queens(N, Qs) :-
         range(1, N, Ns),
         queens(Ns, [], Qs).

     queens(UnplacedQs, SafeQs, Qs) :-
         select(Q, UnplacedQs, UnplacedQs1),
         \+ attack(Q, SafeQs),
         queens(UnplacedQs1, [Q|SafeQs], Qs).
         queens([], Qs, Qs).

     attack(X, Xs) :- attack(X, 1, Xs).

     attack(X, N, [Y|_Ys]) :- X is Y + N.
     attack(X, N, [Y|_Ys]) :- X is Y - N.
     attack(X, N, [_Y|Ys]) :-
         N1 is N + 1,
         attack(X, N1, Ys).

     range(M, N, [M|Ns]) :-
         M < N,
         M1 is M + 1,
         range(M1, N, Ns).
     range(N, N, [N]).

     show_solution(Tcl, L) :-
         reverse(L, LR),
         tcl_eval(Tcl, [show_solution, br(LR)], _),
         tk_do_all_events.

     tk_do_all_events :-
         tk_do_one_event, !,
         tk_do_all_events.
     tk_do_all_events.