diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright (c) 2016, Athan Clark
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Athan Clark nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/pred-set.cabal b/pred-set.cabal
new file mode 100644
--- /dev/null
+++ b/pred-set.cabal
@@ -0,0 +1,24 @@
+Name:                   pred-set
+Version:                0.0.0
+Author:                 Athan Clark <athan.clark@gmail.com>
+Maintainer:             Athan Clark <athan.clark@gmail.com>
+License:                BSD3
+License-File:           LICENSE
+Synopsis:               Simple cached predicates
+Cabal-Version:          >= 1.10
+Build-Type:             Simple
+Category:               Data
+
+Library
+  Default-Language:     Haskell2010
+  HS-Source-Dirs:       src
+  GHC-Options:          -Wall
+  Exposed-Modules:      Data.PredSet.Mutable
+  Build-Depends:        base >= 4.8 && < 5
+                      , hashtables
+                      , hashable
+                      , HSet
+
+Source-Repository head
+  Type:                 git
+  Location:             https://github.com/athanclark/pred-set
diff --git a/src/Data/PredSet/Mutable.hs b/src/Data/PredSet/Mutable.hs
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--- /dev/null
+++ b/src/Data/PredSet/Mutable.hs
@@ -0,0 +1,70 @@
+module Data.PredSet.Mutable where
+
+import Prelude hiding (lookup)
+import Data.Monoid
+
+import           Data.HSet.Mutable (HSet, HKey)
+import qualified Data.HSet.Mutable as HS
+import           Data.HashTable.ST.Cuckoo (HashTable)
+import qualified Data.HashTable.ST.Cuckoo as HT
+import Data.Typeable
+import Data.Hashable
+import Control.Monad.ST
+
+
+
+data CachedPred s k a = CachedPred
+  { cachedPredPred  :: !(k -> Maybe a)
+  , cachedPredCache :: {-# UNPACK #-} !(HashTable s k a)
+  }
+
+newCachedPred :: (k -> Maybe a)
+              -> ST s (CachedPred s k a)
+newCachedPred pred' = CachedPred pred' <$> HT.new
+
+query :: ( Eq k
+         , Hashable k
+         ) => k
+           -> CachedPred s k a
+           -> ST s (Maybe a)
+query k (CachedPred pred' cache) =
+      (\mx -> getFirst $! First mx <> First (pred' k))
+  <$> HT.lookup cache k
+
+
+newtype PredSet s k = PredSet
+  { getPredSet :: HSet s
+  }
+
+newtype PredKey s k a = PredKey
+  { getPredKey :: HKey (CachedPred s k a)
+  }
+
+
+new :: ST s (PredSet s k)
+new = PredSet <$> HS.new
+
+insert :: ( Typeable k
+          , Typeable a
+          , Typeable s
+          ) => (k -> Maybe a)
+            -> PredSet s k
+            -> ST s (PredKey s k a)
+insert pred' (PredSet xs) = do
+  cache <- newCachedPred pred'
+  PredKey <$> HS.insert cache xs
+
+lookup :: ( Eq k
+          , Hashable k
+          , Typeable s
+          , Typeable k
+          , Typeable a
+          ) => PredKey s k a
+            -> k
+            -> PredSet s k
+            -> ST s (Maybe a)
+lookup (PredKey i) k (PredSet xs) = do
+  mCachedPred <- HS.lookup i xs
+  case mCachedPred of
+    Nothing    -> pure Nothing
+    Just cache -> query k cache