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diff --git a/dht/src/Data/Sized.hs b/dht/src/Data/Sized.hs
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+module Data.Sized where
+
+import Data.Typeable
+
+
+-- | Info about a type's serialized length. Either the length is known
+-- independently of the value, or the length depends on the value.
+data Size a
+    = VarSize (a -> Int)
+    | ConstSize !Int
+    deriving Typeable
+
+class Sized a where size :: Size a
+