T2, [H1 | M]).

%% keymerge/3

%% Elements from the first list are prioritized.
keymerge2_1(I, [H1 | T1], E2, H2, T2, M) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            keymerge2_1(I, T1, E2, H2, T2, [H1 | M]);
        E1 ->
            keymerge2_2(I, T1, E1, H2, T2, M, H1)
    end;
keymerge2_1(_I, [], _E2, H2, T2, M) ->
    lists:reverse(T2, [H2 | M]).

keymerge2_2(I, T1, E1, HdM, [H2 | T2], M, H1) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            keymerge2_1(I, T1, E2, H2, T2, [H1, HdM | M]);
        _E2 ->
            keymerge2_2(I, T1, E1, H2, T2, [HdM | M], H1)
    end;
keymerge2_2(_I, T1, _E1, HdM, [], M, H1) ->
    lists:reverse(T1, [H1, HdM | M]).

%% rkeymerge/3

rkeymerge2_1(I, [H1 | T1], E2, H2, T2, M) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            rkeymerge2_2(I, E1, T1, H2, T2, M, H1);
        _E1 ->
            rkeymerge2_1(I, T1, E2, H2, T2, [H1 | M])
    end;
rkeymerge2_1(_I, [], _E2, H2, T2, M) ->
    lists:reverse(T2, [H2 | M]).

rkeymerge2_2(I, E1, T1, HdM, [H2 | T2], M, H1) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            rkeymerge2_2(I, E1, T1, H2, T2, [HdM | M], H1);
        E2 ->
            rkeymerge2_1(I, T1, E2, H2, T2, [H1, HdM | M])
    end;
rkeymerge2_2(_I, _E1, T1, HdM, [], M, H1) ->
    lists:reverse(T1, [H1, HdM | M]).

%% ukeysort/2

%% Ascending.
ukeysplit_1(I, X, EX, Y, EY, [Z | L], R, Rs) ->
    case element(I, Z) of
        EZ when EY == EZ ->
            ukeysplit_1(I, X, EX, Y, EY, L, R, Rs);
	EZ when EY < EZ ->
            ukeysplit_1(I, Y, EY, Z, EZ, L, [X | R], Rs);
        EZ when EX == EZ ->
            ukeysplit_1(I, X, EX, Y, EY, L, R, Rs);
        EZ when EX < EZ ->
            ukeysplit_1(I, Z, EZ, Y, EY, L, [X | R], Rs);
        _EZ when R == [] ->
            ukeysplit_1(I, X, EX, Y, EY, L, [Z], Rs);
        EZ ->
            ukeysplit_1_1(I, X, EX, Y, EY, L, R, Rs, Z, EZ)
    end;
ukeysplit_1(I, X, _EX, Y, _EY, [], R, Rs) ->
    rukeymergel(I, [[Y, X | R] | Rs], []).

ukeysplit_1_1(I, X, EX, Y, EY, [Z | L], R, Rs, S, ES) ->
    case element(I, Z) of
        EZ when EY == EZ ->
            ukeysplit_1_1(I, X, EX, Y, EY, L, R, Rs, S, ES);
        EZ when EY < EZ ->
            ukeysplit_1_1(I, Y, EY, Z, EZ, L, [X | R], Rs, S, ES);
        EZ when EX == EZ ->
            ukeysplit_1_1(I, X, EX, Y, EY, L, R, Rs, S, ES);
        EZ when EX < EZ ->
            ukeysplit_1_1(I, Z, EZ, Y, EY, L, [X | R], Rs, S, ES);
        EZ when ES == EZ ->
            ukeysplit_1_1(I, X, EX, Y, EY, L, R, Rs, S, ES);
        EZ when ES < EZ ->
            ukeysplit_1(I, S, ES, Z, EZ, L, [], [[Y, X | R] | Rs]);
        EZ ->
            ukeysplit_1(I, Z, EZ, S, ES, L, [], [[Y, X | R] | Rs])
    end;
ukeysplit_1_1(I, X, _EX, Y, _EY, [], R, Rs, S, _ES) ->
    rukeymergel(I, [[S], [Y, X | R] | Rs], []).

