diff --git a/src-3.0/GF/Data/MultiMap.hs b/src-3.0/GF/Data/MultiMap.hs
new file mode 100644
index 000000000..e565f433b
--- /dev/null
+++ b/src-3.0/GF/Data/MultiMap.hs
@@ -0,0 +1,47 @@
+module GF.Data.MultiMap where
+
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.Set (Set)
+import qualified Data.Set as Set
+import Prelude hiding (map)
+import qualified Prelude
+
+type MultiMap k a = Map k (Set a)
+
+empty :: MultiMap k a
+empty = Map.empty
+
+keys :: MultiMap k a -> [k]
+keys = Map.keys
+
+elems :: MultiMap k a -> [a]
+elems = concatMap Set.toList . Map.elems
+
+(!) :: Ord k => MultiMap k a -> k -> [a]
+m ! k = Set.toList $ Map.findWithDefault Set.empty k m
+
+member :: (Ord k, Ord a) => k -> a -> MultiMap k a -> Bool
+member k x m = x `Set.member` Map.findWithDefault Set.empty k m
+
+insert :: (Ord k, Ord a) => k -> a -> MultiMap k a -> MultiMap k a
+insert k x m = Map.insertWith Set.union k (Set.singleton x) m
+
+insert' :: (Ord k, Ord a) => k -> a -> MultiMap k a -> Maybe (MultiMap k a)
+insert' k x m | member k x m = Nothing -- FIXME: inefficient
+              | otherwise = Just (insert k x m)
+
+union :: (Ord k, Ord a) => MultiMap k a -> MultiMap k a -> MultiMap k a
+union = Map.unionWith Set.union
+
+size :: MultiMap k a -> Int
+size = sum . Prelude.map Set.size . Map.elems
+
+map :: (Ord a, Ord b) => (a -> b) -> MultiMap k a -> MultiMap k b
+map f = Map.map (Set.map f)
+
+fromList :: (Ord k, Ord a) => [(k,a)] -> MultiMap k a
+fromList xs = Map.fromListWith Set.union [(k, Set.singleton x) | (k,x) <- xs]
+
+toList :: MultiMap k a -> [(k,a)]
+toList m = [(k,x) | (k,s) <- Map.toList m, x <- Set.toList s]
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