1 { q(X,Y) : d(Y) } 1 :- d(X).
1 { q(X,Y) : d(X) } 1 :- d(Y).

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

d(1..queens).

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

% TRY: lpeq-1.19 queens2c.lparse queens2a.lparse | clasp
% TRY: lpeq-1.19 queens2a.lparse queens2c.lparse | clasp