%% Descending.
ukeysplit_2(I, Y, EY, [Z | L], R) ->
    case element(I, Z) of
	EZ when EY == EZ ->
            ukeysplit_2(I, Y, EY, L, R);
	EZ when EY < EZ ->
            ukeysplit_1(I, Y, EY, Z, EZ, L, [], [lists:reverse(R, [])]);
        EZ ->
            ukeysplit_2(I, Z, EZ, L, [Y | R])
    end;
ukeysplit_2(_I, Y, _EY, [], R) ->
    [Y | R].

ukeymergel(I, [T1, [H2 | T2], [H3 | T3] | L], Acc) ->
    %% The fourth argument, [H2 | H3] (=HdM), may confuse type
    %% checkers. Its purpose is to ensure that the tests H2 == HdM
    %% and H3 == HdM in ukeymerge3_1 will always fail as long as M == [].
    M = ukeymerge3_1(I, T1, Acc, [H2 | H3], element(I, H2), H2, T2, [],
                     element(I, H3), H3, T3),
    ukeymergel(I, L, [M | Acc]);
ukeymergel(I, [[H1 | T1], T2 | L], Acc) ->
    ukeymergel(I, L, [ukeymerge2_2(I, T1, element(I, H1), H1, T2, []) | Acc]);
ukeymergel(_I, [L], []) ->
    L;
ukeymergel(I, [L], Acc) ->
    rukeymergel(I, [lists:reverse(L, []) | Acc], []);
ukeymergel(I, [], Acc) ->
    rukeymergel(I, Acc, []).

rukeymergel(I, [[H3 | T3], [H2 | T2], T1 | L], Acc) ->
    M = rukeymerge3_1(I, T1, Acc, [], element(I, H2), H2, T2, [],
                      element(I, H3), H3, T3),
    rukeymergel(I, L, [M | Acc]);
rukeymergel(I, [[H2 | T2], T1 | L], Acc) ->
    rukeymergel(I, L, [rukeymerge2_1(I, T1, element(I,H2), T2, [], H2)|Acc]);
rukeymergel(I, [L], Acc) ->
    ukeymergel(I, [lists:reverse(L, []) | Acc], []);
rukeymergel(I, [], Acc) ->
    ukeymergel(I, Acc, []).

%%% An extra argument, D, just to avoid some move instructions.

%% Take L1 apart.
ukeymerge3_1(I, [H1 | T1], D, HdM, E2, H2, T2, M, E3, H3, T3) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            ukeymerge3_12(I, E1, T1, H1, E2, H2, T2, E3, H3, T3, M, HdM, D);
        E1 when E2 == HdM ->
            ukeymerge3_2(I, E1, T1, H1, T2, HdM, T2, M, E3, H3, T3);
        E1 ->
            ukeymerge3_21(I, E1, T1, H1, E2, H2, T2, E3, H3, T3, M, HdM, T2)
    end;
ukeymerge3_1(I, [], _D, HdM, E2, _H2, T2, M, E3, H3, T3) when E2 == HdM ->
    ukeymerge2_1(I, T2, E3, HdM, T3, M, H3);
ukeymerge3_1(I, [], _D, _HdM, E2, H2, T2, M, E3, H3, T3) when E2 =< E3 ->
    ukeymerge2_1(I, T2, E3, E2, T3, [H2 | M], H3);
ukeymerge3_1(I, [], _D, HdM, E2, H2, T2, M, E3, _H3, T3) when E3 == HdM ->
    ukeymerge2_2(I, T2, E2, H2, T3, M);
ukeymerge3_1(I, [], _D, _HdM, E2, H2, T2, M, _E3, H3, T3) ->
    ukeymerge2_2(I, T2, E2, H2, T3, [H3 | M]).

