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diff --git a/packages/code/hubris/src/Hubris/Collections/LRUCache.hs b/packages/code/hubris/src/Hubris/Collections/LRUCache.hs
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+++ b/packages/code/hubris/src/Hubris/Collections/LRUCache.hs
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+module Hubris.Collections.LRUCache
+  ( empty
+  , insert
+  ) where
+
+import qualified Data.Map as M
+import Data.Monoid (Sum (..))
+
+type LRUCache a = M.Map a (Sum Int)
+
+empty :: LRUCache a
+empty = M.empty
+
+insert :: a -> LRUCache a -> LRUCache a
+insert = M.alter (fmap $ const 0)
+  where