diff --git a/dejafu-tests/Cases/MultiThreaded.hs b/dejafu-tests/Cases/MultiThreaded.hs
index e84b940..43e602b 100644
--- a/dejafu-tests/Cases/MultiThreaded.hs
+++ b/dejafu-tests/Cases/MultiThreaded.hs
@@ -11,6 +11,7 @@ import Test.HUnit.DejaFu (testDejafu)
 
 import Control.Concurrent.Classy
 import Control.Monad.STM.Class
+import Test.DejaFu.Conc (Conc, subconcurrency)
 
 #if __GLASGOW_HASKELL__ < 710
 import Control.Applicative ((<$>), (<*>))
@@ -49,6 +50,13 @@ tests =
   , testGroup "Daemons" . hUnitTestToTests $ test
     [ testDejafu schedDaemon "schedule daemon" $ gives' [0,1]
     ]
+
+  , testGroup "Subconcurrency" . hUnitTestToTests $ test
+    [ testDejafu scDeadlock1 "deadlock1" $ gives' [Left Deadlock, Right ()]
+    , testDejafu scDeadlock2 "deadlock2" $ gives' [(Left Deadlock, ()), (Right (), ())]
+    , testDejafu scSuccess   "success"   $ gives' [Right ()]
+    , testDejafu scIllegal   "illegal"   $ gives  [Left IllegalSubconcurrency]
+    ]
   ]
 
 --------------------------------------------------------------------------------
@@ -207,3 +215,40 @@ schedDaemon = do
   x <- newCRef 0
   _ <- fork $ myThreadId >> writeCRef x 1
   readCRef x
+
+--------------------------------------------------------------------------------
+-- Subconcurrency
+
+-- | Subcomputation deadlocks sometimes.
+scDeadlock1 :: Monad n => Conc n r (Either Failure ())
+scDeadlock1 = do
+  var <- newEmptyMVar
+  subconcurrency $ do
+    void . fork $ putMVar var ()
+    putMVar var ()
+
+-- | Subcomputation deadlocks sometimes, and action after it still
+-- happens.
+scDeadlock2 :: Monad n => Conc n r (Either Failure (), ())
+scDeadlock2 = do
+  var <- newEmptyMVar
+  res <- subconcurrency $ do
+    void . fork $ putMVar var ()
+    putMVar var ()
+  (,) <$> pure res <*> readMVar var
+
+-- | Subcomputation successfully completes.
+scSuccess :: Monad n => Conc n r (Either Failure ())
+scSuccess = do
+  var <- newMVar ()
+  subconcurrency $ do
+    out <- newEmptyMVar
+    void . fork $ takeMVar var >>= putMVar out
+    takeMVar out
+
+-- | Illegal usage
+scIllegal :: Monad n => Conc n r ()
+scIllegal = do
+  var <- newEmptyMVar
+  void . fork $ readMVar var
+  void . subconcurrency $ pure ()
diff --git a/dejafu-tests/Cases/SingleThreaded.hs b/dejafu-tests/Cases/SingleThreaded.hs
index 8ea54ee..698f0d7 100644
--- a/dejafu-tests/Cases/SingleThreaded.hs
+++ b/dejafu-tests/Cases/SingleThreaded.hs
@@ -3,6 +3,7 @@
 module Cases.SingleThreaded where
 
 import Control.Exception (ArithException(..), ArrayException(..))
+import Control.Monad (void)
 import Test.DejaFu (Failure(..), gives, gives')
 import Test.Framework (Test, testGroup)
 import Test.Framework.Providers.HUnit (hUnitTestToTests)
@@ -10,7 +11,7 @@ import Test.HUnit (test)
 import Test.HUnit.DejaFu (testDejafu)
 
 import Control.Concurrent.Classy
-import Control.Monad.STM.Class
+import Test.DejaFu.Conc (Conc, subconcurrency)
 
 import Utils
 
@@ -58,6 +59,12 @@ tests =
     [ testDejafu capsGet "get" $ gives' [True]
     , testDejafu capsSet "set" $ gives' [True]
     ]
+
+  , testGroup "Subconcurrency" . hUnitTestToTests $ test
+    [ testDejafu scDeadlock1 "deadlock1" $ gives' [Left Deadlock]
+    , testDejafu scDeadlock2 "deadlock2" $ gives' [(Left Deadlock, ())]
+    , testDejafu scSuccess   "success"   $ gives' [Right ()]
+    ]
   ]
 
 --------------------------------------------------------------------------------
@@ -252,3 +259,22 @@ capsSet = do
   caps <- getNumCapabilities
   setNumCapabilities $ caps + 1
   (== caps + 1) <$> getNumCapabilities
+
+--------------------------------------------------------------------------------
+-- Subconcurrency
+
+-- | Subcomputation deadlocks.
+scDeadlock1 :: Monad n => Conc n r (Either Failure ())
+scDeadlock1 = subconcurrency (newEmptyMVar >>= readMVar)
+
+-- | Subcomputation deadlocks, and action after it still happens.
+scDeadlock2 :: Monad n => Conc n r (Either Failure (), ())
+scDeadlock2 = do
+  var <- newMVar ()
+  (,) <$> subconcurrency (putMVar var ()) <*> readMVar var
+
+-- | Subcomputation successfully completes.
+scSuccess :: Monad n => Conc n r (Either Failure ())
+scSuccess = do
+  var <- newMVar ()
+  subconcurrency (takeMVar var)
diff --git a/dejafu/Test/DejaFu.hs b/dejafu/Test/DejaFu.hs
index 51c3a2a..28af201 100644
--- a/dejafu/Test/DejaFu.hs
+++ b/dejafu/Test/DejaFu.hs
@@ -240,7 +240,6 @@ module Test.DejaFu
   ) where
 
 import Control.Arrow (first)
-import Control.DeepSeq (NFData(..))
 import Control.Monad (when, unless)
 import Control.Monad.Ref (MonadRef)
 import Control.Monad.ST (runST)
@@ -400,9 +399,6 @@ defaultFail failures = Result False 0 failures ""
 defaultPass :: Result a
 defaultPass = Result True 0 [] ""
 
-instance NFData a => NFData (Result a) where
-  rnf r = rnf (_pass r, _casesChecked r, _failures r, _failureMsg r)
-
 instance Functor Result where
   fmap f r = r { _failures = map (first $ fmap f) $ _failures r }
 
diff --git a/dejafu/Test/DejaFu/Common.hs b/dejafu/Test/DejaFu/Common.hs
index 3a17a9e..ae67cfe 100644
--- a/dejafu/Test/DejaFu/Common.hs
+++ b/dejafu/Test/DejaFu/Common.hs
@@ -60,7 +60,6 @@ module Test.DejaFu.Common
   , MemType(..)
   ) where
 
-import Control.DeepSeq (NFData(..))
 import Control.Exception (MaskingState(..))
 import Data.Dynamic (Dynamic)
 import Data.List (sort, nub, intercalate)
@@ -83,9 +82,6 @@ instance Show ThreadId where
   show (ThreadId (Just n) _) = n
   show (ThreadId Nothing  i) = show i
 
-instance NFData ThreadId where
-  rnf (ThreadId n i) = rnf (n, i)
-
 -- | Every @CRef@ has a unique identifier.
 data CRefId = CRefId (Maybe String) Int
   deriving Eq
@@ -97,9 +93,6 @@ instance Show CRefId where
   show (CRefId (Just n) _) = n
   show (CRefId Nothing  i) = show i
 
-instance NFData CRefId where
-  rnf (CRefId n i) = rnf (n, i)
-
 -- | Every @MVar@ has a unique identifier.
 data MVarId = MVarId (Maybe String) Int
   deriving Eq
@@ -111,9 +104,6 @@ instance Show MVarId where
   show (MVarId (Just n) _) = n
   show (MVarId Nothing  i) = show i
 
-instance NFData MVarId where
-  rnf (MVarId n i) = rnf (n, i)
-
 -- | Every @TVar@ has a unique identifier.
 data TVarId = TVarId (Maybe String) Int
   deriving Eq
@@ -125,9 +115,6 @@ instance Show TVarId where
   show (TVarId (Just n) _) = n
   show (TVarId Nothing  i) = show i
 
-instance NFData TVarId where
-  rnf (TVarId n i) = rnf (n, i)
-
 -- | The ID of the initial thread.
 initialThread :: ThreadId
 initialThread = ThreadId (Just "main") 0
@@ -272,38 +259,10 @@ data ThreadAction =
   -- ^ A '_concMessage' annotation was processed.
   | Stop
   -- ^ Cease execution and terminate.
+  | Subconcurrency
+  -- ^ Start executing an action with @subconcurrency@.
   deriving Show
 
-instance NFData ThreadAction where
-  rnf (Fork t) = rnf t
-  rnf (GetNumCapabilities i) = rnf i
-  rnf (SetNumCapabilities i) = rnf i
-  rnf (NewVar c) = rnf c
-  rnf (PutVar c ts) = rnf (c, ts)
-  rnf (BlockedPutVar c) = rnf c
-  rnf (TryPutVar c b ts) = rnf (c, b, ts)
-  rnf (ReadVar c) = rnf c
-  rnf (BlockedReadVar c) = rnf c
-  rnf (TakeVar c ts) = rnf (c, ts)
-  rnf (BlockedTakeVar c) = rnf c
-  rnf (TryTakeVar c b ts) = rnf (c, b, ts)
-  rnf (NewRef c) = rnf c
-  rnf (ReadRef c) = rnf c
-  rnf (ReadRefCas c) = rnf c
-  rnf (ModRef c) = rnf c
-  rnf (ModRefCas c) = rnf c
-  rnf (WriteRef c) = rnf c
-  rnf (CasRef c b) = rnf (c, b)
-  rnf (CommitRef t c) = rnf (t, c)
-  rnf (STM s ts) = rnf (s, ts)
-  rnf (BlockedSTM s) = rnf s
-  rnf (ThrowTo t) = rnf t
-  rnf (BlockedThrowTo t) = rnf t
-  rnf (SetMasking b m) = b `seq` m `seq` ()
-  rnf (ResetMasking b m) = b `seq` m `seq` ()
-  rnf (Message m) = m `seq` ()
-  rnf a = a `seq` ()
-
 -- | Check if a @ThreadAction@ immediately blocks.
 isBlock :: ThreadAction -> Bool
 isBlock (BlockedThrowTo  _) = True
@@ -401,28 +360,10 @@ data Lookahead =
   -- ^ Will process a _concMessage' annotation.
   | WillStop
   -- ^ Will cease execution and terminate.
+  | WillSubconcurrency
+  -- ^ Will execute an action with @subconcurrency@.
   deriving Show
 
-instance NFData Lookahead where
-  rnf (WillSetNumCapabilities i) = rnf i
-  rnf (WillPutVar c) = rnf c
-  rnf (WillTryPutVar c) = rnf c
-  rnf (WillReadVar c) = rnf c
-  rnf (WillTakeVar c) = rnf c
-  rnf (WillTryTakeVar c) = rnf c
-  rnf (WillReadRef c) = rnf c
-  rnf (WillReadRefCas c) = rnf c
-  rnf (WillModRef c) = rnf c
-  rnf (WillModRefCas c) = rnf c
-  rnf (WillWriteRef c) = rnf c
-  rnf (WillCasRef c) = rnf c
-  rnf (WillCommitRef t c) = rnf (t, c)
-  rnf (WillThrowTo t) = rnf t
-  rnf (WillSetMasking b m) = b `seq` m `seq` ()
-  rnf (WillResetMasking b m) = b `seq` m `seq` ()
-  rnf (WillMessage m) = m `seq` ()
-  rnf l = l `seq` ()
-
 -- | Convert a 'ThreadAction' into a 'Lookahead': \"rewind\" what has
 -- happened. 'Killed' has no 'Lookahead' counterpart.
 rewind :: ThreadAction -> Maybe Lookahead
@@ -462,6 +403,7 @@ rewind LiftIO = Just WillLiftIO
 rewind Return = Just WillReturn
 rewind (Message m) = Just (WillMessage m)
 rewind Stop = Just WillStop
+rewind Subconcurrency = Just WillSubconcurrency
 
 -- | Check if an operation could enable another thread.
 willRelease :: Lookahead -> Bool
@@ -508,17 +450,6 @@ data ActionType =
   -- communication.
   deriving (Eq, Show)
 