%% Take L2 apart.
ukeymerge3_2(I, E1, T1, H1, [H2 | T2], HdM, D, M, E3, H3, T3) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            ukeymerge3_12(I, E1, T1, H1, E2, H2, T2, E3, H3, T3, M, HdM, T1);
        E2 ->
            ukeymerge3_21(I, E1, T1, H1, E2, H2, T2, E3, H3, T3, M, HdM, D)
    end;
ukeymerge3_2(I, E1, T1, H1, [], _HdM, _D, M, E3, H3, T3) when E1 =< E3 ->
    ukeymerge2_1(I, T1, E3, E1, T3, [H1 | M], H3);
ukeymerge3_2(I, E1, T1, H1, [], HdM, _D, M, E3, _H3, T3) when E3 == HdM ->
    ukeymerge2_2(I, T1, E1, H1, T3, M);
ukeymerge3_2(I, E1, T1, H1, [], _HdM, _D, M, _E3, H3, T3) ->
    ukeymerge2_2(I, T1, E1, H1, T3, [H3 | M]).

% E1 =< E2. Inlined.
ukeymerge3_12(I, E1, T1, H1, E2, H2, T2, E3, H3, T3, M, _HdM, D) 
                                                             when E1 =< E3 ->
    ukeymerge3_1(I, T1, D, E1, E2, H2, T2, [H1 | M], E3, H3, T3);
ukeymerge3_12(I, E1, T1, H1, E2, H2, T2, E3, _H3, T3, M, HdM, _D) 
                                                             when E3 == HdM ->
    ukeymerge3_12_3(I, E1, T1, H1, E2, H2, T2, M, T3);
ukeymerge3_12(I, E1, T1, H1, E2, H2, T2, _E3, H3, T3, M, _HdM, _D) ->
    ukeymerge3_12_3(I, E1, T1, H1, E2, H2, T2, [H3 | M], T3).

% E1 =< E2, take L3 apart.
ukeymerge3_12_3(I, E1, T1, H1, E2, H2, T2, M, [H3 | T3]) ->
    case element(I, H3) of
	E3 when E1 =< E3 ->
            ukeymerge3_1(I, T1, T1, E1, E2, H2, T2, [H1 | M], E3, H3, T3);
        _E3 ->
            ukeymerge3_12_3(I, E1, T1, H1, E2, H2, T2, [H3 | M], T3)
    end;
ukeymerge3_12_3(I, E1, T1, H1, E2, H2, T2, M, []) ->
    ukeymerge2_1(I, T1, E2, E1, T2, [H1 | M], H2).

% E1 > E2. Inlined.
ukeymerge3_21(I, E1, T1, H1, E2, H2, T2, E3, H3, T3, M, _HdM, D) 
                                                             when E2 =< E3 ->
    ukeymerge3_2(I, E1, T1, H1, T2, E2, D, [H2 | M], E3, H3, T3);
ukeymerge3_21(I, E1, T1, H1, E2, H2, T2, E3, _H3, T3, M, HdM, _D) 
                                                             when E3 == HdM ->
    ukeymerge3_21_3(I, E1, T1, H1, E2, H2, T2, M, T3);
ukeymerge3_21(I, E1, T1, H1, E2, H2, T2, _E3, H3, T3, M, _HdM, _D) ->
    ukeymerge3_21_3(I, E1, T1, H1, E2, H2, T2, [H3 | M], T3).

% E1 > E2, take L3 apart.
ukeymerge3_21_3(I, E1, T1, H1, E2, H2, T2, M, [H3 | T3]) ->
    case element(I, H3) of
        E3 when E2 =< E3 ->
            ukeymerge3_2(I, E1, T1, H1, T2, E2, T2, [H2 | M], E3, H3, T3);
        _E3 ->
            ukeymerge3_21_3(I, E1, T1, H1, E2, H2, T2, [H3 | M], T3)
    end;
ukeymerge3_21_3(I, E1, T1, H1, _E2, H2, T2, M, []) ->
    ukeymerge2_2(I, T1, E1, H1, T2, [H2 | M]).

%%% Two extra arguments, D1 and D2, just to avoid some move instructions.

