safe(Q1X-Q1Y, Q2X-Q2Y) :-
	between(0, 7, Q1Y),
	between(0, 7, Q2Y),
	Q1X =\= Q2X,
	Q1Y =\= Q2Y,
	abs(Q1X - Q2X) =\= abs(Q1Y - Q2Y).
safe(_, []).
safe(QN, [Q | QS]) :- safe(QN, [Q | QS], 1).
safe(_, [], _).
safe(QN, [Q | QS], QX) :- safe(0-QN, QX-Q), safe(QN, QS, QX + 1).

add(Qs, QsN) :-
	QsN = [Q | Qs],
	safe(Q, Qs).

add(1, Qs, QsN) :- add(Qs, QsN).
add(N, Qs, QsN) :- add(Qs, QA), add(M, QA, QsN), succ(M, N).

solution(Qs) :- add(8, [], Qs).