diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
--- /dev/null
+++ b/CHANGELOG.md
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,19 @@
+Copyright (c) 2014 Julian Fleischer
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
diff --git a/README.md b/README.md
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--- /dev/null
+++ b/README.md
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/hydrogen-multimap.cabal b/hydrogen-multimap.cabal
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--- /dev/null
+++ b/hydrogen-multimap.cabal
@@ -0,0 +1,37 @@
+name:                 hydrogen-multimap
+version:              0.1
+homepage:             https://scravy.de/hydrogen-multimap/
+synopsis:             Hydrogen Multimap
+license:              MIT
+license-file:         LICENSE
+extra-source-files:   CHANGELOG.md, README.md
+author:               Julian Fleischer
+maintainer:           julian@scravy.de
+category:             Language
+build-type:           Simple
+cabal-version:        >=1.14
+
+source-repository head
+    type:             git
+    location:         https://github.com/scravy/hydrogen-multimap
+
+library
+  exposed-modules:    Hydrogen.MultiMap
+  build-depends:      base ==4.7.*
+                      , containers ==0.5.*
+  hs-source-dirs:     src
+  ghc-options:        -Wall
+  default-language:   Haskell2010
+  default-extensions:  CPP
+                       , DeriveDataTypeable
+                       , DeriveFoldable
+                       , DeriveFunctor
+                       , DeriveGeneric
+                       , DeriveTraversable
+                       , GADTs
+                       , LambdaCase
+                       , MultiWayIf
+                       , NoImplicitPrelude
+                       , ScopedTypeVariables
+                       , TypeFamilies
+
diff --git a/src/Hydrogen/MultiMap.hs b/src/Hydrogen/MultiMap.hs
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--- /dev/null
+++ b/src/Hydrogen/MultiMap.hs
@@ -0,0 +1,127 @@
+module Hydrogen.MultiMap (
+    MultiMap
+  , empty
+  , null
+  , keys
+  , elems
+  , numKeys
+  , numElems
+  , lookup
+  , member
+  , insert
+  , delete
+  , update
+  , adjust
+  , toMap
+  , fromMap
+  , fromList
+  , fromList'
+  , fromSet
+  , toList
+  , toList'
+  , union
+  ) where
+
+import Prelude hiding (lookup, foldr, null)
+
+import Data.Foldable (Foldable, foldr)
+import Data.Traversable (Traversable)
+import Data.Typeable (Typeable)
+
+import GHC.Generics (Generic)
+
+import Data.Map (Map)
+import Data.Set (Set)
+
+import qualified Data.Map as Map
+import qualified Data.List as List
+
+data MultiMap k v = MultiMap (Map k [v]) Int
+  deriving (Eq, Ord, Functor, Foldable, Traversable, Generic, Typeable)
+
+instance (Show k, Show v) => Show (MultiMap k v) where
+    show (MultiMap m _) = show m
+
+count :: Map k [v] -> Int
+count = foldr (\vs s -> length vs + s) 0
+
+empty :: MultiMap k v
+empty = MultiMap Map.empty 0
+
+numKeys :: MultiMap k v -> Int
+numKeys (MultiMap m _) = Map.size m
+
+numElems :: MultiMap k v -> Int
+numElems (MultiMap _ s) = s
+
+null :: MultiMap k v -> Bool
+null (MultiMap m _) = Map.null m
+
+keys :: MultiMap k v -> [k]
+keys (MultiMap m _) = Map.keys m
+
+elems :: MultiMap k v -> [[v]]
+elems (MultiMap m _) = Map.elems m
+
+lookup :: Ord k => k -> MultiMap k v -> [v]
+lookup k (MultiMap m _) = maybe [] id $ Map.lookup k m
+
+member :: Ord k => k -> MultiMap k v -> Bool
+member k = not . List.null . lookup k
+
+insert :: Ord k => k -> v -> MultiMap k v -> MultiMap k v
+insert k v mm@(MultiMap m s) = MultiMap (Map.insert k set' m) s'
+  where
+    set = lookup k mm
+    set' = v : set
+    s' = s - length set + length set'
+
+delete :: Ord k => k -> MultiMap k v -> MultiMap k v
+delete k mm@(MultiMap m s) = MultiMap (Map.delete k m) s'
+  where
+    s' = s - length (lookup k mm)
+
+update :: Ord k => k -> [v] -> MultiMap k v -> MultiMap k v
+update k vs mm@(MultiMap m s)
+    | List.null vs = MultiMap (Map.delete k m) s'
+    | otherwise = MultiMap (Map.insert k vs m) s'
+  where
+    s' = s - length (lookup k mm) + length vs
+
+adjust :: Ord k => ([v] -> [v]) -> k -> MultiMap k v -> MultiMap k v
+adjust f k mm@(MultiMap m s)
+    | List.null set' = MultiMap (Map.delete k m) s'
+    | otherwise = MultiMap (Map.insert k set' m) s'
+  where
+    set = lookup k mm
+    set' = f set
+    s' = s - length set + length set'
+
+toMap :: MultiMap k v -> Map k [v]
+toMap (MultiMap m _) = m
+
+fromMap :: Map k [v] -> MultiMap k v
+fromMap m = MultiMap m (count m)
+
+toList :: MultiMap k v -> [(k, [v])]
+toList (MultiMap m _) = Map.toList m
+
+toList' :: MultiMap k v -> [(k, v)]
+toList' = concat . map (\(k, vs) -> [(k, v) | v <- vs]) . toList
+
+fromList :: Ord k => [(k, [v])] -> MultiMap k v
+fromList xs = MultiMap (Map.fromList xs) (foldr (\x s -> length (snd x) + s) 0 xs)
+
+fromList' :: Ord k => [(k, v)] -> MultiMap k v
+fromList' = foldr (uncurry insert) empty
+
+fromSet :: Ord k => (k -> [v]) -> Set k -> MultiMap k v
+fromSet f s = MultiMap m (count m)
+  where
+    m = Map.fromSet f s
+
+union :: Ord k => MultiMap k v -> MultiMap k v -> MultiMap k v
+union (MultiMap m1 s1) (MultiMap m2 s2) = MultiMap (Map.unionWith (++) m1 m2) (s1 + s2)
+
+
+