%% Take L1 apart.
rukeymerge3_1(I, [H1 | T1], D1, D2, E2, H2, T2, M, E3, H3, T3) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
	    rukeymerge3_12a(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M);
        E1 ->
            rukeymerge3_21a(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D1, D2)
    end;
rukeymerge3_1(I, [], _D1, _D2, E2, H2, T2, M, E3, H3, T3) when E2 =< E3 ->
    rukeymerge2_2(I, T2, E2, T3, M, E3, H3, H2);
rukeymerge3_1(I, [], _D1, _D2, _E2, H2, T2, M, E3, H3, T3) ->
    rukeymerge2_1(I, T2, E3, T3, [H2 | M], H3).

% E1 =< E2. Inlined.
rukeymerge3_12a(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M) when E2 =< E3 ->
    rukeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, M, E3, H3, T3);
rukeymerge3_12a(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M) ->
    rukeymerge3_2(I, E1, H1, T1, T2, H2, E2, M, E3, H3, T3).

% E1 > E2. Inlined
rukeymerge3_21a(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, _D1, _D2) 
                                                              when E1 =< E3 ->
    rukeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, M, E3, H3, T3);
rukeymerge3_21a(I, _E1, H1, T1, E2, H2, T2, E3, H3, T3, M, D1, D2) ->
    rukeymerge3_1(I, T1, D1, D2, E2, H2, T2, [H1 | M], E3, H3, T3).

%% Take L2 apart. E2M > E3. E2M > E2.
rukeymerge3_2(I, E1, H1, T1, [H2 | T2], H2M, E2M, M, E3, H3, T3) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            % E2M > E1.
	    rukeymerge3_12b(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, H2M);
        E2 when E1 == E2M ->
            rukeymerge3_1(I, T1, H1, T1, E2, H2, T2, [H1 | M], E3, H3, T3);
        E2 ->
            % E2M > E1.
	    rukeymerge3_21b(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, H2M)
    end;
rukeymerge3_2(I, E1, H1, T1, [], _H2M, E2M, M, E3, H3, T3) when E1 == E2M ->
    rukeymerge2_1(I, T1, E3, T3, [H1 | M], H3);
rukeymerge3_2(I, E1, H1, T1, [], H2M, _E2M, M, E3, H3, T3) when E1 =< E3 ->
    rukeymerge2_2(I, T1, E1, T3, [H2M | M], E3, H3, H1);
rukeymerge3_2(I, _E1, H1, T1, [], H2M, _E2M, M, E3, H3, T3) ->
    rukeymerge2_1(I, T1, E3, T3, [H1, H2M | M], H3).

% E1 =< E2. Inlined.
rukeymerge3_12b(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, H2M) 
                                                             when E2 =< E3 ->
    rukeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, [H2M | M], E3, H3, T3);
rukeymerge3_12b(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M, H2M) ->
    rukeymerge3_2(I, E1, H1, T1, T2, H2, E2, [H2M | M], E3, H3, T3).

% E1 > E2. Inlined
rukeymerge3_21b(I, E1, H1, T1, E2, H2, T2, E3, H3, T3, M,H2M) when E1 =< E3 ->
    rukeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, [H2M | M], E3, H3, T3);
rukeymerge3_21b(I, _E1, H1, T1, E2, H2, T2, E3, H3, T3, M, H2M) ->
    rukeymerge3_1(I, T1, H1, T1, E2, H2, T2, [H1, H2M | M], E3, H3, T3).

% E1 =< E2, take L3 apart.
rukeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, M, E3M, H3M, [H3 | T3]) ->
    case element(I, H3) of
	E3 when E2 =< E3 ->
            rukeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, [H3M | M], E3, H3, T3);
        E3 when E2 == E3M ->
            rukeymerge3_2(I, E1, H1, T1, T2, H2, E2, M, E3, H3, T3);
        E3 ->
            rukeymerge3_2(I, E1, H1, T1, T2, H2, E2, [H3M | M], E3, H3, T3)
    end;
rukeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, M, E3M, _H3M, []) when E2 == E3M ->
    rukeymerge2_2(I, T1, E1, T2, M, E2, H2, H1);
rukeymerge3_12_3(I, E1, H1, T1, E2, H2, T2, M, _E3M, H3M, []) ->
    rukeymerge2_2(I, T1, E1, T2, [H3M | M], E2, H2, H1).

