% The relation permute(X,Y) is satisfied when Y is a permutation of X (or vice
% versa). The relation var(X) is a builtin which is satisfied when X is
% an unbound variable. Essentially, we want to evaluate permute when its
% first argument is known, and the second is the result. The second rule
% simply flips then two when this is not the case.
permute(X,Y) :- var(X), permute(Y,X).
permute(L1, [L2H | L2T]) :- not(var(L1)),
append(L1F1, [L2H | L1F2], L1), append(L1F1, L1F2, L1S),
% The relation ordered(X) is satisfied when the members of X are arranged in
% ascending order.
ordered([A,B|C]) :- A =< B, ordered([B|C]).
% Perform sorting straight from the definition.
sorted(X,Y) :- permute(X,Y), ordered(Y).