-instance NFData ActionType where
-  rnf (UnsynchronisedRead  r) = rnf r
-  rnf (UnsynchronisedWrite r) = rnf r
-  rnf (PartiallySynchronisedCommit r) = rnf r
-  rnf (PartiallySynchronisedWrite  r) = rnf r
-  rnf (PartiallySynchronisedModify  r) = rnf r
-  rnf (SynchronisedModify  r) = rnf r
-  rnf (SynchronisedRead    c) = rnf c
-  rnf (SynchronisedWrite   c) = rnf c
-  rnf a = a `seq` ()
-
 -- | Check if an action imposes a write barrier.
 isBarrier :: ActionType -> Bool
 isBarrier (SynchronisedModify _) = True
@@ -611,13 +542,6 @@ data TAction =
   -- ^ Terminate successfully and commit effects.
   deriving (Eq, Show)
 
-instance NFData TAction where
-  rnf (TRead  v) = rnf v
-  rnf (TWrite v) = rnf v
-  rnf (TCatch  s m) = rnf (s, m)
-  rnf (TOrElse s m) = rnf (s, m)
-  rnf a = a `seq` ()
-
 -------------------------------------------------------------------------------
 -- Traces
 
@@ -640,11 +564,6 @@ data Decision =
   -- ^ Pre-empt the running thread, and switch to another.
   deriving (Eq, Show)
 
-instance NFData Decision where
-  rnf (Start    tid) = rnf tid
-  rnf (SwitchTo tid) = rnf tid
-  rnf d = d `seq` ()
-
 -- | Pretty-print a trace, including a key of the thread IDs (not
 -- including thread 0). Each line of the key is indented by two
 -- spaces.
@@ -675,15 +594,15 @@ showTrace trc = intercalate "\n" $ concatMap go trc : strkey where
 preEmpCount :: [(Decision, ThreadAction)]
             -> (Decision, Lookahead)
             -> Int
-preEmpCount ts (d, _) = go initialThread Nothing ts where
-  go _ (Just Yield) ((SwitchTo t, a):rest) = go t (Just a) rest
-  go tid prior ((SwitchTo t, a):rest)
+preEmpCount (x:xs) (d, _) = go initialThread x xs where
+  go _ (_, Yield) (r@(SwitchTo t, _):rest) = go t r rest
+  go tid prior (r@(SwitchTo t, _):rest)
     | isCommitThread t = go tid prior (skip rest)
-    | otherwise = 1 + go t (Just a) rest
-  go _   _ ((Start t,  a):rest) = go t   (Just a) rest
-  go tid _ ((Continue, a):rest) = go tid (Just a) rest
+    | otherwise = 1 + go t r rest
+  go _   _ (r@(Start t,  _):rest) = go t   r rest
+  go tid _ (r@(Continue, _):rest) = go tid r rest
   go _ prior [] = case (prior, d) of
-    (Just Yield, SwitchTo _) -> 0
+    ((_, Yield), SwitchTo _) -> 0
     (_, SwitchTo _) -> 1
     _ -> 0
 
@@ -694,6 +613,7 @@ preEmpCount ts (d, _) = go initialThread Nothing ts where
   skip = dropWhile (not . isContextSwitch . fst)
   isContextSwitch Continue = False
   isContextSwitch _ = True
+preEmpCount [] _ = 0
 
 -------------------------------------------------------------------------------
 -- Failures
@@ -716,18 +636,19 @@ data Failure =
   -- ^ The computation became blocked indefinitely on @TVar@s.
   | UncaughtException
   -- ^ An uncaught exception bubbled to the top of the computation.
+  | IllegalSubconcurrency
+  -- ^ Calls to @subconcurrency@ were nested, or attempted when
+  -- multiple threads existed.
   deriving (Eq, Show, Read, Ord, Enum, Bounded)
 
-instance NFData Failure where
-  rnf f = f `seq` ()
-
 -- | Pretty-print a failure
 showFail :: Failure -> String
-showFail Abort             = "[abort]"
-showFail Deadlock          = "[deadlock]"
-showFail STMDeadlock       = "[stm-deadlock]"
-showFail InternalError     = "[internal-error]"
+showFail Abort = "[abort]"
+showFail Deadlock = "[deadlock]"
+showFail STMDeadlock = "[stm-deadlock]"
+showFail InternalError = "[internal-error]"
 showFail UncaughtException = "[exception]"
+showFail IllegalSubconcurrency = "[illegal-subconcurrency]"
 
 -------------------------------------------------------------------------------
 -- Memory Models
@@ -751,9 +672,6 @@ data MemType =
   -- created.
   deriving (Eq, Show, Read, Ord, Enum, Bounded)
 
-instance NFData MemType where
-  rnf m = m `seq` ()
-
 -------------------------------------------------------------------------------
 -- Utilities
 
diff --git a/dejafu/Test/DejaFu/Conc.hs b/dejafu/Test/DejaFu/Conc.hs
index 6ecdc25..58cae52 100755
--- a/dejafu/Test/DejaFu/Conc.hs
+++ b/dejafu/Test/DejaFu/Conc.hs
@@ -28,6 +28,7 @@ module Test.DejaFu.Conc
   , Failure(..)
   , MemType(..)
   , runConcurrent
+  , subconcurrency
 
   -- * Execution traces
   , Trace
@@ -49,12 +50,11 @@ import Control.Exception (MaskingState(..))
 import qualified Control.Monad.Base as Ba
 import qualified Control.Monad.Catch as Ca
 import qualified Control.Monad.IO.Class as IO
-import Control.Monad.Ref (MonadRef, newRef, readRef, writeRef)
+import Control.Monad.Ref (MonadRef,)
 import Control.Monad.ST (ST)
 import Data.Dynamic (toDyn)
+import qualified Data.Foldable as F
 import Data.IORef (IORef)
-import qualified Data.Map.Strict as M
-import Data.Maybe (fromJust)
 import Data.STRef (STRef)
 import Test.DejaFu.Schedule
 
@@ -62,13 +62,12 @@ import qualified Control.Monad.Conc.Class as C
 import Test.DejaFu.Common
 import Test.DejaFu.Conc.Internal
 import Test.DejaFu.Conc.Internal.Common
-import Test.DejaFu.Conc.Internal.Threading
 import Test.DejaFu.STM
 
 {-# ANN module ("HLint: ignore Avoid lambda" :: String) #-}
 {-# ANN module ("HLint: ignore Use const"    :: String) #-}
 
-newtype Conc n r a = C { unC :: M n r (STMLike n r) a } deriving (Functor, Applicative, Monad)
+newtype Conc n r a = C { unC :: M n r a } deriving (Functor, Applicative, Monad)
 
 -- | A 'MonadConc' implementation using @ST@, this should be preferred
 -- if you do not need 'liftIO'.
@@ -77,10 +76,10 @@ type ConcST t = Conc (ST t) (STRef t)
 -- | A 'MonadConc' implementation using @IO@.
 type ConcIO = Conc IO IORef
 
-toConc :: ((a -> Action n r (STMLike n r)) -> Action n r (STMLike n r)) -> Conc n r a
+toConc :: ((a -> Action n r) -> Action n r) -> Conc n r a
 toConc = C . cont
 
-wrap :: (M n r (STMLike n r) a -> M n r (STMLike n r) a) -> Conc n r a -> Conc n r a
+wrap :: (M n r a -> M n r a) -> Conc n r a -> Conc n r a
 wrap f = C . f . unC
 
 instance IO.MonadIO ConcIO where
@@ -181,20 +180,15 @@ runConcurrent :: MonadRef r n
               -> s
               -> Conc n r a
               -> n (Either Failure a, s, Trace)
-runConcurrent sched memtype s (C conc) = do
-  ref <- newRef Nothing
+runConcurrent sched memtype s ma = do
+  (res, s', trace) <- runConcurrency sched memtype s (unC ma)
+  pure (res, s', F.toList trace)
 
-  let c = runCont conc (AStop . writeRef ref . Just . Right)
-  let threads = launch' Unmasked initialThread (const c) M.empty
-
-  (s', trace) <- runThreads runTransaction
-                           sched
-                           memtype
-                           s
-                           threads
-                           initialIdSource
-                           ref
-
-  out <- readRef ref
-
-  pure (fromJust out, s', reverse trace)
+-- | Run a concurrent computation and return its result.
+--
+-- This can only be called in the main thread, when no other threads
+-- exist. Calls to 'subconcurrency' cannot be nested. Violating either
+-- of these conditions will result in the computation failing with
+-- @IllegalSubconcurrency@.
+subconcurrency :: Conc n r a -> Conc n r (Either Failure a)
+subconcurrency ma = toConc (ASub (unC ma))
diff --git a/dejafu/Test/DejaFu/Conc/Internal.hs b/dejafu/Test/DejaFu/Conc/Internal.hs
index b3a2af7..7b821e8 100755
--- a/dejafu/Test/DejaFu/Conc/Internal.hs
+++ b/dejafu/Test/DejaFu/Conc/Internal.hs
@@ -16,19 +16,23 @@
 module Test.DejaFu.Conc.Internal where
 
 import Control.Exception (MaskingState(..), toException)
-import Control.Monad.Ref (MonadRef, newRef, writeRef)
+import Control.Monad.Ref (MonadRef, newRef, readRef, writeRef)
+import qualified Data.Foldable as F
 import Data.Functor (void)
 import Data.List (sort)
 import Data.List.NonEmpty (NonEmpty(..), fromList)
 import qualified Data.Map.Strict as M
-import Data.Maybe (fromJust, isJust, isNothing, listToMaybe)
+import Data.Maybe (fromJust, isJust, isNothing)
+import Data.Monoid ((<>))
+import Data.Sequence (Seq, (<|))
+import qualified Data.Sequence as Seq
 
 import Test.DejaFu.Common
 import Test.DejaFu.Conc.Internal.Common
 import Test.DejaFu.Conc.Internal.Memory
 import Test.DejaFu.Conc.Internal.Threading
 import Test.DejaFu.Schedule
-import Test.DejaFu.STM (Result(..))
+import Test.DejaFu.STM (Result(..), runTransaction)
 
 {-# ANN module ("HLint: ignore Use record patterns" :: String) #-}
 {-# ANN module ("HLint: ignore Use const"           :: String) #-}
@@ -36,40 +40,69 @@ import Test.DejaFu.STM (Result(..))
 --------------------------------------------------------------------------------
 -- * Execution
 
+-- | 'Trace' but as a sequence.
+type SeqTrace
+  = Seq (Decision, [(ThreadId, NonEmpty Lookahead)], ThreadAction)
+
+-- | Run a concurrent computation with a given 'Scheduler' and initial
+-- state, returning a failure reason on error. Also returned is the
+-- final state of the scheduler, and an execution trace.
+runConcurrency :: MonadRef r n
+               => Scheduler g
+               -> MemType
+               -> g
+               -> M n r a
+               -> n (Either Failure a, g, SeqTrace)
+runConcurrency sched memtype g ma = do
+  ref <- newRef Nothing
+
+  let c = runCont ma (AStop . writeRef ref . Just . Right)
+  let threads = launch' Unmasked initialThread (const c) M.empty
+  let ctx = Context { cSchedState = g, cIdSource = initialIdSource, cThreads = threads, cWriteBuf = emptyBuffer, cCaps = 2 }
+
+  (finalCtx, trace) <- runThreads sched memtype ref ctx
+  out <- readRef ref
+  pure (fromJust out, cSchedState finalCtx, trace)
+
+-- | The context a collection of threads are running in.
+data Context n r g = Context
+  { cSchedState :: g
+  , cIdSource   :: IdSource
+  , cThreads    :: Threads n r
+  , cWriteBuf   :: WriteBuffer r
+  , cCaps       :: Int
+  }
+
 -- | Run a collection of threads, until there are no threads left.
---
--- Note: this returns the trace in reverse order, because it's more
--- efficient to prepend to a list than append. As this function isn't
--- exposed to users of the library, this is just an internal gotcha to
--- watch out for.
-runThreads :: MonadRef r n => (forall x. s x -> IdSource -> n (Result x, IdSource, TTrace))
-           -> Scheduler g -> MemType -> g -> Threads n r s -> IdSource -> r (Maybe (Either Failure a)) -> n (g, Trace)
-runThreads runstm sched memtype origg origthreads idsrc ref = go idsrc [] Nothing origg origthreads emptyBuffer 2 where
-  go idSource sofar prior g threads wb caps
-    | isTerminated  = stop g
-    | isDeadlocked  = die g Deadlock
-    | isSTMLocked   = die g STMDeadlock
-    | isAborted     = die g' Abort
-    | isNonexistant = die g' InternalError
-    | isBlocked     = die g' InternalError
+runThreads :: MonadRef r n
+           => Scheduler g -> MemType -> r (Maybe (Either Failure a)) -> Context n r g -> n (Context n r g, SeqTrace)
+runThreads sched memtype ref = go Seq.empty [] Nothing where
+  -- sofar is the 'SeqTrace', sofarSched is the @[(Decision,
+  -- ThreadAction)]@ trace the scheduler needs.
+  go sofar sofarSched prior ctx
+    | isTerminated  = stop ctx
+    | isDeadlocked  = die Deadlock ctx
+    | isSTMLocked   = die STMDeadlock ctx
+    | isAborted     = die Abort $ ctx { cSchedState = g' }
+    | isNonexistant = die InternalError $ ctx { cSchedState = g' }
+    | isBlocked     = die InternalError $ ctx { cSchedState = g' }
     | otherwise = do
-      stepped <- stepThread runstm memtype (_continuation $ fromJust thread) idSource chosen threads wb caps
+      stepped <- stepThread sched memtype chosen (_continuation $ fromJust thread) $ ctx { cSchedState = g' }
       case stepped of
-        Right (threads', idSource', act, wb', caps') -> loop threads' idSource' act wb' caps'
-
+        Right (ctx', actOrTrc) -> loop actOrTrc ctx'
         Left UncaughtException
-          | chosen == initialThread -> die g' UncaughtException
-          | otherwise -> loop (kill chosen threads) idSource Killed wb caps
-
-        Left failure -> die g' failure
+          | chosen == initialThread -> die UncaughtException $ ctx { cSchedState = g' }
+          | otherwise -> loop (Right Killed) $ ctx { cThreads = kill chosen threadsc, cSchedState = g' }
+        Left failure -> die failure $ ctx { cSchedState = g' }
 
