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diff --git a/src/Much/MappedSets.hs b/src/Much/MappedSets.hs
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+module Much.MappedSets (invert, mk) where
+
+import Control.Arrow
+import Data.Map.Strict (Map)
+import qualified Data.Map.Strict as Map
+import Data.Maybe
+import Data.Set (Set)
+import qualified Data.Set as Set
+
+
+mk :: (Ord a, Ord b) => [(a, [b])] -> Map a (Set b)
+mk =
+ Map.fromList . map (second Set.fromList)
+
+
+invert :: (Ord a, Ord b) => Map a (Set b) -> Map b (Set a)
+invert =
+ Map.foldrWithKey invert1 Map.empty
+
+
+invert1 :: (Ord a, Ord b) => a -> Set b -> Map b (Set a) -> Map b (Set a)
+invert1 k v a =
+ Set.foldr (upsert k) a v
+
+
+upsert :: (Ord a, Ord b) => a -> b -> Map b (Set a) -> Map b (Set a)
+upsert k =
+ Map.alter (Just . Set.insert k . fromMaybe Set.empty)