import Control.SetFunctions

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

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

-- Solution to the n queens problem:
-- Here: weak encapsulation via set function is important!!!
queens :: Int -> [Int]
queens n | isEmpty ((set1 unsafe) p) = p
 where p = perm [1..n]