% N-queens problem by Ilkka Niemelä;
% modified by Tomi Janhunen to place queens row-by-row
%
% d(1..n) describe the dimensions of the board
% q(X,Y) gives the legal positions of the queens

d(1..n).

% Place a queen on each row

{ q(X,Y) : d(X) } :- d(Y).
:- q(X,Y), q(Z,Y), d(X;Y;Z), X<Z.

% Make sure they don't threaten each other

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