% E1 > E2, take L3 apart.
rukeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, M, E3M, H3M, [H3 | T3]) ->
    case element(I, H3) of
	E3 when E1 =< E3 ->
            rukeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, [H3M | M], E3, H3, T3);
        E3 when E1 == E3M ->
            rukeymerge3_1(I, T1, H1, T1, E2, H2, T2, [H1 | M], E3, H3, T3);
        E3 ->
            rukeymerge3_1(I, T1, H1, T1, E2, H2, T2, [H1,H3M | M], E3, H3, T3)
    end;
rukeymerge3_21_3(I, E1, H1, T1, E2, H2, T2, M, E3M, _H3M, []) when E1 == E3M ->
    rukeymerge2_1(I, T1, E2, T2, [H1 | M], H2);
rukeymerge3_21_3(I, _E1, H1, T1, E2, H2, T2, M, _E3M, H3M, []) ->
    rukeymerge2_1(I, T1, E2, T2, [H1, H3M | M], H2).

%% ukeymerge/3

%% Elements from the first list are kept and prioritized.
ukeymerge2_1(I, [H1 | T1], E2, HdM, T2, M, H2) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            ukeymerge2_1(I, T1, E2, E1, T2, [H1 | M], H2);
        E1 when E2 == HdM ->
            ukeymerge2_2(I, T1, E1, H1, T2, M);
        E1 ->
            ukeymerge2_2(I, T1, E1, H1, T2, [H2 | M])
    end;
ukeymerge2_1(_I, [], E2, HdM, T2, M, _H2) when E2 == HdM ->
    lists:reverse(T2, M);
ukeymerge2_1(_I, [], _E2, _HdM, T2, M, H2) ->
    lists:reverse(T2, [H2 | M]).

ukeymerge2_2(I, T1, E1, H1, [H2 | T2], M) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            ukeymerge2_1(I, T1, E2, E1, T2, [H1 | M], H2);
        _E2 ->
            ukeymerge2_2(I, T1, E1, H1, T2, [H2 | M])
    end;
ukeymerge2_2(_I, T1, _E1, H1, [], M) ->
    lists:reverse(T1, [H1 | M]).

%% rukeymerge/3

rukeymerge2_1(I, [H1 | T1], E2, T2, M, H2) ->
    case element(I, H1) of
	E1 when E1 =< E2 ->
            rukeymerge2_2(I, T1, E1, T2, M, E2, H2, H1);
        _E1 ->
            rukeymerge2_1(I, T1, E2, T2, [H1 | M], H2)
    end;
rukeymerge2_1(_I, [], _E2, T2, M, H2) ->
    lists:reverse(T2, [H2 | M]).

rukeymerge2_2(I, T1, E1, [H2 | T2], M, E2M, H2M, H1) ->
    case element(I, H2) of
	E2 when E1 =< E2 ->
            rukeymerge2_2(I, T1, E1, T2, [H2M | M], E2, H2, H1);
        E2 when E1 == E2M ->
            rukeymerge2_1(I, T1, E2, T2, [H1 | M], H2);
        E2 ->
            rukeymerge2_1(I, T1, E2, T2, [H1, H2M | M], H2)
    end;
rukeymerge2_2(_I, T1, E1, [], M, E2M, _H2M, H1) when E1 == E2M ->
    lists:reverse(T1, [H1 | M]);
rukeymerge2_2(_I, T1, _E1, [], M, _E2M, H2M, H1) ->
    lists:reverse(T1, [H1, H2M | M]).

