diff --git a/src/Forest/OrderedMap.hs b/src/Forest/OrderedMap.hs
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+++ b/src/Forest/OrderedMap.hs
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+module Forest.OrderedMap
+  ( OrderedMap
+  -- * Construction
+  , empty
+  , singleton
+  , fromSet
+  -- ** From lists
+  , fromList
+  , fromListWith
+  , fromListWithKey
+  -- ** From maps
+  , fromMap
+  , fromMapWithOrder
+  -- * Insertion
+  , append
+  , appendWith
+  , appendWithKey
+  , appendLookupWithKey
+  , prepend
+  , prependWith
+  , prependWithKey
+  , prependLookupWithKey
+  -- * Deletion/Update
+  , delete
+  , adjust
+  , adjustWithKey
+  , update
+  , updateWithKey
+  -- * Query
+  -- ** Lookup
+  , Forest.OrderedMap.lookup
+  , (!?)
+  , (!)
+  , findWithDefault
+  , member
+  , notMember
+  , lookupLT
+  , lookupGT
+  , lookupLE
+  , lookupGE
+  -- ** Size
+  , Forest.OrderedMap.null
+  , size
+  -- * Traversal
+  -- ** Map
+  , Forest.OrderedMap.map
+  , mapWithKey
+  -- * Conversion
+  , elems
+  , keys
+  , assocs
+  , keysSet
+  -- ** Lists
+  , toList
+  -- ** Maps
+  , toMap
+  -- * Filter
+  , Forest.OrderedMap.filter
+  ) where
+
+import qualified Data.List       as L
+import qualified Data.Map.Strict as Map
+import qualified Data.Set        as Set
+
+data OrderedMap k a = OrderedMap
+  { omMap   :: Map.Map k a
+  , omOrder :: [k]
+  }
+
+instance (Ord k, Show k, Show a) => Show (OrderedMap k a) where
+  show m = "fromList " ++ show (toList m)
+
+-- Invariants of this data type:
+--
+-- 1. The 'omOrder' list contains each key from 'omMap' exactly once.
+-- 2. The 'omOrder' list contains no other values.
+--
+-- All functions operating on an 'OrderedMap' may assume these invariants. All
+-- modifications must preserve these invariants.
+
+-- | Like 'Map.empty'.
+empty :: OrderedMap k a
+empty = OrderedMap
+  { omMap   = Map.empty
+  , omOrder = []
+  }
+
+-- | Like 'Map.singleton'.
+singleton :: k -> a -> OrderedMap k a
+singleton k a = OrderedMap
+  { omMap   = Map.singleton k a
+  , omOrder = [k]
+  }
+
+-- | Like 'Map.fromSet'. Uses 'Set.toAscList' as the order.
+fromSet :: (k -> a) -> Set.Set k -> OrderedMap k a
+fromSet f kSet = OrderedMap
+  { omMap   = Map.fromSet f kSet
+  , omOrder = Set.toAscList kSet
+  }
+
+fromMap :: Map.Map k a -> OrderedMap k a
+fromMap m = OrderedMap
+  { omMap   = m
+  , omOrder = Map.keys m
+  }
+
+-- | Create a new 'OrderedMap' from a 'Map.Map' using a list of keys to specify
+-- the order. If the list of keys contains a key multiple times, only the last
+-- occurrence is counted. The keys missing from the list of keys are appended at
+-- the end in ascending order.
+fromMapWithOrder :: Ord k => Map.Map k a -> [k] -> OrderedMap k a
+fromMapWithOrder m l =
+  let kSet = Map.keysSet m
+      onlyExistingKeys = L.filter (`Set.member` kSet) l
+      deduplicatedKeys = fst $ keepLastInstances onlyExistingKeys
+      missingKeysSet = kSet Set.\\ Set.fromList deduplicatedKeys
+      order = if Set.null missingKeysSet
+        then deduplicatedKeys -- For the extra performance :P
+        else deduplicatedKeys ++ Set.toAscList missingKeysSet
+  in  OrderedMap {omMap = m, omOrder = order}
+
+keepLastInstances :: Ord a => [a] -> ([a], Set.Set a)
+keepLastInstances [] = ([], Set.empty)
+keepLastInstances (x:xs)
+  | x `Set.member` subset = (sublist, subset)
+  | otherwise = (x : sublist, Set.insert x subset)
+  where
+    (sublist, subset) = keepLastInstances xs
+
+-- | Like 'Map.fromList'. Uses the last occurrence of each key as the order.
+fromList :: Ord k => [(k, a)] -> OrderedMap k a
+-- This function could've been implemented as @fromListWith const@, but that
+-- might lead to a performance penalty. I don't know though ¯\_(ツ)_/¯
+fromList pairs = OrderedMap
+  { omMap   = Map.fromList pairs
+  , omOrder = fst $ keepLastInstances $ L.map fst pairs
+  }
+
+-- | Like 'Map.fromListWith'. Uses the last occurrence of each key as the order.
+fromListWith :: Ord k => (a -> a -> a) -> [(k, a)] -> OrderedMap k a
+fromListWith f = fromListWithKey $ const f
+
+-- | Like 'Map.fromListWithKey'. Uses the last occurrence of each key as the
+-- order.
