diff --git a/Acme/RealWorld.hs b/Acme/RealWorld.hs
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--- /dev/null
+++ b/Acme/RealWorld.hs
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+-- |
+-- Module:      Acme.RealWorld
+-- Copyright:   (c) Joseph Adams 2011
+-- Maintainer:  joeyadams3.14159@gmail.com
+-- Portability: GHC-only
+--
+-- Primitives for manipulating the state of the universe.
+
+{-# LANGUAGE MagicHash, UnboxedTuples #-}
+module Acme.RealWorld (
+    -- * The RealWorld type
+    RealWorld,
+
+    -- * Universe manipulation primitives
+    getWorld,
+    putWorld,
+    execIO,
+
+    -- * Derived combinators
+    hypothetically,
+) where
+
+import Control.Exception (bracket)
+import GHC.IO
+import GHC.Exts
+
+fromState :: State# RealWorld -> RealWorld
+fromState = unsafeCoerce#
+
+toState :: RealWorld -> State# RealWorld
+toState = unsafeCoerce#
+
+-- | Retrieve the current state of the universe.
+getWorld :: IO RealWorld
+getWorld = IO (\s -> (# s, fromState s #))
+
+-- | Set the current state of the universe.  Program values are not affected by
+-- this operation, but the rest of the universe is.
+--
+-- 'putWorld' may not be called on the same state twice (this is enforced by
+-- the runtime system).  Otherwise, it would be possible to trap the universe
+-- in a temporal loop:
+--
+-- >getWorld >>= forever . putWorld
+putWorld :: RealWorld -> IO ()
+putWorld s' = IO (\_ -> (# toState s', () #))
+
+-- | Given an action, construct a function that, given a state of the universe,
+-- returns the state of the universe after the action has occurred.
+--
+-- Example:
+--
+-- >main = do
+-- >    let f = execIO $ putStrLn "Second"
+-- >        g = execIO $ putStrLn "First"
+-- >        getWorld >>= putWorld . f . g
+execIO :: IO a -> RealWorld -> RealWorld
+execIO (IO k) = \w -> case k (toState w) of (# s', _ #) -> fromState s'
+
+-- | Perform an action and return its value, but undo any side effects to the
+-- universe.  Thus, it appears to return instantly, regardless of how long the
+-- action would take to run.
+--
+-- The caller must ensure that the program /would/ have enough time to perform
+-- the computation.  Otherwise, either an exception will be thrown, or the
+-- operation will block because it never gets a chance to restore the original
+-- state of the universe.
+hypothetically :: IO a -> IO a
+hypothetically action = bracket getWorld putWorld (\_ -> action)
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
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+Copyright (c) 2011, Joseph Adams
+
+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 Joseph Adams 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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+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/acme-realworld.cabal b/acme-realworld.cabal
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+++ b/acme-realworld.cabal
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+name:                acme-realworld
+version:             0.1
+synopsis:            Primitives for manipulating the state of the universe
+description:
+    GHC represents an IO action internally as a function from one state of the
+    world to the next:
+    .
+    >State# RealWorld -> (# State# RealWorld, a #)
+    .
+    This module provides a very useful capability stemming directly from this
+    representation: the ability to save the current state of the universe and
+    restore it later.  This provides a way to \"undo\" certain types of side
+    effects in the IO monad:
+    .
+    >import Acme.Missiles
+    >import Acme.RealWorld
+    >
+    >main :: IO ()
+    >main = do
+    >    -- Save the current state of the universe
+    >    world_as_we_know_it <- getWorld
+    >
+    >    -- Cause serious international side effects
+    >    launchMissiles
+    >
+    >    -- After realizing that was a terrible, terrible mistake, restore the
+    >    -- pre-war state of the universe.
+    >    putWorld world_as_we_know_it
+license:             BSD3
+license-file:        LICENSE
+author:              Joey Adams
+maintainer:          joeyadams3.14159@gmail.com
+copyright:           Copyright (c) Joseph Adams 2011
+category:            Acme
+build-type:          Simple
+cabal-version:       >=1.8
+
+library
+    exposed-modules:    Acme.RealWorld
+    build-depends:      base == 4.*