import Control.SetFunctions

perm :: [a] -> [a]
perm []     = []
perm (x:xs) = insert x (perm xs)
 where insert z ys     = z : ys
       insert z (y:ys) = y : insert z ys

unsafe :: [Int] -> Bool
unsafe xs | xs =:= _ ++ [i] ++ zs ++ [j] ++ _
          = abs (i-j) =:= length zs + 1
 where i,j,zs free

-- solving the n queens problem:
queens :: Int -> [Int]
queens n | isEmpty ((set1 unsafe) p) = p
 where p = perm [1 .. n]