%% sort/2

%% Ascending.
fsplit_1(Y, X, Fun, [Z | L], R, Rs) ->
    case Fun(Y, Z) of
        true -> 
            fsplit_1(Z, Y, Fun, L, [X | R], Rs);
        false ->
            case Fun(X, Z) of
                true ->
                    fsplit_1(Y, Z, Fun, L, [X | R], Rs);
                false when R == [] ->
                    fsplit_1(Y, X, Fun, L, [Z], Rs);
                false ->
                    fsplit_1_1(Y, X, Fun, L, R, Rs, Z)
            end
    end;
fsplit_1(Y, X, Fun, [], R, Rs) ->
    rfmergel([[Y, X | R] | Rs], [], Fun, asc).

fsplit_1_1(Y, X, Fun, [Z | L], R, Rs, S) ->
    case Fun(Y, Z) of
        true ->
            fsplit_1_1(Z, Y, Fun, L, [X | R], Rs, S);
        false ->
            case Fun(X, Z) of
                true ->
                    fsplit_1_1(Y, Z, Fun, L, [X | R], Rs, S);
                false ->
                    case Fun(S, Z) of
                        true ->
                            fsplit_1(Z, S, Fun, L, [], [[Y, X | R] | Rs]);
                        false ->
                            fsplit_1(S, Z, Fun, L, [], [[Y, X | R] | Rs])
                    end
            end
    end;
fsplit_1_1(Y, X, Fun, [], R, Rs, S) ->
    rfmergel([[S], [Y, X | R] | Rs], [], Fun, asc).

%% Descending.
fsplit_2(Y, X, Fun, [Z | L], R, Rs) ->
    case Fun(Y, Z) of
        false -> 
            fsplit_2(Z, Y, Fun, L, [X | R], Rs);
        true ->
            case Fun(X, Z) of
                false ->
                    fsplit_2(Y, Z, Fun, L, [X | R], Rs);
                true when R == [] ->
                    fsplit_2(Y, X, Fun, L, [Z], Rs);
                true ->
                    fsplit_2_1(Y, X, Fun, L, R, Rs, Z)
            end
    end;
fsplit_2(Y, X, Fun, [], R, Rs) ->
    fmergel([[Y, X | R] | Rs], [], Fun, desc).

fsplit_2_1(Y, X, Fun, [Z | L], R, Rs, S) ->
    case Fun(Y, Z) of
        false ->
            fsplit_2_1(Z, Y, Fun, L, [X | R], Rs, S);
        true ->
            case Fun(X, Z) of
                false ->
                    fsplit_2_1(Y, Z, Fun, L, [X | R], Rs, S);
                true ->
                    case Fun(S, Z) of
                        false ->
                            fsplit_2(Z, S, Fun, L, [], [[Y, X | R] | Rs]);
                        true ->
                            fsplit_2(S, Z, Fun, L, [], [[Y, X | R] | Rs])
                    end
            end
    end;
fsplit_2_1(Y, X, Fun, [], R, Rs, S) ->
    fmergel([[S], [Y, X | R] | Rs], [], Fun, desc).

fmergel([T1, [H2 | T2] | L], Acc, Fun, asc) ->
    fmergel(L, [fmerge2_1(T1, H2, Fun, T2, []) | Acc], Fun, asc);
fmergel([[H2 | T2], T1 | L], Acc, Fun, desc) ->
    fmergel(L, [fmerge2_1(T1, H2, Fun, T2, []) | Acc], Fun, desc);
fmergel([L], [], _Fun, _O) ->
    L;
fmergel([L], Acc, Fun, O) ->
    rfmergel([lists:reverse(L, []) | Acc], [], Fun, O);
fmergel([], Acc, Fun, O) ->
    rfmergel(Acc, [], Fun, O).

rfmergel([[H2 | T2], T1 | L], Acc, Fun, asc) ->
    rfmergel(L, [rfmerge2_1(T1, H2, Fun, T2, []) | Acc], Fun, asc);
rfmergel([T1, [H2 | T2] | L], Acc, Fun, desc) ->
    rfmergel(L, [rfmerge2_1(T1, H2, Fun, T2, []) | Acc], Fun, desc);
rfmergel([L], Acc, Fun, O) ->
    fmergel([lists:reverse(L, []) | Acc], [], Fun, O);
rfmergel([], Acc, Fun, O) ->
    fmergel(Acc, [], Fun, O).

