import Data.Set (Set)
import qualified Data.Set as Set

kSubsetPermutations :: Ord a => Int -> Set a -> [[a]]
kSubsetPermutations 0 _  = []
kSubsetPermutations 1 xs = map (\x -> [x]) $ Set.toList xs
kSubsetPermutations k xs = concatMap f $ Set.toList xs where
    f x = map (x :) $ kSubsetPermutations (k-1) $ Set.delete x xs