diff --git a/COPYING b/COPYING
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--- /dev/null
+++ b/COPYING
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+		   GNU LESSER GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
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diff --git a/Data/BitSet.hs b/Data/BitSet.hs
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+-- | An efficient membership-testing module, for types that can be mapped into
+-- @Int@s.
+--
+-- The implementation is quite simple: we rely on the @Bits Integer@ instance
+-- from @Data.Bits@ for the three main operations.  An advantage of this
+-- library is the phantom parameter used in the BitSet type.  Since there is
+-- no exported way to construct a value of type `BitSet' directly, the
+-- interface we expose ensures client code will not typecheck if it confuses
+-- two bit sets intended to keep track of different types.
+--
+-- It is important that the values you intend to keep track of start from 0
+-- and go up.  Each @Int@ mapped to be hash corresponds to that bit location in
+-- an @Integer@, and thus requires that @Integer@ to have at least that many
+-- bits.  Don't shoot yourself in the foot.
+module Data.BitSet
+    ( Hash(..)
+    , BitSet
+    , empty
+    , insert
+    , delete
+    , member
+    ) where
+
+import Data.Bits
+
+import Test.QuickCheck
+
+-- | Map a value to an non-negative @Int@.
+--
+-- For the implementation to give reliable results, it must be that if @hash x
+-- == hash y@, @x@ and @y@ are equivalent under the relevant relation used in
+-- the client code.  (We don't depend on equality, but the client code will
+-- certainly want to use the above sort of inference.)
+--
+-- In fact, if your equivalence relation is @==@, the following quickcheck
+-- test should pass, for arbitrary @x@ and @y@:
+--
+-- @prop_hash x y =
+--     if hash x == hash y then x == y
+--     else x /= y
+--     && if x == y then hash x == hash y
+--        else hash x /= hash y@
+class Hash a where
+    hash :: a -> Int
+
+-- | The @Show@ instance kind of sucks.  It just shows the @Integer@
+-- representation.  A good show would probably show all the present hashes.
+newtype BitSet a = BS { unBS :: Integer }
+    deriving (Eq)
+
+instance Show (BitSet a) where
+    show s = "BitSet " ++ show (unBS s)
+
+-- | The empty bit set.
+empty :: BitSet a
+
+-- | /O(setBit on Integer)/ Insert an item into the bit set.
+insert :: Hash a => a -> BitSet a -> BitSet a
+
+-- | /O(clearBit on Integer)/ Delete an item from the bit set.
+delete :: Hash a => a -> BitSet a -> BitSet a
+
+-- | /O(testBit on Integer)/ Ask whether the item is in the bit set.
+member :: Hash a => a -> BitSet a -> Bool
+
+
+-- * Implementation
+
+empty = BS 0
+
+{-# INLINE insert #-}
+insert x s = BS $ setBit (unBS s) (hash x)
+
+{-# INLINE delete #-}
+delete x s = BS $ clearBit (unBS s) (hash x)
+
+{-# INLINE member #-}
+member x s = testBit (unBS s) (hash x)
+
+-- * Default instances
+
+instance Hash Int where
+    hash = id
+
+instance Hash Integer where
+    hash = fromIntegral
+
+-- Needs UndecidableInstances?
+-- instance Integral a => Hash a where
+--     hash = fromIntegral
+
+-- * Quickcheck properties
+
+instance Arbitrary (BitSet a) where
+    arbitrary = sized $ \n -> return $ BS $ fromIntegral n
+
+prop_insert x s = xa `member` insert xa s
+    where xa = abs x :: Int
+
+prop_delete x s = not $ xa `member` delete xa s
+    where xa = abs x :: Int
+
+prop_insDelIdempotent x = empty == (delete xa . insert xa $ empty)
+    where xa = abs x :: Int
+
+prop_delDelIdempotent x s = delete xa s == (delete xa . delete xa $ s)
+    where xa = abs x :: Int
+
+prop_insInsIdempotent x s = insert xa s == (insert xa . insert xa $ s)
+    where xa = abs x :: Int
+
+prop_extensional xs = and $ map (`member` s) xsa
+    where s = foldr insert empty xsa
+          xsa = map abs xs :: [Int]
+
+prop_empty x = not $ xa `member` empty
+    where xa = abs x :: Int
diff --git a/Setup.lhs b/Setup.lhs
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--- /dev/null
+++ b/Setup.lhs
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+#! /usr/bin/env runhaskell
+
+> import Distribution.Simple
+> main = defaultMain
diff --git a/bitset.cabal b/bitset.cabal
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+Name:                bitset
+Version:             0.5
+Synopsis:            A functional data structure for efficient membership testing.
+Description:         A bit set is a data structure for keeping membership information efficiently.  This implementation uses Data.Bits on Integers, and thus should achieve reasonable speed.
+Category:            Data Structures
+License:             LGPL
+License-file:        COPYING
+Author:              Denis Bueno
+Maintainer:          Denis Bueno <dbueno@gmail.com>
+Stability:           provisional
+Build-Depends:       base, QuickCheck
+Build-type:          Simple
+
+Exposed-modules:     Data.BitSet
+Ghc-options:         -W