% N-queens problem by Ilkka Niemelä;
% modified by Tomi Janhunen to place queens column-by-column
%
% q(t,X,Y) gives the legal positions of the queens
% d(1..queens): facts for the dimension of the board.

% Place a queen on each column

negq(X,Y2) :- q(X,Y), d(X), d(Y), d(Y2), Y2 != Y.
q(X,Y) :- not negq(X,Y), not q(X,Y2): d(Y2): Y2 != Y , d(X), d(Y).

hide negq(X,Y).

% Make sure they don't threaten each other

:- d(X), d(Y), d(X1), q(X,Y), q(X1,Y), X1 != X.
:- d(X), d(Y), d(X1), d(Y1), q(X,Y), q(X1,Y1),
   X != X1,  Y != Y1, abs(X - X1) == abs(Y - Y1).

d(1..queens).

% Typical command line
% lparse -1 -d none -c queens=8 queens.lp  | smodels