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#show queen/2.
#program board(n).
#external attack(n,1..n,h).
#external attack(1..n,n,v).
target(n, X, X, n, b,n) :- X = 1..n-1. % diagonal b
target(Y, n-1,n, Y-1,b,n) :- Y = 2..n-1. % diagonal b
target(X, n-1,X+1,n, f,n) :- X = 1..n-1. % diagonal f
target(n-1,Y, n, Y+1,f,n) :- Y = 1..n-2. % diagonal f
target(X, n, X-1,n, h,n) :- X = 2..n. % horizontal
target(n, Y, n-1,Y, h,n) :- Y = 1..n-1. % horizontal
target(Y, X, Y, X-1,v,n) :- target(X,Y,X-1,Y,h,n). % vertical
{ queen(1..n,n); queen(n,1..n-1) }.
attack(X',Y',D) :- target(X,Y,X',Y',D,n), queen(X,Y).
attack(X',Y',D) :- target(X,Y,X',Y',D,n), attack(X,Y,D).
:- target(X,Y,X',Y',D,n), attack(X',Y',D), queen(X',Y').
:- not queen(1,n), not attack(1,n,h).
:- not queen(n,1), not attack(n,1,v).
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