     where
-      (choice, g')  = sched (map (\(d,_,a) -> (d,a)) $ reverse sofar) ((\p (_,_,a) -> (p,a)) <$> prior <*> listToMaybe sofar) (fromList $ map (\(t,l:|_) -> (t,l)) runnable') g
+      (choice, g')  = sched sofarSched prior (fromList $ map (\(t,l:|_) -> (t,l)) runnable') (cSchedState ctx)
       chosen        = fromJust choice
       runnable'     = [(t, nextActions t) | t <- sort $ M.keys runnable]
       runnable      = M.filter (isNothing . _blocking) threadsc
       thread        = M.lookup chosen threadsc
-      threadsc      = addCommitThreads wb threads
+      threadsc      = addCommitThreads (cWriteBuf ctx) threads
+      threads       = cThreads ctx
       isAborted     = isNothing choice
       isBlocked     = isJust . _blocking $ fromJust thread
       isNonexistant = isNothing thread
@@ -87,299 +120,262 @@ runThreads runstm sched memtype origg origthreads idsrc ref = go idsrc [] Nothin
           _ -> thrd
 
       decision
-        | Just chosen == prior = Continue
-        | prior `notElem` map (Just . fst) runnable' = Start chosen
+        | Just chosen == (fst <$> prior) = Continue
+        | (fst <$> prior) `notElem` map (Just . fst) runnable' = Start chosen
         | otherwise = SwitchTo chosen
 
       nextActions t = lookahead . _continuation . fromJust $ M.lookup t threadsc
 
-      stop outg = pure (outg, sofar)
-      die  outg reason = writeRef ref (Just $ Left reason) >> stop outg
+      stop finalCtx = pure (finalCtx, sofar)
+      die reason finalCtx = writeRef ref (Just $ Left reason) >> stop finalCtx
 
-      loop threads' idSource' act wb' =
-        let sofar' = ((decision, runnable', act) : sofar)
-            threads'' = if (interruptible <$> M.lookup chosen threads') /= Just False then unblockWaitingOn chosen threads' else threads'
-        in go idSource' sofar' (Just chosen) g' (delCommitThreads threads'') wb'
+      loop trcOrAct ctx' =
+        let (act, trc) = case trcOrAct of
+              Left (a, as) -> (a, (decision, runnable', a) <| as)
+              Right a      -> (a, Seq.singleton (decision, runnable', a))
+            threads' = if (interruptible <$> M.lookup chosen (cThreads ctx')) /= Just False
+                       then unblockWaitingOn chosen (cThreads ctx')
+                       else cThreads ctx'
+            sofar' = sofar <> trc
+            sofarSched' = sofarSched <> map (\(d,_,a) -> (d,a)) (F.toList trc)
+            prior' = Just (chosen, act)
+        in go sofar' sofarSched' prior' $ ctx' { cThreads = delCommitThreads threads' }
 
 --------------------------------------------------------------------------------
 -- * Single-step execution
 
 -- | Run a single thread one step, by dispatching on the type of
 -- 'Action'.
-stepThread :: forall n r s. MonadRef r n
-  => (forall x. s x -> IdSource -> n (Result x, IdSource, TTrace))
-  -- ^ Run a 'MonadSTM' transaction atomically.
+stepThread :: forall n r g. MonadRef r n
+  => Scheduler g
+  -- ^ The scheduler.
   -> MemType
-  -- ^ The memory model
-  -> Action n r s
-  -- ^ Action to step
-  -> IdSource
-  -- ^ Source of fresh IDs
+  -- ^ The memory model to use.
   -> ThreadId
   -- ^ ID of the current thread
-  -> Threads n r s
-  -- ^ Current state of threads
-  -> WriteBuffer r
-  -- ^ @CRef@ write buffer
-  -> Int
-  -- ^ The number of capabilities
-  -> n (Either Failure (Threads n r s, IdSource, ThreadAction, WriteBuffer r, Int))
-stepThread runstm memtype action idSource tid threads wb caps = case action of
-  AFork    n a b   -> stepFork        n a b
-  AMyTId   c       -> stepMyTId       c
-  AGetNumCapabilities   c -> stepGetNumCapabilities c
-  ASetNumCapabilities i c -> stepSetNumCapabilities i c
-  AYield   c       -> stepYield       c
-  ANewVar  n c     -> stepNewVar      n c
-  APutVar  var a c -> stepPutVar      var a c
-  ATryPutVar var a c -> stepTryPutVar var a c
-  AReadVar var c   -> stepReadVar     var c
-  ATakeVar var c   -> stepTakeVar     var c
-  ATryTakeVar var c -> stepTryTakeVar var c
-  ANewRef  n a c   -> stepNewRef      n a c
-  AReadRef ref c   -> stepReadRef     ref c
-  AReadRefCas ref c -> stepReadRefCas ref c
-  AModRef  ref f c -> stepModRef      ref f c
-  AModRefCas ref f c -> stepModRefCas ref f c
-  AWriteRef ref a c -> stepWriteRef   ref a c
-  ACasRef ref tick a c -> stepCasRef ref tick a c
-  ACommit  t c     -> stepCommit      t c
-  AAtom    stm c   -> stepAtom        stm c
-  ALift    na      -> stepLift        na
-  AThrow   e       -> stepThrow       e
-  AThrowTo t e c   -> stepThrowTo     t e c
-  ACatching h ma c -> stepCatching    h ma c
-  APopCatching a   -> stepPopCatching a
-  AMasking m ma c  -> stepMasking     m ma c
-  AResetMask b1 b2 m c -> stepResetMask b1 b2 m c
-  AReturn     c    -> stepReturn c
-  AMessage    m c  -> stepMessage m c
-  AStop       na   -> stepStop na
+  -> Action n r
+  -- ^ Action to step
+  -> Context n r g
+  -- ^ The execution context.
+  -> n (Either Failure (Context n r g, Either (ThreadAction, SeqTrace) ThreadAction))
+stepThread sched memtype tid action ctx = case action of
+    -- start a new thread, assigning it the next 'ThreadId'
+    AFork n a b -> pure . Right $
+        let threads' = launch tid newtid a (cThreads ctx)
+            (idSource', newtid) = nextTId n (cIdSource ctx)
+        in (ctx { cThreads = goto (b newtid) tid threads', cIdSource = idSource' }, Right (Fork newtid))
 
-  where
-    -- | Start a new thread, assigning it the next 'ThreadId'
-    --
-    -- Explicit type signature needed for GHC 8. Looks like the
-    -- impredicative polymorphism checks got stronger.
-    stepFork :: String
-             -> ((forall b. M n r s b -> M n r s b) -> Action n r s)
-             -> (ThreadId -> Action n r s)
-             -> n (Either Failure (Threads n r s, IdSource, ThreadAction, WriteBuffer r, Int))
-    stepFork n a b = return $ Right (goto (b newtid) tid threads', idSource', Fork newtid, wb, caps) where
-      threads' = launch tid newtid a threads
-      (idSource', newtid) = nextTId n idSource
+    -- get the 'ThreadId' of the current thread
+    AMyTId c -> simple (goto (c tid) tid (cThreads ctx)) MyThreadId
 
-    -- | Get the 'ThreadId' of the current thread
-    stepMyTId c = simple (goto (c tid) tid threads) MyThreadId
+    -- get the number of capabilities
+    AGetNumCapabilities c -> simple (goto (c (cCaps ctx)) tid (cThreads ctx)) $ GetNumCapabilities (cCaps ctx)
 
-    -- | Get the number of capabilities
-    stepGetNumCapabilities c = simple (goto (c caps) tid threads) $ GetNumCapabilities caps
+    -- set the number of capabilities
+    ASetNumCapabilities i c -> pure . Right $
+      (ctx { cThreads = goto c tid (cThreads ctx), cCaps = i }, Right (SetNumCapabilities i))
 
-    -- | Set the number of capabilities
-    stepSetNumCapabilities i c = return $ Right (goto c tid threads, idSource, SetNumCapabilities i, wb, i)
+    -- yield the current thread
+    AYield c -> simple (goto c tid (cThreads ctx)) Yield
 
-    -- | Yield the current thread
-    stepYield c = simple (goto c tid threads) Yield
+    -- create a new @MVar@, using the next 'MVarId'.
+    ANewVar n c -> do
+      let (idSource', newmvid) = nextMVId n (cIdSource ctx)
+      ref <- newRef Nothing
+      let mvar = MVar newmvid ref
+      pure $ Right (ctx { cThreads = goto (c mvar) tid (cThreads ctx), cIdSource = idSource' }, Right (NewVar newmvid))
 
-    -- | Put a value into a @MVar@, blocking the thread until it's
-    -- empty.
-    stepPutVar cvar@(MVar cvid _) a c = synchronised $ do
-      (success, threads', woken) <- putIntoMVar cvar a c tid threads
+    -- put a value into a @MVar@, blocking the thread until it's empty.
+    APutVar cvar@(MVar cvid _) a c -> synchronised $ do
+      (success, threads', woken) <- putIntoMVar cvar a c tid (cThreads ctx)
       simple threads' $ if success then PutVar cvid woken else BlockedPutVar cvid
 
-    -- | Try to put a value into a @MVar@, without blocking.
-    stepTryPutVar cvar@(MVar cvid _) a c = synchronised $ do
-      (success, threads', woken) <- tryPutIntoMVar cvar a c tid threads
+    -- try to put a value into a @MVar@, without blocking.
+    ATryPutVar cvar@(MVar cvid _) a c -> synchronised $ do
+      (success, threads', woken) <- tryPutIntoMVar cvar a c tid (cThreads ctx)
       simple threads' $ TryPutVar cvid success woken
 
-    -- | Get the value from a @MVar@, without emptying, blocking the
+    -- get the value from a @MVar@, without emptying, blocking the
     -- thread until it's full.
-    stepReadVar cvar@(MVar cvid _) c = synchronised $ do
-      (success, threads', _) <- readFromMVar cvar c tid threads
+    AReadVar cvar@(MVar cvid _) c -> synchronised $ do
+      (success, threads', _) <- readFromMVar cvar c tid (cThreads ctx)
       simple threads' $ if success then ReadVar cvid else BlockedReadVar cvid
 
-    -- | Take the value from a @MVar@, blocking the thread until it's
+    -- take the value from a @MVar@, blocking the thread until it's
     -- full.
-    stepTakeVar cvar@(MVar cvid _) c = synchronised $ do
-      (success, threads', woken) <- takeFromMVar cvar c tid threads
+    ATakeVar cvar@(MVar cvid _) c -> synchronised $ do
+      (success, threads', woken) <- takeFromMVar cvar c tid (cThreads ctx)
       simple threads' $ if success then TakeVar cvid woken else BlockedTakeVar cvid
 
