Fix combination bug

src/Multiset.hs

@@ -1,13 +1,18 @@
 {-# LANGUAGE TupleSections #-}
 module Multiset where
 import qualified Data.Map.Strict as M
+import qualified Data.IntMap.Strict as IM
+import qualified Data.IntSet as IS
 import Control.Monad
+import Data.List (subsequences)
 
 powlistN :: Int -> [a] ->[[a]]
 powlistN _ [] = [[]]
 powlistN n (x:xs) 
     | n <=0     = [[]]
     | otherwise = ((x:) <$> powlistN (n-1) xs) ++ powlistN n xs
+powlistN' :: Int -> [a] ->[[a]]
+powlistN' k = filter ( ( k >= ) . length ) . subsequences
 
 -- exponential, so don't use it on long lists
 powlist :: [a] -> [[a]]
@@ -15,3 +20,8 @@ powlist = filterM (const [True,False])
 
 toMultiset :: Ord a => [a] -> M.Map a Int
 toMultiset = foldr (uncurry (M.insertWith (+)) . (, 1)) M.empty
+
+invertIntMap :: Ord a => IM.IntMap a -> M.Map a IS.IntSet
+invertIntMap = IM.foldrWithKey 
+    (\k x -> M.insertWith IS.union x (IS.singleton k))
+    M.empty