%%  n-queens problem
%%  Domain predicate "d" specifies the number of queens and the dimensions 
%%  of the board. The main predicate is the predicate "q". Atom "q(X,Y)" 
%%  represents the fact that there is a queen in the position (X,Y). 

% Domain predicate: the dimension of the board. 
d(1..n).

% Exactly one queen in each row
1 { q(X,Y):d(Y) } 1 :- d(X).

% Exactly one queen in each column
1 { q(X,Y):d(X) } 1 :- d(Y).

% No two queens on an diagonal
:- d(X;Y;X1;Y1), q(X,Y), q(X1,Y1), 
        X < X1,  Y != Y1, abs(X - X1) == abs(Y - Y1).