-    -- | Try to take the value from a @MVar@, without blocking.
-    stepTryTakeVar cvar@(MVar cvid _) c = synchronised $ do
-      (success, threads', woken) <- tryTakeFromMVar cvar c tid threads
+    -- try to take the value from a @MVar@, without blocking.
+    ATryTakeVar cvar@(MVar cvid _) c -> synchronised $ do
+      (success, threads', woken) <- tryTakeFromMVar cvar c tid (cThreads ctx)
       simple threads' $ TryTakeVar cvid success woken
 
-    -- | Read from a @CRef@.
-    stepReadRef cref@(CRef crid _) c = do
+    -- create a new @CRef@, using the next 'CRefId'.
+    ANewRef n a c -> do
+      let (idSource', newcrid) = nextCRId n (cIdSource ctx)
+      ref <- newRef (M.empty, 0, a)
+      let cref = CRef newcrid ref
+      pure $ Right (ctx { cThreads = goto (c cref) tid (cThreads ctx), cIdSource = idSource' }, Right (NewRef newcrid))
+
+    -- read from a @CRef@.
+    AReadRef cref@(CRef crid _) c -> do
       val <- readCRef cref tid
-      simple (goto (c val) tid threads) $ ReadRef crid
+      simple (goto (c val) tid (cThreads ctx)) $ ReadRef crid
 
-    -- | Read from a @CRef@ for future compare-and-swap operations.
-    stepReadRefCas cref@(CRef crid _) c = do
+    -- read from a @CRef@ for future compare-and-swap operations.
+    AReadRefCas cref@(CRef crid _) c -> do
       tick <- readForTicket cref tid
-      simple (goto (c tick) tid threads) $ ReadRefCas crid
+      simple (goto (c tick) tid (cThreads ctx)) $ ReadRefCas crid
 
-    -- | Modify a @CRef@.
-    stepModRef cref@(CRef crid _) f c = synchronised $ do
+    -- modify a @CRef@.
+    AModRef cref@(CRef crid _) f c -> synchronised $ do
       (new, val) <- f <$> readCRef cref tid
       writeImmediate cref new
-      simple (goto (c val) tid threads) $ ModRef crid
+      simple (goto (c val) tid (cThreads ctx)) $ ModRef crid
 
-    -- | Modify a @CRef@ using a compare-and-swap.
-    stepModRefCas cref@(CRef crid _) f c = synchronised $ do
+    -- modify a @CRef@ using a compare-and-swap.
+    AModRefCas cref@(CRef crid _) f c -> synchronised $ do
       tick@(Ticket _ _ old) <- readForTicket cref tid
       let (new, val) = f old
       void $ casCRef cref tid tick new
-      simple (goto (c val) tid threads) $ ModRefCas crid
+      simple (goto (c val) tid (cThreads ctx)) $ ModRefCas crid
 
-    -- | Write to a @CRef@ without synchronising
-    stepWriteRef cref@(CRef crid _) a c = case memtype of
-      -- Write immediately.
+    -- write to a @CRef@ without synchronising.
+    AWriteRef cref@(CRef crid _) a c -> case memtype of
+      -- write immediately.
       SequentialConsistency -> do
         writeImmediate cref a
-        simple (goto c tid threads) $ WriteRef crid
-
-      -- Add to buffer using thread id.
+        simple (goto c tid (cThreads ctx)) $ WriteRef crid
+      -- add to buffer using thread id.
       TotalStoreOrder -> do
-        wb' <- bufferWrite wb (tid, Nothing) cref a
-        return $ Right (goto c tid threads, idSource, WriteRef crid, wb', caps)
-
-      -- Add to buffer using both thread id and cref id
+        wb' <- bufferWrite (cWriteBuf ctx) (tid, Nothing) cref a
+        pure $ Right (ctx { cThreads = goto c tid (cThreads ctx), cWriteBuf = wb' }, Right (WriteRef crid))
+      -- add to buffer using both thread id and cref id
       PartialStoreOrder -> do
-        wb' <- bufferWrite wb (tid, Just crid) cref a
-        return $ Right (goto c tid threads, idSource, WriteRef crid, wb', caps)
+        wb' <- bufferWrite (cWriteBuf ctx) (tid, Just crid) cref a
+        pure $ Right (ctx { cThreads = goto c tid (cThreads ctx), cWriteBuf = wb' }, Right (WriteRef crid))
 
-    -- | Perform a compare-and-swap on a @CRef@.
-    stepCasRef cref@(CRef crid _) tick a c = synchronised $ do
+    -- perform a compare-and-swap on a @CRef@.
+    ACasRef cref@(CRef crid _) tick a c -> synchronised $ do
       (suc, tick') <- casCRef cref tid tick a
-      simple (goto (c (suc, tick')) tid threads) $ CasRef crid suc
+      simple (goto (c (suc, tick')) tid (cThreads ctx)) $ CasRef crid suc
 
-    -- | Commit a @CRef@ write
-    stepCommit t c = do
+    -- commit a @CRef@ write
+    ACommit t c -> do
       wb' <- case memtype of
-        -- Shouldn't ever get here
+        -- shouldn't ever get here
         SequentialConsistency ->
           error "Attempting to commit under SequentialConsistency"
+        -- commit using the thread id.
+        TotalStoreOrder -> commitWrite (cWriteBuf ctx) (t, Nothing)
+        -- commit using the cref id.
+        PartialStoreOrder -> commitWrite (cWriteBuf ctx) (t, Just c)
+      pure $ Right (ctx { cWriteBuf = wb' }, Right (CommitRef t c))
 
-        -- Commit using the thread id.
-        TotalStoreOrder -> commitWrite wb (t, Nothing)
-
-        -- Commit using the cref id.
-        PartialStoreOrder -> commitWrite wb (t, Just c)
-
-      return $ Right (threads, idSource, CommitRef t c, wb', caps)
-
-    -- | Run a STM transaction atomically.
-    stepAtom stm c = synchronised $ do
-      (res, idSource', trace) <- runstm stm idSource
+    -- run a STM transaction atomically.
+    AAtom stm c -> synchronised $ do
+      (res, idSource', trace) <- runTransaction stm (cIdSource ctx)
       case res of
         Success _ written val ->
-          let (threads', woken) = wake (OnTVar written) threads
-          in return $ Right (goto (c val) tid threads', idSource', STM trace woken, wb, caps)
+          let (threads', woken) = wake (OnTVar written) (cThreads ctx)
+          in pure $ Right (ctx { cThreads = goto (c val) tid threads', cIdSource = idSource' }, Right (STM trace woken))
         Retry touched ->
-          let threads' = block (OnTVar touched) tid threads
-          in return $ Right (threads', idSource', BlockedSTM trace, wb, caps)
+          let threads' = block (OnTVar touched) tid (cThreads ctx)
+          in pure $ Right (ctx { cThreads = threads', cIdSource = idSource'}, Right (BlockedSTM trace))
         Exception e -> do
           res' <- stepThrow e
-          return $ case res' of
-            Right (threads', _, _, _, _) -> Right (threads', idSource', Throw, wb, caps)
+          pure $ case res' of
+            Right (ctx', _) -> Right (ctx' { cIdSource = idSource' }, Right Throw)
             Left err -> Left err
 
-    -- | Run a subcomputation in an exception-catching context.
-    stepCatching h ma c = simple threads' Catching where
-      a     = runCont ma      (APopCatching . c)
-      e exc = runCont (h exc) (APopCatching . c)
+    -- lift an action from the underlying monad into the @Conc@
+    -- computation.
+    ALift na -> do
+      a <- na
+      simple (goto a tid (cThreads ctx)) LiftIO
 
-      threads' = goto a tid (catching e tid threads)
-
-    -- | Pop the top exception handler from the thread's stack.
-    stepPopCatching a = simple threads' PopCatching where
-      threads' = goto a tid (uncatching tid threads)
-
-    -- | Throw an exception, and propagate it to the appropriate
+    -- throw an exception, and propagate it to the appropriate
     -- handler.
-    stepThrow e =
-      case propagate (toException e) tid threads of
-        Just threads' -> simple threads' Throw
-        Nothing -> return $ Left UncaughtException
+    AThrow e -> stepThrow e
 
-    -- | Throw an exception to the target thread, and propagate it to
+    -- throw an exception to the target thread, and propagate it to
     -- the appropriate handler.
-    stepThrowTo t e c = synchronised $
-      let threads' = goto c tid threads
-          blocked  = block (OnMask t) tid threads
-      in case M.lookup t threads of
+    AThrowTo t e c -> synchronised $
+      let threads' = goto c tid (cThreads ctx)
+          blocked  = block (OnMask t) tid (cThreads ctx)
+      in case M.lookup t (cThreads ctx) of
            Just thread
              | interruptible thread -> case propagate (toException e) t threads' of
                Just threads'' -> simple threads'' $ ThrowTo t
                Nothing
-                 | t == initialThread -> return $ Left UncaughtException
+                 | t == initialThread -> pure $ Left UncaughtException
                  | otherwise -> simple (kill t threads') $ ThrowTo t
              | otherwise -> simple blocked $ BlockedThrowTo t
            Nothing -> simple threads' $ ThrowTo t
 
-    -- | Execute a subcomputation with a new masking state, and give
-    -- it a function to run a computation with the current masking
-    -- state.
-    --
-    -- Explicit type sig necessary for checking in the prescence of
-    -- 'umask', sadly.
-    stepMasking :: MaskingState
-                -> ((forall b. M n r s b -> M n r s b) -> M n r s a)
-                -> (a -> Action n r s)
-                -> n (Either Failure (Threads n r s, IdSource, ThreadAction, WriteBuffer r, Int))
-    stepMasking m ma c = simple threads' $ SetMasking False m where
-      a = runCont (ma umask) (AResetMask False False m' . c)
+    -- run a subcomputation in an exception-catching context.
+    ACatching h ma c ->
+      let a        = runCont ma      (APopCatching . c)
+          e exc    = runCont (h exc) (APopCatching . c)
+          threads' = goto a tid (catching e tid (cThreads ctx))
+      in simple threads' Catching
 
-      m' = _masking . fromJust $ M.lookup tid threads
-      umask mb = resetMask True m' >> mb >>= \b -> resetMask False m >> return b
-      resetMask typ ms = cont $ \k -> AResetMask typ True ms $ k ()
+    -- pop the top exception handler from the thread's stack.
+    APopCatching a ->
+      let threads' = goto a tid (uncatching tid (cThreads ctx))
+      in simple threads' PopCatching
 
-      threads' = goto a tid (mask m tid threads)
+    -- execute a subcomputation with a new masking state, and give it
+    -- a function to run a computation with the current masking state.
+    AMasking m ma c ->
+      let a = runCont (ma umask) (AResetMask False False m' . c)
+          m' = _masking . fromJust $ M.lookup tid (cThreads ctx)
+          umask mb = resetMask True m' >> mb >>= \b -> resetMask False m >> pure b
+          resetMask typ ms = cont $ \k -> AResetMask typ True ms $ k ()
+          threads' = goto a tid (mask m tid (cThreads ctx))
+      in simple threads' $ SetMasking False m
 
-    -- | Reset the masking thread of the state.
-    stepResetMask b1 b2 m c = simple threads' act where
-      act      = (if b1 then SetMasking else ResetMasking) b2 m
-      threads' = goto c tid (mask m tid threads)
 
-    -- | Create a new @MVar@, using the next 'MVarId'.
-    stepNewVar n c = do
-      let (idSource', newmvid) = nextMVId n idSource
-      ref <- newRef Nothing
-      let mvar = MVar newmvid ref
-      return $ Right (goto (c mvar) tid threads, idSource', NewVar newmvid, wb, caps)
+    -- reset the masking thread of the state.
+    AResetMask b1 b2 m c ->
+      let act      = (if b1 then SetMasking else ResetMasking) b2 m
+          threads' = goto c tid (mask m tid (cThreads ctx))
+      in simple threads' act
 
-    -- | Create a new @CRef@, using the next 'CRefId'.
-    stepNewRef n a c = do
-      let (idSource', newcrid) = nextCRId n idSource
-      ref <- newRef (M.empty, 0, a)
-      let cref = CRef newcrid ref
-      return $ Right (goto (c cref) tid threads, idSource', NewRef newcrid, wb, caps)
+    -- execute a 'return' or 'pure'.
+    AReturn c -> simple (goto c tid (cThreads ctx)) Return
 
