#const diags  = queens * 2 - 1.

d(1..queens).
diag(1..diags).

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

d1(X,Y,X - Y + queens) :- d(X;Y).
d2(X,Y,X + Y - 1)      :- d(X;Y).

 :- diag(D), 2 { q(X,Y) : d1(X,Y,D) }.
 :- diag(D), 2 { q(X,Y) : d2(X,Y,D) }.

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