#options -d none
#external q(X,Y).

d(1..q). q(X,Y) :- d(X;Y). %% for grounding

% Make sure queens don't threaten each other

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