Nqueens without definition better than with

[Broes De Cat]

The nqueens theory with the constraints ! x1 y1 x2 y2 : Queen(x1,y1) & Queen(x2,y2) & x1<x2 => abs(x1-x2) ~= abs(y1-y2).

now works a lot faster than the constraints { Diag(x1,y1,x2,y2) <- x1 < x2 & abs(x1-x2) = abs(y1-y2). } ! x1 y1 x2 y2 : Diag(x1,y1,x2,y2) => ~(Queen(x1,y1) & Queen(x2,y2)).

[Broes De Cat]

Duplicate of #253.