diff --git a/Hufflepdf.cabal b/Hufflepdf.cabal
index 710156b..c6e2477 100644
--- a/Hufflepdf.cabal
+++ b/Hufflepdf.cabal
@@ -17,6 +17,7 @@ cabal-version:       >=1.10
 
 library
   exposed-modules:     PDF
+                     , PDF.Box
                      , PDF.CMap
                      , PDF.Content
                      , PDF.EOL
diff --git a/src/PDF/Box.hs b/src/PDF/Box.hs
new file mode 100644
index 0000000..f88c2e1
--- /dev/null
+++ b/src/PDF/Box.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE UndecidableInstances #-}
+module PDF.Box (
+      Box(..)
+    , modifyAt
+  ) where
+
+import Control.Monad.State (MonadState(..))
+
+class Box m i a b | m a i -> b where
+  r :: i -> a -> m b
+  w :: i -> a -> b -> m a
+
+
+modifyAt :: (MonadState a m, Box m i a b) => i -> (b -> m b) -> m ()
+modifyAt i f = do
+  a <- get
+  r i a >>= f >>= w i a >>= put
+
+instance (Box m i a b, Box m j b c) => Box m (i, j) a c