1 { queen(I,1..n) } 1 :- I = 1..n. 1 { queen(1..n,J) } 1 :- J = 1..n. :- 2 { queen(I,J) : D = I+J+1 }, D=1..2*n-1. :- 2 { queen(I,J) : D = I-J+n }, D=1..2*n-1.