%% merge/3 

%% Elements from the first list are prioritized.
fmerge2_1([H1 | T1], H2, Fun, T2, M) ->
    case Fun(H1, H2) of
        true ->
            fmerge2_1(T1, H2, Fun, T2, [H1 | M]);
        false ->
            fmerge2_2(H1, T1, Fun, T2, [H2 | M])
    end;
fmerge2_1([], H2, _Fun, T2, M) ->
    lists:reverse(T2, [H2 | M]).

fmerge2_2(H1, T1, Fun, [H2 | T2], M) ->
    case Fun(H1, H2) of
        true ->
            fmerge2_1(T1, H2, Fun, T2, [H1 | M]);
        false ->
            fmerge2_2(H1, T1, Fun, T2, [H2 | M])
    end;
fmerge2_2(H1, T1, _Fun, [], M) ->
    lists:reverse(T1, [H1 | M]).

%% rmerge/3

rfmerge2_1([H1 | T1], H2, Fun, T2, M) ->
    case Fun(H1, H2) of
        true ->
            rfmerge2_2(H1, T1, Fun, T2, [H2 | M]);
        false ->
            rfmerge2_1(T1, H2, Fun, T2, [H1 | M])
    end;
rfmerge2_1([], H2, _Fun, T2, M) ->
    lists:reverse(T2, [H2 | M]).

rfmerge2_2(H1, T1, Fun, [H2 | T2], M) ->
    case Fun(H1, H2) of
        true ->
            rfmerge2_2(H1, T1, Fun, T2, [H2 | M]);
        false ->
            rfmerge2_1(T1, H2, Fun, T2, [H1 | M])
    end;
rfmerge2_2(H1, T1, _Fun, [], M) ->
    lists:reverse(T1, [H1 | M]).

%% usort/2

%% Ascending. X < Y
ufsplit_1(Y, X, Fun, [Z | L], R, Rs) ->
    case Fun(Y, Z) of
        true ->
            case Fun(Z, Y) of
                true -> % Z equal to Y
                    ufsplit_1(Y, X, Fun, L, R, Rs);
                false ->
                    ufsplit_1(Z, Y, Fun, L, [X | R], Rs)
            end;
        false ->
            case Fun(X, Z) of
                true ->
                    case Fun(Z, X) of
                        true -> % Z equal to X
                            ufsplit_1(Y, X, Fun, L, R, Rs);
                        false ->
                            ufsplit_1(Y, Z, Fun, L, [X | R], Rs)
                    end;
                false when R == [] ->
                    ufsplit_1(Y, X, Fun, L, [Z], Rs);
                false ->
                    ufsplit_1_1(Y, X, Fun, L, R, Rs, Z)
            end
    end;
ufsplit_1(Y, X, Fun, [], R, Rs) ->
    rufmergel([[Y, X | R] | Rs], [], Fun).

%% X < Y
ufsplit_1_1(Y, X, Fun, [Z | L], R, Rs, S) ->
    case Fun(Y, Z) of
        true ->
            case Fun(Z, Y) of
                true -> % Z equal to Y
                    ufsplit_1_1(Y, X, Fun, L, R, Rs, S);
                false ->
                    ufsplit_1_1(Z, Y, Fun, L, [X | R], Rs, S)
            end;
        false ->
            case Fun(X, Z) of
                true ->
                    case Fun(Z, X) of
                        true -> % Z equal to X
                            ufsplit_1_1(Y, X, Fun, L, R, Rs, S);
                        false ->
                            ufsplit_1_1(Y, Z, Fun, L, [X | R], Rs, S)
                    end;
                false ->
                    case Fun(S, Z) of
                        true ->
                            case Fun(Z, S) of
                                true -> % Z equal to S
                                    ufsplit_1_1(Y, X, Fun, L, R, Rs, S);
                                false ->
                                    ufsplit_1(Z, S, Fun, L, [], [[Y, X | R] | Rs])
                            end;
                        false ->
                            ufsplit_1(S, Z, Fun, L, [], [[Y, X | R] | Rs])
                    end
            end
    end;
ufsplit_1_1(Y, X, Fun, [], R, Rs, S) ->
    rufmergel([[S], [Y, X | R] | Rs], [], Fun).

