Add file

src/Multiset.hs

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+{-# LANGUAGE TupleSections #-}
+module Multiset where
+import qualified Data.Map.Strict as M
+import Control.Monad
+
+powlistN :: Int -> [a] ->[[a]]
+powlistN _ [] = [[]]
+powlistN n (x:xs) 
+    | n <=0     = [[]]
+    | otherwise = ((x:) <$> powlistN (n-1) xs) ++ powlistN n xs
+
+-- exponential, so don't use it on long lists
+powlist :: [a] -> [[a]]
+powlist = filterM (const [True,False])
+
+toMultiset :: Ord a => [a] -> M.Map a Int
+toMultiset = foldr (uncurry $ M.insertWith (+)) M.empty . map (,1)