-    -- | Lift an action from the underlying monad into the @Conc@
-    -- computation.
-    stepLift na = do
-      a <- na
-      simple (goto a tid threads) LiftIO
+    -- add a message to the trace.
+    AMessage m c  -> simple (goto c tid (cThreads ctx)) (Message m)
 
-    -- | Execute a 'return' or 'pure'.
-    stepReturn c = simple (goto c tid threads) Return
+    -- kill the current thread.
+    AStop na -> na >> simple (kill tid (cThreads ctx)) Stop
 
-    -- | Add a message to the trace.
-    stepMessage m c = simple (goto c tid threads) (Message m)
+    -- run a subconcurrent computation.
+    ASub ma c
+      | M.size (cThreads ctx) > 1 -> pure (Left IllegalSubconcurrency)
+      | otherwise -> do
+          (res, g', trace) <- runConcurrency sched memtype (cSchedState ctx) ma
+          pure $ Right (ctx { cThreads = goto (c res) tid (cThreads ctx), cSchedState = g' }, Left (Subconcurrency, trace))
+  where
 
-    -- | Kill the current thread.
-    stepStop na = na >> simple (kill tid threads) Stop
+    -- this is not inline in the long @case@ above as it's needed by
+    -- @AAtom@, @AThrow@, and @AThrowTo@.
+    stepThrow e =
+      case propagate (toException e) tid (cThreads ctx) of
+        Just threads' -> simple threads' Throw
+        Nothing -> pure $ Left UncaughtException
 
-    -- | Helper for actions which don't touch the 'IdSource' or
-    -- 'WriteBuffer'
-    simple threads' act = return $ Right (threads', idSource, act, wb, caps)
+    -- helper for actions which only change the threads.
+    simple threads' act = pure $ Right (ctx { cThreads = threads' }, Right act)
 
-    -- | Helper for actions impose a write barrier.
+    -- helper for actions impose a write barrier.
     synchronised ma = do
-      writeBarrier wb
+      writeBarrier (cWriteBuf ctx)
       res <- ma
 
       return $ case res of
-        Right (threads', idSource', act', _, caps') -> Right (threads', idSource', act', emptyBuffer, caps')
+        Right (ctx', act) -> Right (ctx' { cWriteBuf = emptyBuffer }, act)
         _ -> res
diff --git a/dejafu/Test/DejaFu/Conc/Internal/Common.hs b/dejafu/Test/DejaFu/Conc/Internal/Common.hs
index b69b62d..aab7eae 100755
--- a/dejafu/Test/DejaFu/Conc/Internal/Common.hs
+++ b/dejafu/Test/DejaFu/Conc/Internal/Common.hs
@@ -18,6 +18,7 @@ import Data.Dynamic (Dynamic)
 import Data.Map.Strict (Map)
 import Data.List.NonEmpty (NonEmpty, fromList)
 import Test.DejaFu.Common
+import Test.DejaFu.STM (STMLike)
 
 {-# ANN module ("HLint: ignore Use record patterns" :: String) #-}
 
@@ -32,16 +33,16 @@ import Test.DejaFu.Common
 -- current expression of threads and exception handlers very difficult
 -- (perhaps even not possible without significant reworking), so I
 -- abandoned the attempt.
-newtype M n r s a = M { runM :: (a -> Action n r s) -> Action n r s }
+newtype M n r a = M { runM :: (a -> Action n r) -> Action n r }
 
-instance Functor (M n r s) where
+instance Functor (M n r) where
     fmap f m = M $ \ c -> runM m (c . f)
 
-instance Applicative (M n r s) where
+instance Applicative (M n r) where
     pure x  = M $ \c -> AReturn $ c x
     f <*> v = M $ \c -> runM f (\g -> runM v (c . g))
 
-instance Monad (M n r s) where
+instance Monad (M n r) where
     return  = pure
     m >>= k = M $ \c -> runM m (\x -> runM (k x) c)
 
@@ -82,11 +83,11 @@ data Ticket a = Ticket
   }
 
 -- | Construct a continuation-passing operation from a function.
-cont :: ((a -> Action n r s) -> Action n r s) -> M n r s a
+cont :: ((a -> Action n r) -> Action n r) -> M n r a
 cont = M
 
 -- | Run a CPS computation with the given final computation.
-runCont :: M n r s a -> (a -> Action n r s) -> Action n r s
+runCont :: M n r a -> (a -> Action n r) -> Action n r
 runCont = runM
 
 --------------------------------------------------------------------------------
@@ -96,49 +97,51 @@ runCont = runM
 -- only occur as a result of an action, and they cover (most of) the
 -- primitives of the concurrency. 'spawn' is absent as it is
 -- implemented in terms of 'newEmptyMVar', 'fork', and 'putMVar'.
-data Action n r s =
-    AFork  String ((forall b. M n r s b -> M n r s b) -> Action n r s) (ThreadId -> Action n r s)
-  | AMyTId (ThreadId -> Action n r s)
+data Action n r =
+    AFork  String ((forall b. M n r b -> M n r b) -> Action n r) (ThreadId -> Action n r)
+  | AMyTId (ThreadId -> Action n r)
 
-  | AGetNumCapabilities (Int -> Action n r s)
-  | ASetNumCapabilities Int (Action n r s)
+  | AGetNumCapabilities (Int -> Action n r)
+  | ASetNumCapabilities Int (Action n r)
 
-  | forall a. ANewVar String (MVar r a -> Action n r s)
-  | forall a. APutVar     (MVar r a) a (Action n r s)
-  | forall a. ATryPutVar  (MVar r a) a (Bool -> Action n r s)
-  | forall a. AReadVar    (MVar r a) (a -> Action n r s)
-  | forall a. ATakeVar    (MVar r a) (a -> Action n r s)
-  | forall a. ATryTakeVar (MVar r a) (Maybe a -> Action n r s)
+  | forall a. ANewVar String (MVar r a -> Action n r)
+  | forall a. APutVar     (MVar r a) a (Action n r)
+  | forall a. ATryPutVar  (MVar r a) a (Bool -> Action n r)
+  | forall a. AReadVar    (MVar r a) (a -> Action n r)
+  | forall a. ATakeVar    (MVar r a) (a -> Action n r)
+  | forall a. ATryTakeVar (MVar r a) (Maybe a -> Action n r)
 
-  | forall a.   ANewRef String a (CRef r a -> Action n r s)
-  | forall a.   AReadRef    (CRef r a) (a -> Action n r s)
-  | forall a.   AReadRefCas (CRef r a) (Ticket a -> Action n r s)
-  | forall a b. AModRef     (CRef r a) (a -> (a, b)) (b -> Action n r s)
-  | forall a b. AModRefCas  (CRef r a) (a -> (a, b)) (b -> Action n r s)
-  | forall a.   AWriteRef   (CRef r a) a (Action n r s)
-  | forall a.   ACasRef     (CRef r a) (Ticket a) a ((Bool, Ticket a) -> Action n r s)
+  | forall a.   ANewRef String a (CRef r a -> Action n r)
+  | forall a.   AReadRef    (CRef r a) (a -> Action n r)
+  | forall a.   AReadRefCas (CRef r a) (Ticket a -> Action n r)
+  | forall a b. AModRef     (CRef r a) (a -> (a, b)) (b -> Action n r)
+  | forall a b. AModRefCas  (CRef r a) (a -> (a, b)) (b -> Action n r)
+  | forall a.   AWriteRef   (CRef r a) a (Action n r)
+  | forall a.   ACasRef     (CRef r a) (Ticket a) a ((Bool, Ticket a) -> Action n r)
 
   | forall e.   Exception e => AThrow e
-  | forall e.   Exception e => AThrowTo ThreadId e (Action n r s)
-  | forall a e. Exception e => ACatching (e -> M n r s a) (M n r s a) (a -> Action n r s)
-  | APopCatching (Action n r s)
-  | forall a. AMasking MaskingState ((forall b. M n r s b -> M n r s b) -> M n r s a) (a -> Action n r s)
-  | AResetMask Bool Bool MaskingState (Action n r s)
+  | forall e.   Exception e => AThrowTo ThreadId e (Action n r)
+  | forall a e. Exception e => ACatching (e -> M n r a) (M n r a) (a -> Action n r)
+  | APopCatching (Action n r)
+  | forall a. AMasking MaskingState ((forall b. M n r b -> M n r b) -> M n r a) (a -> Action n r)
+  | AResetMask Bool Bool MaskingState (Action n r)
 
-  | AMessage    Dynamic (Action n r s)
+  | AMessage    Dynamic (Action n r)
 
-  | forall a. AAtom (s a) (a -> Action n r s)
-  | ALift (n (Action n r s))
-  | AYield  (Action n r s)
-  | AReturn (Action n r s)
+  | forall a. AAtom (STMLike n r a) (a -> Action n r)
+  | ALift (n (Action n r))
+  | AYield  (Action n r)
+  | AReturn (Action n r)
   | ACommit ThreadId CRefId
   | AStop (n ())
 
+  | forall a. ASub (M n r a) (Either Failure a -> Action n r)
+
 --------------------------------------------------------------------------------
 -- * Scheduling & Traces
 
 -- | Look as far ahead in the given continuation as possible.
-lookahead :: Action n r s -> NonEmpty Lookahead
+lookahead :: Action n r -> NonEmpty Lookahead
 lookahead = fromList . lookahead' where
   lookahead' (AFork _ _ _)           = [WillFork]
   lookahead' (AMyTId _)              = [WillMyThreadId]
@@ -170,3 +173,4 @@ lookahead = fromList . lookahead' where
   lookahead' (AYield k)              = WillYield : lookahead' k
   lookahead' (AReturn k)             = WillReturn : lookahead' k
   lookahead' (AStop _)               = [WillStop]
+  lookahead' (ASub _ _)              = [WillSubconcurrency]
diff --git a/dejafu/Test/DejaFu/Conc/Internal/Memory.hs b/dejafu/Test/DejaFu/Conc/Internal/Memory.hs
index 2165cad..a63a763 100755
--- a/dejafu/Test/DejaFu/Conc/Internal/Memory.hs
+++ b/dejafu/Test/DejaFu/Conc/Internal/Memory.hs
@@ -126,7 +126,7 @@ writeBarrier (WriteBuffer wb) = mapM_ flush $ M.elems wb where
   flush = mapM_ $ \(BufferedWrite _ cref a) -> writeImmediate cref a
 
 -- | Add phantom threads to the thread list to commit pending writes.
-addCommitThreads :: WriteBuffer r -> Threads n r s -> Threads n r s
+addCommitThreads :: WriteBuffer r -> Threads n r -> Threads n r
 addCommitThreads (WriteBuffer wb) ts = ts <> M.fromList phantoms where
   phantoms = [ (ThreadId Nothing $ negate tid, mkthread $ fromJust c)
              | ((k, b), tid) <- zip (M.toList wb) [1..]
@@ -136,41 +136,41 @@ addCommitThreads (WriteBuffer wb) ts = ts <> M.fromList phantoms where
   go EmptyL = Nothing
 
 -- | Remove phantom threads.
-delCommitThreads :: Threads n r s -> Threads n r s
+delCommitThreads :: Threads n r -> Threads n r
 delCommitThreads = M.filterWithKey $ \k _ -> k >= initialThread
 
 --------------------------------------------------------------------------------
 -- * Manipulating @MVar@s
 
 -- | Put into a @MVar@, blocking if full.
-putIntoMVar :: MonadRef r n => MVar r a -> a -> Action n r s
-            -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+putIntoMVar :: MonadRef r n => MVar r a -> a -> Action n r
+            -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 putIntoMVar cvar a c = mutMVar True cvar a (const c)
 
 -- | Try to put into a @MVar@, not blocking if full.
-tryPutIntoMVar :: MonadRef r n => MVar r a -> a -> (Bool -> Action n r s)
-               -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+tryPutIntoMVar :: MonadRef r n => MVar r a -> a -> (Bool -> Action n r)
+               -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 tryPutIntoMVar = mutMVar False
 
 -- | Read from a @MVar@, blocking if empty.
-readFromMVar :: MonadRef r n => MVar r a -> (a -> Action n r s)
-            -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+readFromMVar :: MonadRef r n => MVar r a -> (a -> Action n r)
+            -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 readFromMVar cvar c = seeMVar False True cvar (c . fromJust)
 