%% Descending.
ufsplit_2(Y, [Z | L], Fun, R) ->
    case Fun(Y, Z) of
        true ->
            case Fun(Z, Y) of
                true -> % Z equal to Y
                    ufsplit_2(Y, L, Fun, R);
                false ->
                    ufsplit_1(Z, Y, Fun, L, [], [lists:reverse(R, [])])
            end;
        false ->
            ufsplit_2(Z, L, Fun, [Y | R])
    end;
ufsplit_2(Y, [], _Fun, R) ->
    [Y | R].

ufmergel([[H1 | T1], T2 | L], Acc, Fun) ->
    ufmergel(L, [ufmerge2_2(H1, T1, Fun, T2, []) | Acc], Fun);
ufmergel([L], [], _Fun) ->
    L;
ufmergel([L], Acc, Fun) ->
    rufmergel([lists:reverse(L, []) | Acc], [], Fun);
ufmergel([], Acc, Fun) ->
    rufmergel(Acc, [], Fun).

rufmergel([[H2 | T2], T1 | L], Acc, Fun) ->
    rufmergel(L, [rufmerge2_1(T1, H2, Fun, T2, []) | Acc], Fun);
rufmergel([L], Acc, Fun) ->
    ufmergel([lists:reverse(L, []) | Acc], [], Fun);
rufmergel([], Acc, Fun) ->
    ufmergel(Acc, [], Fun).

%% umerge/3

%% Elements from the first list are kept and prioritized.
%% HdM before H2.
ufmerge2_1([H1 | T1], H2, Fun, T2, M, HdM) ->
    case Fun(H1, H2) of
        true ->
            ufmerge2_1(T1, H2, Fun, T2, [H1 | M], H1);
        false ->
            case Fun(H2, HdM) of
                true -> % HdM equal to H2
                    ufmerge2_2(H1, T1, Fun, T2, M);
                false ->
                    ufmerge2_2(H1, T1, Fun, T2, [H2 | M])
            end
    end;
ufmerge2_1([], H2, Fun, T2, M, HdM) ->
    case Fun(H2, HdM) of
        true -> % HdM equal to H2
            lists:reverse(T2, M);
        false ->
            lists:reverse(T2, [H2 | M])
    end.

ufmerge2_2(H1, T1, Fun, [H2 | T2], M) ->
    case Fun(H1, H2) of
        true ->
            ufmerge2_1(T1, H2, Fun, T2, [H1 | M], H1);
        false ->
            ufmerge2_2(H1, T1, Fun, T2, [H2 | M])
    end;
ufmerge2_2(H1, T1, _Fun, [], M) ->
    lists:reverse(T1, [H1 | M]).

%% rumerge/3

rufmerge2_1([H1 | T1], H2, Fun, T2, M) ->
    case Fun(H1, H2) of
        true ->
            rufmerge2_2(H1, T1, Fun, T2, M, H2);
        false ->
            rufmerge2_1(T1, H2, Fun, T2, [H1 | M])
    end;
rufmerge2_1([], H2, _Fun, T2, M) ->
    lists:reverse(T2, [H2 | M]).

%% H1 before H2M
rufmerge2_2(H1, T1, Fun, [H2 | T2], M, H2M) ->
    case Fun(H1, H2) of
        true ->
            rufmerge2_2(H1, T1, Fun, T2, [H2M | M], H2);
        false ->
            case Fun(H2M, H1) of
                true -> % H2M equal to H1
                    rufmerge2_1(T1, H2, Fun, T2, [H1 | M]);
                false ->
                    rufmerge2_1(T1, H2, Fun, T2, [H1, H2M | M])
            end
    end;
rufmerge2_2(H1, T1, Fun, [], M, H2M) ->
    case Fun(H2M, H1) of
        true -> 
            lists:reverse(T1, [H1 | M]);
        false ->
            lists:reverse(T1, [H1, H2M | M])
    end.