diff --git a/src/PDF/Box.hs b/src/PDF/Box.hs
index f43a3f8..efb2cc8 100644
--- a/src/PDF/Box.hs
+++ b/src/PDF/Box.hs
@@ -18,8 +18,10 @@ import Control.Monad.Except (ExceptT(..), runExceptT)
 import Control.Monad.Fail (MonadFail(..))
 import Control.Monad.State (MonadState(..))
 --import Control.Monad.Reader (MonadReader(..))
+import Data.Id (Id, IdMap)
+import qualified Data.Id as Id (insert, lookup)
 import Data.Map (Map)
-import qualified Data.Map as Map (insert, lookup, member)
+import qualified Data.Map as Map (insert, lookup)
 import Data.OrderedMap (OrderedMap)
 import qualified Data.OrderedMap as OrderedMap (lookup, set)
 import Prelude hiding (fail)
@@ -63,15 +65,16 @@ instance MonadFail m => Box m Index [a] a where
 
 instance (Ord k, MonadFail m) => Box m k (Map k a) a where
   r k = maybe (fail "Unknown key") return . Map.lookup k
-  w k a aMap
-    | Map.member k aMap =
-        return $ Map.insert k a aMap
-    | otherwise = fail "Unknown key"
+  w k a = return . Map.insert k a
 
 instance (Ord k, MonadFail m) => Box m k (OrderedMap k a) a where
   r k = maybe (fail "Unknown key") return . OrderedMap.lookup k
   w k a orderedMap = r k orderedMap >> return (OrderedMap.set k a orderedMap)
 
+instance MonadFail m => Box m (Id k) (IdMap k a) a where
+  r k = maybe (fail "Unknown key") return . Id.lookup k
+  w k a = return . Id.insert k a
+
 instance (Monad m, Box Maybe i a b) => Box m (Maybe_ i) a (Maybe b) where
   r (Maybe_ i) = return . r i
   w (Maybe_ i) (Just b) a =