% 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([],[]). permute(X,Y) :- var(X), permute(Y,X). permute(L1, [L2H | L2T]) :- not(var(L1)), append(L1F1, [L2H | L1F2], L1), append(L1F1, L1F2, L1S), permute(L1S, L2T). % The relation ordered(X) is satisfied when the members of X are arranged in % ascending order. ordered([]). ordered([_]). ordered([A,B|C]) :- A =< B, ordered([B|C]). % Perform sorting straight from the definition. sorted(X,Y) :- permute(X,Y), ordered(Y).