%%% SORTING

%%% Stupid sort:

sort1(L1,L2) :- permutation1(L1,L2),sorted(L2),!.

permutation1([],[]).
permutation1(L,[H|T]) :- 
    append(V,[H|U],L),
    append(V,U,W),
    permutation1(W,T).

sorted([]).
sorted([X]).
sorted([X,Y|L]) :- order(X,Y), sorted([Y|L]).

order(X,Y) :- X =< Y.
order2(X,Y) :- X @=< Y.

/***** Example run:

| ?- permutation1([1,2,3],L).

L = [1,2,3] ? ;

L = [1,3,2] ? ;

L = [2,1,3] ? ;

L = [2,3,1] ? ;

L = [3,1,2] ? ;

L = [3,2,1] ? ;

*****/


%%%% For more sorting algorithms, see Section 7.7.