 -- | Take from a @MVar@, blocking if empty.
-takeFromMVar :: MonadRef r n => MVar r a -> (a -> Action n r s)
-             -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+takeFromMVar :: MonadRef r n => MVar r a -> (a -> Action n r)
+             -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 takeFromMVar cvar c = seeMVar True True cvar (c . fromJust)
 
 -- | Try to take from a @MVar@, not blocking if empty.
-tryTakeFromMVar :: MonadRef r n => MVar r a -> (Maybe a -> Action n r s)
-                -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+tryTakeFromMVar :: MonadRef r n => MVar r a -> (Maybe a -> Action n r)
+                -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 tryTakeFromMVar = seeMVar True False
 
 -- | Mutate a @MVar@, in either a blocking or nonblocking way.
 mutMVar :: MonadRef r n
-        => Bool -> MVar r a -> a -> (Bool -> Action n r s)
-        -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+        => Bool -> MVar r a -> a -> (Bool -> Action n r)
+        -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 mutMVar blocking (MVar cvid ref) a c threadid threads = do
   val <- readRef ref
 
@@ -191,8 +191,8 @@ mutMVar blocking (MVar cvid ref) a c threadid threads = do
 -- | Read a @MVar@, in either a blocking or nonblocking
 -- way.
 seeMVar :: MonadRef r n
-        => Bool -> Bool -> MVar r a -> (Maybe a -> Action n r s)
-        -> ThreadId -> Threads n r s -> n (Bool, Threads n r s, [ThreadId])
+        => Bool -> Bool -> MVar r a -> (Maybe a -> Action n r)
+        -> ThreadId -> Threads n r -> n (Bool, Threads n r, [ThreadId])
 seeMVar emptying blocking (MVar cvid ref) c threadid threads = do
   val <- readRef ref
 
diff --git a/dejafu/Test/DejaFu/Conc/Internal/Threading.hs b/dejafu/Test/DejaFu/Conc/Internal/Threading.hs
index 63925d6..2302a67 100644
--- a/dejafu/Test/DejaFu/Conc/Internal/Threading.hs
+++ b/dejafu/Test/DejaFu/Conc/Internal/Threading.hs
@@ -27,22 +27,22 @@ import qualified Data.Map.Strict as M
 -- * Threads
 
 -- | Threads are stored in a map index by 'ThreadId'.
-type Threads n r s = Map ThreadId (Thread n r s)
+type Threads n r = Map ThreadId (Thread n r)
 
 -- | All the state of a thread.
-data Thread n r s = Thread
-  { _continuation :: Action n r s
+data Thread n r = Thread
+  { _continuation :: Action n r
   -- ^ The next action to execute.
   , _blocking     :: Maybe BlockedOn
   -- ^ The state of any blocks.
-  , _handlers     :: [Handler n r s]
+  , _handlers     :: [Handler n r]
   -- ^ Stack of exception handlers
   , _masking      :: MaskingState
   -- ^ The exception masking state.
   }
 
 -- | Construct a thread with just one action
-mkthread :: Action n r s -> Thread n r s
+mkthread :: Action n r -> Thread n r
 mkthread c = Thread c Nothing [] Unmasked
 
 --------------------------------------------------------------------------------
@@ -53,7 +53,7 @@ mkthread c = Thread c Nothing [] Unmasked
 data BlockedOn = OnMVarFull MVarId | OnMVarEmpty MVarId | OnTVar [TVarId] | OnMask ThreadId deriving Eq
 
 -- | Determine if a thread is blocked in a certain way.
-(~=) :: Thread n r s -> BlockedOn -> Bool
+(~=) :: Thread n r -> BlockedOn -> Bool
 thread ~= theblock = case (_blocking thread, theblock) of
   (Just (OnMVarFull  _), OnMVarFull  _) -> True
   (Just (OnMVarEmpty _), OnMVarEmpty _) -> True
@@ -65,11 +65,11 @@ thread ~= theblock = case (_blocking thread, theblock) of
 -- * Exceptions
 
 -- | An exception handler.
-data Handler n r s = forall e. Exception e => Handler (e -> Action n r s)
+data Handler n r = forall e. Exception e => Handler (e -> Action n r)
 
 -- | Propagate an exception upwards, finding the closest handler
 -- which can deal with it.
-propagate :: SomeException -> ThreadId -> Threads n r s -> Maybe (Threads n r s)
+propagate :: SomeException -> ThreadId -> Threads n r -> Maybe (Threads n r)
 propagate e tid threads = case M.lookup tid threads >>= go . _handlers of
   Just (act, hs) -> Just $ except act hs tid threads
   Nothing -> Nothing
@@ -79,40 +79,40 @@ propagate e tid threads = case M.lookup tid threads >>= go . _handlers of
     go (Handler h:hs) = maybe (go hs) (\act -> Just (act, hs)) $ h <$> fromException e
 
 -- | Check if a thread can be interrupted by an exception.
-interruptible :: Thread n r s -> Bool
+interruptible :: Thread n r -> Bool
 interruptible thread = _masking thread == Unmasked || (_masking thread == MaskedInterruptible && isJust (_blocking thread))
 
 -- | Register a new exception handler.
-catching :: Exception e => (e -> Action n r s) -> ThreadId -> Threads n r s -> Threads n r s
+catching :: Exception e => (e -> Action n r) -> ThreadId -> Threads n r -> Threads n r
 catching h = M.alter $ \(Just thread) -> Just $ thread { _handlers = Handler h : _handlers thread }
 
 -- | Remove the most recent exception handler.
-uncatching :: ThreadId -> Threads n r s -> Threads n r s
+uncatching :: ThreadId -> Threads n r -> Threads n r
 uncatching = M.alter $ \(Just thread) -> Just $ thread { _handlers = tail $ _handlers thread }
 
 -- | Raise an exception in a thread.
-except :: Action n r s -> [Handler n r s] -> ThreadId -> Threads n r s -> Threads n r s
+except :: Action n r -> [Handler n r] -> ThreadId -> Threads n r -> Threads n r
 except act hs = M.alter $ \(Just thread) -> Just $ thread { _continuation = act, _handlers = hs, _blocking = Nothing }
 
 -- | Set the masking state of a thread.
-mask :: MaskingState -> ThreadId -> Threads n r s -> Threads n r s
+mask :: MaskingState -> ThreadId -> Threads n r -> Threads n r
 mask ms = M.alter $ \(Just thread) -> Just $ thread { _masking = ms }
 
 --------------------------------------------------------------------------------
 -- * Manipulating threads
 
 -- | Replace the @Action@ of a thread.
-goto :: Action n r s -> ThreadId -> Threads n r s -> Threads n r s
+goto :: Action n r -> ThreadId -> Threads n r -> Threads n r
 goto a = M.alter $ \(Just thread) -> Just (thread { _continuation = a })
 
 -- | Start a thread with the given ID, inheriting the masking state
 -- from the parent thread. This ID must not already be in use!
-launch :: ThreadId -> ThreadId -> ((forall b. M n r s b -> M n r s b) -> Action n r s) -> Threads n r s -> Threads n r s
+launch :: ThreadId -> ThreadId -> ((forall b. M n r b -> M n r b) -> Action n r) -> Threads n r -> Threads n r
 launch parent tid a threads = launch' ms tid a threads where
   ms = fromMaybe Unmasked $ _masking <$> M.lookup parent threads
 
 -- | Start a thread with the given ID and masking state. This must not already be in use!
-launch' :: MaskingState -> ThreadId -> ((forall b. M n r s b -> M n r s b) -> Action n r s) -> Threads n r s -> Threads n r s
+launch' :: MaskingState -> ThreadId -> ((forall b. M n r b -> M n r b) -> Action n r) -> Threads n r -> Threads n r
 launch' ms tid a = M.insert tid thread where
   thread = Thread { _continuation = a umask, _blocking = Nothing, _handlers = [], _masking = ms }
 
@@ -120,11 +120,11 @@ launch' ms tid a = M.insert tid thread where
   resetMask typ m = cont $ \k -> AResetMask typ True m $ k ()
 
 -- | Kill a thread.
-kill :: ThreadId -> Threads n r s -> Threads n r s
+kill :: ThreadId -> Threads n r -> Threads n r
 kill = M.delete
 
 -- | Block a thread.
-block :: BlockedOn -> ThreadId -> Threads n r s -> Threads n r s
+block :: BlockedOn -> ThreadId -> Threads n r -> Threads n r
 block blockedOn = M.alter doBlock where
   doBlock (Just thread) = Just $ thread { _blocking = Just blockedOn }
   doBlock _ = error "Invariant failure in 'block': thread does NOT exist!"
@@ -132,7 +132,7 @@ block blockedOn = M.alter doBlock where
 -- | Unblock all threads waiting on the appropriate block. For 'TVar'
 -- blocks, this will wake all threads waiting on at least one of the
 -- given 'TVar's.
-wake :: BlockedOn -> Threads n r s -> (Threads n r s, [ThreadId])
+wake :: BlockedOn -> Threads n r -> (Threads n r, [ThreadId])
 wake blockedOn threads = (unblock <$> threads, M.keys $ M.filter isBlocked threads) where
   unblock thread
     | isBlocked thread = thread { _blocking = Nothing }
diff --git a/dejafu/Test/DejaFu/SCT.hs b/dejafu/Test/DejaFu/SCT.hs
index dfdd3e7..b3bd510 100755
--- a/dejafu/Test/DejaFu/SCT.hs
+++ b/dejafu/Test/DejaFu/SCT.hs
@@ -79,9 +79,8 @@ module Test.DejaFu.SCT
   , sctLengthBound
   ) where
 
-import Control.DeepSeq (NFData(..))
 import Control.Monad.Ref (MonadRef)
-import Data.List (nub)
+import Data.List (foldl')
 import qualified Data.Map.Strict as M
 import Data.Maybe (isJust, fromJust)
 import qualified Data.Set as S
@@ -140,17 +139,16 @@ cBound (Bounds pb fb lb) =
 --
 -- If no bounds are enabled, just backtrack to the given point.
 cBacktrack :: Bounds -> BacktrackFunc
-cBacktrack (Bounds Nothing Nothing Nothing) bs i t = backtrackAt (const False) False bs i t
-cBacktrack (Bounds pb fb lb) bs i t = lBack . fBack $ pBack bs where
-  pBack backs = if isJust pb then pBacktrack   backs i t else backs
-  fBack backs = if isJust fb then fBacktrack   backs i t else backs
-  lBack backs = if isJust lb then lBacktrack backs i t else backs
+cBacktrack (Bounds (Just _) _ _) = pBacktrack
+cBacktrack (Bounds _ (Just _) _) = fBacktrack
+cBacktrack (Bounds _ _ (Just _)) = lBacktrack
+cBacktrack _ = backtrackAt (\_ _ -> False)
 
 -------------------------------------------------------------------------------
 -- Pre-emption bounding
 
 newtype PreemptionBound = PreemptionBound Int
-  deriving (NFData, Enum, Eq, Ord, Num, Real, Integral, Read, Show)
+  deriving (Enum, Eq, Ord, Num, Real, Integral, Read, Show)
 
 -- | A sensible default preemption bound: 2.
 --
@@ -181,31 +179,26 @@ pBound (PreemptionBound pb) ts dl = preEmpCount ts dl <= pb
 -- the same state being reached multiple times, but is needed because
 -- of the artificial dependency imposed by the bound.
 pBacktrack :: BacktrackFunc
-pBacktrack bs i tid =
-  maybe id (\j' b -> backtrack True b j' tid) j $ backtrack False bs i tid
+pBacktrack bs = backtrackAt (\_ _ -> False) bs . concatMap addConservative where
+  addConservative o@(i, _, tid) = o : case conservative i of
+    Just j  -> [(j, True, tid)]
+    Nothing -> []
 
-  where
-    -- Index of the conservative point
-    j = goJ . reverse . pairs $ zip [0..i-1] bs where
-      goJ (((_,b1), (j',b2)):rest)
-        | bcktThreadid b1 /= bcktThreadid b2
-          && not (isCommitRef . snd $ bcktDecision b1)
-          && not (isCommitRef . snd $ bcktDecision b2) = Just j'
-        | otherwise = goJ rest
-      goJ [] = Nothing
-
-    -- List of adjacent pairs
-    {-# INLINE pairs #-}
-    pairs = zip <*> tail
-
-    -- Add a backtracking point.
-    backtrack = backtrackAt $ const False
+  -- index of conservative point
+  conservative i = go (reverse (take (i-1) bs)) (i-1) where
+    go _ (-1) = Nothing
+    go (b1:rest@(b2:_)) j
+      | bcktThreadid b1 /= bcktThreadid b2
+        && not (isCommitRef $ bcktAction b1)
+        && not (isCommitRef $ bcktAction b2) = Just j
+      | otherwise = go rest (j-1)
+    go _ _ = Nothing
 
