% by Tomi Janhunen (adopted from Ilkka Niemelä's encoding)

d(1..queens).

queens {q(X,Y):d(X;Y)} queens.

:- q(X,Y1), q(X,Y2), Y1!=Y2, d(X;Y1;Y2).
:- q(X1,Y), q(X2,Y), X1!=X2, d(X1;X2;Y).
:- q(X1,Y1), q(X2,Y2), #abs(X1-X2)==#abs(Y1-Y2),
   X1!=X2, Y1!=Y2, d(X1;Y1;X2;Y2).

#hide.
#show q(X,Y).