+fromListWithKey :: Ord k => (k -> a -> a -> a) -> [(k, a)] -> OrderedMap k a
+fromListWithKey f pairs = OrderedMap
+  { omMap   = Map.fromListWithKey f pairs
+  , omOrder = fst $ keepLastInstances $ L.map fst pairs
+  }
+
+append :: Ord k => k -> a -> OrderedMap k a -> OrderedMap k a
+append = appendWith const
+
+appendWith :: Ord k => (a -> a -> a) -> k -> a -> OrderedMap k a -> OrderedMap k a
+appendWith f = appendWithKey $ const f
+
+appendWithKey :: Ord k => (k -> a -> a -> a) -> k -> a -> OrderedMap k a -> OrderedMap k a
+appendWithKey f k a = snd . appendLookupWithKey f k a
+
+appendLookupWithKey :: Ord k => (k -> a -> a -> a) -> k -> a -> OrderedMap k a -> (Maybe a, OrderedMap k a)
+appendLookupWithKey f k a m =
+  let (maybePrevA, newMap) = Map.insertLookupWithKey f k a $ omMap m
+      newOrder = case maybePrevA of
+        Nothing -> omOrder m ++ [k]
+        Just _  -> omOrder m
+  in  (maybePrevA, OrderedMap {omMap = newMap , omOrder = newOrder})
+
+prepend :: Ord k => k -> a -> OrderedMap k a -> OrderedMap k a
+prepend = prependWith const
+
+prependWith :: Ord k => (a -> a -> a) -> k -> a -> OrderedMap k a -> OrderedMap k a
+prependWith f = prependWithKey $ const f
+
+prependWithKey :: Ord k => (k -> a -> a -> a) -> k -> a -> OrderedMap k a -> OrderedMap k a
+prependWithKey f k a = snd . prependLookupWithKey f k a
+
+prependLookupWithKey :: Ord k => (k -> a -> a -> a) -> k -> a -> OrderedMap k a -> (Maybe a, OrderedMap k a)
+prependLookupWithKey f k a m =
+  let (maybePrevA, newMap) = Map.insertLookupWithKey f k a $ omMap m
+      newOrder = case maybePrevA of
+        Nothing -> k : omOrder m
+        Just _  -> omOrder m
+  in  (maybePrevA, OrderedMap {omMap = newMap , omOrder = newOrder})
+
+delete :: Ord k => k -> OrderedMap k a -> OrderedMap k a
+delete k m = m
+  { omMap   = Map.delete k $ omMap m
+  , omOrder = L.delete k $ omOrder m
+  }
+
+adjust :: Ord k => (a -> a) -> k -> OrderedMap k a -> OrderedMap k a
+adjust f = adjustWithKey $ const f
+
+adjustWithKey :: Ord k => (k -> a -> a) -> k -> OrderedMap k a -> OrderedMap k a
+adjustWithKey f k m = m {omMap = Map.adjustWithKey f k $ omMap m}
+
+update :: Ord k => (a -> Maybe a) -> k -> OrderedMap k a -> OrderedMap k a
+update f = updateWithKey $ const f
+
+updateWithKey :: Ord k => (k -> a -> Maybe a) -> k -> OrderedMap k a -> OrderedMap k a
+updateWithKey f k m =
+  let newMap = Map.updateWithKey f k $ omMap m
+      newOrder = case newMap Map.!? k of
+        Nothing -> L.delete k $ omOrder m
+        Just _  -> omOrder m
+  in  OrderedMap {omMap = newMap, omOrder = newOrder}
+
+lookup :: Ord k => k -> OrderedMap k a -> Maybe a
+lookup k = Map.lookup k . omMap
+
+infixl 9 !?
+(!?) :: Ord k => OrderedMap k a -> k -> Maybe a
+m !? k = omMap m Map.!? k
+
+infixl 9 !
+(!) :: Ord k => OrderedMap k a -> k -> a
+m ! k = omMap m Map.! k
+
+findWithDefault :: Ord k => a -> k -> OrderedMap k a -> a
+findWithDefault a k = Map.findWithDefault a k . omMap
+
+member :: Ord k => k -> OrderedMap k a -> Bool
+member k = Map.member k . omMap
+
+notMember :: Ord k => k -> OrderedMap k a -> Bool
+notMember k = Map.notMember k . omMap
+
+lookupLT :: Ord k => k -> OrderedMap k v -> Maybe (k, v)
+lookupLT k = Map.lookupLT k . omMap
+
+lookupGT :: Ord k => k -> OrderedMap k v -> Maybe (k, v)
+lookupGT k = Map.lookupGT k . omMap
+
+lookupLE :: Ord k => k -> OrderedMap k v -> Maybe (k, v)
+lookupLE k = Map.lookupLE k . omMap
+
+lookupGE :: Ord k => k -> OrderedMap k v -> Maybe (k, v)
+lookupGE k = Map.lookupGE k . omMap
+
+null :: OrderedMap k a -> Bool
+null = Map.null . omMap
+
+size :: OrderedMap k a -> Int
+size = Map.size . omMap
+
+map :: (a -> b) -> OrderedMap k a -> OrderedMap k b
+map f = mapWithKey $ const f
+
+mapWithKey :: (k -> a -> b) -> OrderedMap k a -> OrderedMap k b
+mapWithKey f m = m {omMap = Map.mapWithKey f $ omMap m}
+
+elems :: Ord k => OrderedMap k a -> [a]
+elems = L.map snd . assocs
+
+keys :: OrderedMap k a -> [k]
+keys = omOrder
+
+assocs :: Ord k => OrderedMap k a -> [(k, a)]
+assocs = toList
+
+keysSet :: OrderedMap k a -> Set.Set k
+keysSet = Map.keysSet . omMap
+
+toList :: Ord k => OrderedMap k a -> [(k, a)]
+toList m = L.map (\k -> (k, omMap m Map.! k)) $ omOrder m
+
+toMap :: OrderedMap k a -> Map.Map k a
+toMap = omMap
+
+filter :: Ord k => (a -> Bool) -> OrderedMap k a -> OrderedMap k a
+filter f m =
+  let newMap = Map.filter f $ omMap m
+      newOrder = L.filter (`Set.member` Map.keysSet newMap) $ omOrder m
+  in  OrderedMap {omMap = newMap, omOrder = newOrder}