 -------------------------------------------------------------------------------
 -- Fair bounding
 
 newtype FairBound = FairBound Int
-  deriving (NFData, Enum, Eq, Ord, Num, Real, Integral, Read, Show)
+  deriving (Enum, Eq, Ord, Num, Real, Integral, Read, Show)
 
 -- | A sensible default fair bound: 5.
 --
@@ -233,15 +226,15 @@ fBound (FairBound fb) ts (_, l) = maxYieldCountDiff ts l <= fb
 -- | Add a backtrack point. If the thread isn't runnable, or performs
 -- a release operation, add all runnable threads.
 fBacktrack :: BacktrackFunc
-fBacktrack bs i t = backtrackAt check False bs i t where
+fBacktrack = backtrackAt check where
   -- True if a release operation is performed.
-  check b = Just True == (willRelease <$> M.lookup t (bcktRunnable b))
+  check t b = Just True == (willRelease <$> M.lookup t (bcktRunnable b))
 
 -------------------------------------------------------------------------------
 -- Length bounding
 
 newtype LengthBound = LengthBound Int
-  deriving (NFData, Enum, Eq, Ord, Num, Real, Integral, Read, Show)
+  deriving (Enum, Eq, Ord, Num, Real, Integral, Read, Show)
 
 -- | A sensible default length bound: 250.
 --
@@ -269,7 +262,7 @@ lBound (LengthBound lb) ts _ = length ts < lb
 -- | Add a backtrack point. If the thread isn't runnable, add all
 -- runnable threads.
 lBacktrack :: BacktrackFunc
-lBacktrack = backtrackAt (const False) False
+lBacktrack = backtrackAt (\_ _ -> False)
 
 -------------------------------------------------------------------------------
 -- DPOR
@@ -313,7 +306,7 @@ sctBounded memtype bf backtrack conc = go initialState where
 
       if schedIgnore s
       then go newDPOR
-      else ((res, trace):) <$> go (pruneCommits $ addBacktracks bpoints newDPOR)
+      else ((res, trace):) <$> go (addBacktracks bpoints newDPOR)
 
     Nothing -> pure []
 
@@ -332,32 +325,11 @@ sctBounded memtype bf backtrack conc = go initialState where
   -- Incorporate the new backtracking steps into the DPOR tree.
   addBacktracks = incorporateBacktrackSteps bf
 
--------------------------------------------------------------------------------
--- Post-processing
-
--- | Remove commits from the todo sets where every other action will
--- result in a write barrier (and so a commit) occurring.
---
--- To get the benefit from this, do not execute commit actions from
--- the todo set until there are no other choises.
-pruneCommits :: DPOR -> DPOR
-pruneCommits bpor
-  | not onlycommits || not alldonesync = go bpor
-  | otherwise = go bpor { dporTodo = M.empty }
-
-  where
-    go b = b { dporDone = pruneCommits <$> dporDone bpor }
-
-    onlycommits = all (<initialThread) . M.keys $ dporTodo bpor
-    alldonesync = all barrier . M.elems $ dporDone bpor
-
-    barrier = isBarrier . simplifyAction . fromJust . dporAction
-
 -------------------------------------------------------------------------------
 -- Dependency function
 
 -- | Check if an action is dependent on another.
-dependent :: MemType -> DepState -> (ThreadId, ThreadAction) -> (ThreadId, ThreadAction) -> Bool
+dependent :: MemType -> DepState -> ThreadId -> ThreadAction -> ThreadId -> ThreadAction -> Bool
 -- This is basically the same as 'dependent'', but can make use of the
 -- additional information in a 'ThreadAction' to make different
 -- decisions in a few cases:
@@ -381,14 +353,14 @@ dependent :: MemType -> DepState -> (ThreadId, ThreadAction) -> (ThreadId, Threa
 --  - Dependency of STM transactions can be /greatly/ improved here,
 --    as the 'Lookahead' does not know which @TVar@s will be touched,
 --    and so has to assume all transactions are dependent.
-dependent _ _ (_, SetNumCapabilities a) (_, GetNumCapabilities b) = a /= b
-dependent _ ds (_, ThrowTo t) (t2, a) = t == t2 && canInterrupt ds t2 a
-dependent memtype ds (t1, a1) (t2, a2) = case rewind a2 of
+dependent _ _ _ (SetNumCapabilities a) _ (GetNumCapabilities b) = a /= b
+dependent _ ds _ (ThrowTo t) t2 a = t == t2 && canInterrupt ds t2 a
+dependent memtype ds t1 a1 t2 a2 = case rewind a2 of
   Just l2
     | isSTM a1 && isSTM a2
       -> not . S.null $ tvarsOf a1 `S.intersection` tvarsOf a2
     | not (isBlock a1 && isBarrier (simplifyLookahead l2)) ->
-      dependent' memtype ds (t1, a1) (t2, l2)
+      dependent' memtype ds t1 a1 t2 l2
   _ -> dependentActions memtype ds (simplifyAction a1) (simplifyAction a2)
 
   where
@@ -400,8 +372,8 @@ dependent memtype ds (t1, a1) (t2, a2) = case rewind a2 of
 --
 -- Termination of the initial thread is handled specially in the DPOR
 -- implementation.
-dependent' :: MemType -> DepState -> (ThreadId, ThreadAction) -> (ThreadId, Lookahead) -> Bool
-dependent' memtype ds (t1, a1) (t2, l2) = case (a1, l2) of
+dependent' :: MemType -> DepState -> ThreadId -> ThreadAction -> ThreadId -> Lookahead -> Bool
+dependent' memtype ds t1 a1 t2 l2 = case (a1, l2) of
   -- Worst-case assumption: all IO is dependent.
   (LiftIO, WillLiftIO) -> True
 
@@ -496,6 +468,7 @@ yieldCount tid ts l = go initialThread ts where
     | t == tid && willYield l = 1
     | otherwise = 0
 
+  {-# INLINE go' #-}
   go' t t' act rest
     | t == tid && didYield act = 1 + go t' rest
     | otherwise = go t' rest
@@ -505,10 +478,14 @@ yieldCount tid ts l = go initialThread ts where
 maxYieldCountDiff :: [(Decision, ThreadAction)]
   -> Lookahead
   -> Int
-maxYieldCountDiff ts l = maximum yieldCountDiffs where
-  yieldsBy tid = yieldCount tid ts l
-  yieldCounts = [yieldsBy tid | tid <- nub $ allTids ts]
-  yieldCountDiffs = [y1 - y2 | y1 <- yieldCounts, y2 <- yieldCounts]
+maxYieldCountDiff ts l = go 0 yieldCounts where
+  go m (yc:ycs) =
+    let m' = m `max` foldl' (go' yc) 0 ycs
+    in go m' ycs
+  go m [] = m
+  go' yc0 m yc = m `max` abs (yc0 - yc)
+
+  yieldCounts = [yieldCount t ts l | t <- allTids ts]
 
   -- All the threads created during the lifetime of the system.
   allTids ((_, act):rest) =
diff --git a/dejafu/Test/DejaFu/SCT/Internal.hs b/dejafu/Test/DejaFu/SCT/Internal.hs
index 657c020..3696cd1 100644
--- a/dejafu/Test/DejaFu/SCT/Internal.hs
+++ b/dejafu/Test/DejaFu/SCT/Internal.hs
@@ -11,14 +11,14 @@
 -- interface of this library.
 module Test.DejaFu.SCT.Internal where
 
-import Control.DeepSeq (NFData(..), force)
 import Control.Exception (MaskingState(..))
 import Data.Char (ord)
-import Data.List (foldl', intercalate, partition, sortBy)
+import Data.Function (on)
+import qualified Data.Foldable as F
+import Data.List (intercalate, nubBy, partition, sortOn)
 import Data.List.NonEmpty (NonEmpty(..), toList)
-import Data.Ord (Down(..), comparing)
 import Data.Map.Strict (Map)
-import Data.Maybe (fromJust, isJust, isNothing, mapMaybe)
+import Data.Maybe (catMaybes, fromJust, isNothing)
 import qualified Data.Map.Strict as M
 import Data.Set (Set)
 import qualified Data.Set as S
@@ -51,16 +51,7 @@ data DPOR = DPOR
   , dporAction   :: Maybe ThreadAction
   -- ^ What happened at this step. This will be 'Nothing' at the root,
   -- 'Just' everywhere else.
-  }
-
-instance NFData DPOR where
-  rnf dpor = rnf ( dporRunnable dpor
-                 , dporTodo     dpor
-                 , dporDone     dpor
-                 , dporSleep    dpor
-                 , dporTaken    dpor
-                 , dporAction   dpor
-                 )
+  } deriving Show
 
 -- | One step of the execution, including information for backtracking
 -- purposes. This backtracking information is used to generate new
@@ -68,7 +59,9 @@ instance NFData DPOR where
 data BacktrackStep = BacktrackStep
   { bcktThreadid   :: ThreadId
   -- ^ The thread running at this step
-  , bcktDecision   :: (Decision, ThreadAction)
+  , bcktDecision   :: Decision
+  -- ^ What was decided at this step.
+  , bcktAction     :: ThreadAction
   -- ^ What happened at this step.
   , bcktRunnable   :: Map ThreadId Lookahead
   -- ^ The threads runnable at this step
@@ -77,15 +70,7 @@ data BacktrackStep = BacktrackStep
   -- alternatives were added conservatively due to the bound.
   , bcktState      :: DepState
   -- ^ Some domain-specific state at this point.
-  }
-
-instance NFData BacktrackStep where
-  rnf b = rnf ( bcktThreadid   b
-              , bcktDecision   b
-              , bcktRunnable   b
-              , bcktBacktracks b
-              , bcktState      b
-              )
+  } deriving Show
 
 -- | Initial DPOR state, given an initial thread ID. This initial
 -- thread should exist and be runnable at the start of execution.
@@ -127,24 +112,17 @@ findSchedulePrefix predicate idx dpor0
                     (ts, c, slp) = allPrefixes !! i
                 in Just (ts, c, slp, g)
   where
-    allPrefixes = go (initialDPORThread dpor0) dpor0
+    allPrefixes = go dpor0
 
-    go tid dpor =
+    go dpor =
           -- All the possible prefix traces from this point, with
           -- updated DPOR subtrees if taken from the done list.
-      let prefixes = concatMap go' (M.toList $ dporDone dpor) ++ here dpor
-          -- Sort by number of preemptions, in descending order.
-          cmp = Down . preEmps tid dpor . (\(a,_,_) -> a)
-          sorted = sortBy (comparing cmp) prefixes
+      let prefixes = here dpor : map go' (M.toList $ dporDone dpor)
+      in case concatPartition (\(t:_,_,_) -> predicate t) prefixes of
+           ([], choices) -> choices
+           (choices, _)  -> choices
 
-      in if null prefixes
-         then []
-         else case partition (\(t:_,_,_) -> predicate t) sorted of
-                ([], []) -> err "findSchedulePrefix" "empty prefix list!" 
-                ([], choices) -> choices
-                (choices, _)  -> choices
-
-    go' (tid, dpor) = (\(ts,c,slp) -> (tid:ts,c,slp)) <$> go tid dpor
+    go' (tid, dpor) = (\(ts,c,slp) -> (tid:ts,c,slp)) <$> go dpor
 
     -- Prefix traces terminating with a to-do decision at this point.
     here dpor = [([t], c, sleeps dpor) | (t, c) <- M.toList $ dporTodo dpor]
@@ -154,16 +132,10 @@ findSchedulePrefix predicate idx dpor0
     -- explored.
     sleeps dpor = dporSleep dpor `M.union` dporTaken dpor
 
