:- module(queen). :- export queen/2. queen(Data, Out) :- qperm(Data, Out), safe(Out). qperm([], []). qperm([X|Y], [U|V]) :- qdelete(U, X, Y, Z), qperm(Z, V). qdelete(A, A, L, L). qdelete(X, A, [H|T], [A|R]) :- qdelete(X, H, T, R). safe([]). safe([N|L]) :- nodiag(L, N, 1), safe(L). nodiag([], _, _). nodiag([N|L], B, D) :- D =\= N - B, D =\= B - N, D1 is D + 1, nodiag(L, B, D1).