-    -- The number of pre-emptive context switches
-    preEmps tid dpor (t:ts) =
-      let rest = preEmps t (fromJust . M.lookup t $ dporDone dpor) ts
-      in  if tid `S.member` dporRunnable dpor then 1 + rest else rest
-    preEmps _ _ [] = 0::Int
-
 -- | Add a new trace to the tree, creating a new subtree branching off
 -- at the point where the \"to-do\" decision was made.
 incorporateTrace
-  :: (DepState -> (ThreadId, ThreadAction) -> (ThreadId, ThreadAction) -> Bool)
+  :: (DepState -> ThreadId -> ThreadAction -> ThreadId -> ThreadAction -> Bool)
   -- ^ Dependency function
   -> Bool
   -- ^ Whether the \"to-do\" point which was used to create this new
@@ -176,7 +148,7 @@ incorporateTrace
 incorporateTrace dependency conservative trace dpor0 = grow initialDepState (initialDPORThread dpor0) trace dpor0 where
   grow state tid trc@((d, _, a):rest) dpor =
     let tid'   = tidOf tid d
-        state' = updateDepState state (tid', a)
+        state' = updateDepState state tid' a
     in case M.lookup tid' (dporDone dpor) of
          Just dpor' ->
            let done = M.insert tid' (grow state' tid' rest dpor') (dporDone dpor)
@@ -193,8 +165,8 @@ incorporateTrace dependency conservative trace dpor0 = grow initialDepState (ini
 
   -- Construct a new subtree corresponding to a trace suffix.
   subtree state tid sleep ((_, _, a):rest) =
-    let state' = updateDepState state (tid, a)
-        sleep' = M.filterWithKey (\t a' -> not $ dependency state' (tid, a) (t,a')) sleep
+    let state' = updateDepState state tid a
+        sleep' = M.filterWithKey (\t a' -> not $ dependency state' tid a t a') sleep
     in DPOR
         { dporRunnable = S.fromList $ case rest of
             ((_, runnable, _):_) -> map fst runnable
@@ -225,7 +197,7 @@ incorporateTrace dependency conservative trace dpor0 = grow initialDepState (ini
 -- runnable, a dependency is imposed between this final action and
 -- everything else.
 findBacktrackSteps
-  :: (DepState -> (ThreadId, ThreadAction) -> (ThreadId, Lookahead) -> Bool)
+  :: (DepState -> ThreadId -> ThreadAction -> ThreadId -> Lookahead -> Bool)
   -- ^ Dependency function.
   -> BacktrackFunc
   -- ^ Backtracking function. Given a list of backtracking points, and
@@ -244,17 +216,16 @@ findBacktrackSteps
   -> Trace
   -- ^ The execution trace.
   -> [BacktrackStep]
-findBacktrackSteps _ _ _ bcktrck
-  | Sq.null bcktrck = const []
-findBacktrackSteps dependency backtrack boundKill bcktrck = go initialDepState S.empty initialThread [] (Sq.viewl bcktrck) where
+findBacktrackSteps dependency backtrack boundKill = go initialDepState S.empty initialThread [] . F.toList where
   -- Walk through the traces one step at a time, building up a list of
   -- new backtracking points.
-  go state allThreads tid bs ((e,i):<is) ((d,_,a):ts) =
+  go state allThreads tid bs ((e,i):is) ((d,_,a):ts) =
     let tid' = tidOf tid d
-        state' = updateDepState state (tid', a)
+        state' = updateDepState state tid' a
         this = BacktrackStep
           { bcktThreadid   = tid'
-          , bcktDecision   = (d, a)
+          , bcktDecision   = d
+          , bcktAction     = a
           , bcktRunnable   = M.fromList . toList $ e
           , bcktBacktracks = M.fromList $ map (\i' -> (i', False)) i
           , bcktState      = state'
@@ -263,30 +234,41 @@ findBacktrackSteps dependency backtrack boundKill bcktrck = go initialDepState S
         runnable = S.fromList (M.keys $ bcktRunnable this)
         allThreads' = allThreads `S.union` runnable
         killsEarly = null ts && boundKill
-    in go state' allThreads' tid' bs' (Sq.viewl is) ts
+    in go state' allThreads' tid' bs' is ts
   go _ _ _ bs _ _ = bs
 
   -- Find the prior actions dependent with this one and add
   -- backtracking points.
   doBacktrack killsEarly allThreads enabledThreads bs =
     let tagged = reverse $ zip [0..] bs
-        idxs   = [ (head is, u)
+        idxs   = [ (head is, False, u)
                  | (u, n) <- enabledThreads
                  , v <- S.toList allThreads
                  , u /= v
                  , let is = idxs' u n v tagged
                  , not $ null is]
 
-        idxs' u n v = mapMaybe go' where
-          go' (i, b)
+        idxs' u n v = catMaybes . go' True where
+          {-# INLINE go' #-}
+          go' final ((i,b):rest)
+            -- Don't cross subconcurrency boundaries
+            | isSubC final b = []
             -- If this is the final action in the trace and the
             -- execution was killed due to nothing being within bounds
             -- (@killsEarly == True@) assume worst-case dependency.
-            | bcktThreadid b == v && (killsEarly || isDependent b) = Just i
-            | otherwise = Nothing
+            | bcktThreadid b == v && (killsEarly || isDependent b) = Just i : go' False rest
+            | otherwise = go' False rest
+          go' _ [] = []
 
-          isDependent b = dependency (bcktState b) (bcktThreadid b, snd $ bcktDecision b) (u, n)
-    in foldl' (\b (i, u) -> backtrack b i u) bs idxs
+          {-# INLINE isSubC #-}
+          isSubC final b = case bcktAction b of
+            Stop -> not final && bcktThreadid b == initialThread
+            Subconcurrency -> bcktThreadid b == initialThread
+            _ -> False
+
+          {-# INLINE isDependent #-}
+          isDependent b = dependency (bcktState b) (bcktThreadid b) (bcktAction b) u n
+    in backtrack bs idxs
 
 -- | Add new backtracking points, if they have not already been
 -- visited, fit into the bound, and aren't in the sleep set.
@@ -302,10 +284,9 @@ incorporateBacktrackSteps bv = go Nothing [] where
   go priorTid pref (b:bs) bpor =
     let bpor' = doBacktrack priorTid pref b bpor
         tid   = bcktThreadid b
-        pref' = pref ++ [bcktDecision b]
+        pref' = pref ++ [(bcktDecision b, bcktAction b)]
         child = go (Just tid) pref' bs . fromJust $ M.lookup tid (dporDone bpor)
     in bpor' { dporDone = M.insert tid child $ dporDone bpor' }
-
   go _ _ [] bpor = bpor
 
   doBacktrack priorTid pref b bpor =
@@ -343,16 +324,7 @@ data SchedState = SchedState
   , schedDepState  :: DepState
   -- ^ State used by the dependency function to determine when to
   -- remove decisions from the sleep set.
-  }
-
-instance NFData SchedState where
-  rnf s = rnf ( schedSleep     s
-              , schedPrefix    s
-              , schedBPoints   s
-              , schedIgnore    s
-              , schedBoundKill s
-              , schedDepState  s
-              )
+  } deriving Show
 
 -- | Initial scheduler state for a given prefix
 initialSchedState :: Map ThreadId ThreadAction
@@ -378,52 +350,60 @@ type BoundFunc
 -- | A backtracking step is a point in the execution where another
 -- decision needs to be made, in order to explore interesting new
 -- schedules. A backtracking /function/ takes the steps identified so
--- far and a point and a thread to backtrack to, and inserts at least
--- that backtracking point. More may be added to compensate for the
--- effects of the bounding function. For example, under pre-emption
--- bounding a conservative backtracking point is added at the prior
--- context switch.
+-- far and a list of points and thread at that point to backtrack
+-- to. More points be added to compensate for the effects of the
+-- bounding function. For example, under pre-emption bounding a
+-- conservative backtracking point is added at the prior context
+-- switch. The bool is whether the point is conservative. Conservative
+-- points are always explored, whereas non-conservative ones might be
+-- skipped based on future information.
 --
 -- In general, a backtracking function should identify one or more
 -- backtracking points, and then use @backtrackAt@ to do the actual
 -- work.
 type BacktrackFunc
-  = [BacktrackStep] -> Int -> ThreadId -> [BacktrackStep]
+  = [BacktrackStep] -> [(Int, Bool, ThreadId)] -> [BacktrackStep]
 
 -- | Add a backtracking point. If the thread isn't runnable, add all
 -- runnable threads. If the backtracking point is already present,
 -- don't re-add it UNLESS this would make it conservative.
 backtrackAt
-  :: (BacktrackStep -> Bool)
+  :: (ThreadId -> BacktrackStep -> Bool)
   -- ^ If this returns @True@, backtrack to all runnable threads,
   -- rather than just the given thread.
-  -> Bool
-  -- ^ Is this backtracking point conservative? Conservative points
-  -- are always explored, whereas non-conservative ones might be
-  -- skipped based on future information.
   -> BacktrackFunc
-backtrackAt toAll conservative bs i tid = go bs i where
-  go bx@(b:rest) 0
+backtrackAt toAll bs0 = backtrackAt' . nubBy ((==) `on` fst') . sortOn fst' where
+  fst' (x,_,_) = x
+
+  backtrackAt' ((i,c,t):is) = go i bs0 i c t is
+  backtrackAt' [] = bs0
+
+  go i0 (b:bs) 0 c tid is
     -- If the backtracking point is already present, don't re-add it,
     -- UNLESS this would force it to backtrack (it's conservative)
     -- where before it might not.
-    | not (toAll b) && tid `M.member` bcktRunnable b =
+    | not (toAll tid b) && tid `M.member` bcktRunnable b =
       let val = M.lookup tid $ bcktBacktracks b
-      in if isNothing val || (val == Just False && conservative)
-         then b { bcktBacktracks = backtrackTo b } : rest
-         else bx
-
+          b' = if isNothing val || (val == Just False && c)
+            then b { bcktBacktracks = backtrackTo tid c b }
+            else b
+      in b' : case is of
+        ((i',c',t'):is') -> go i' bs (i'-i0-1) c' t' is'
+        [] -> bs
     -- Otherwise just backtrack to everything runnable.
-    | otherwise = b { bcktBacktracks = backtrackAll b } : rest
-
-  go (b:rest) n = b : go rest (n-1)
-  go [] _ = error "backtrackAt: Ran out of schedule whilst backtracking!"
+    | otherwise =
+      let b' = b { bcktBacktracks = backtrackAll c b }
+      in b' : case is of
+        ((i',c',t'):is') -> go i' bs (i'-i0-1) c' t' is'
+        [] -> bs
+  go i0 (b:bs) i c tid is = b : go i0 bs (i-1) c tid is
+  go _ [] _ _ _ _ = err "backtrackAt" "ran out of schedule whilst backtracking!"
 
   -- Backtrack to a single thread
-  backtrackTo = M.insert tid conservative . bcktBacktracks
+  backtrackTo tid c = M.insert tid c . bcktBacktracks
 
   -- Backtrack to all runnable threads
-  backtrackAll = M.map (const conservative) . bcktRunnable
+  backtrackAll c = M.map (const c) . bcktRunnable
 
 -- | DPOR scheduler: takes a list of decisions, and maintains a trace
 -- including the runnable threads, and the alternative choices allowed
@@ -433,17 +413,14 @@ backtrackAt toAll conservative bs i tid = go bs i where
 -- the prior thread if it's (1) still runnable and (2) hasn't just
 -- yielded. Furthermore, threads which /will/ yield are ignored in
 -- preference of those which will not.
---
--- This forces full evaluation of the result every step, to avoid any
--- possible space leaks.
 dporSched
-  :: (DepState -> (ThreadId, ThreadAction) -> (ThreadId, ThreadAction) -> Bool)
+  :: (DepState -> ThreadId -> ThreadAction -> ThreadId -> ThreadAction -> Bool)
   -- ^ Dependency function.
   -> BoundFunc
   -- ^ Bound function: returns true if that schedule prefix terminated
   -- with the lookahead decision fits within the bound.
   -> Scheduler SchedState
-dporSched dependency inBound trc prior threads s = force schedule where
+dporSched dependency inBound trc prior threads s = schedule where
   -- Pick a thread to run.
   schedule = case schedPrefix s of
     -- If there is a decision available, make it
@@ -455,7 +432,7 @@ dporSched dependency inBound trc prior threads s = force schedule where
     [] ->
       let choices  = restrictToBound initialise
           checkDep t a = case prior of
-            Just (tid, act) -> dependency (schedDepState s) (tid, act) (t, a)
+            Just (tid, act) -> dependency (schedDepState s) tid act t a
             Nothing -> False
           ssleep'  = M.filterWithKey (\t a -> not $ checkDep t a) $ schedSleep s
           choices' = filter (`notElem` M.keys ssleep') choices
@@ -470,7 +447,7 @@ dporSched dependency inBound trc prior threads s = force schedule where
     { schedBPoints  = schedBPoints s |> (threads, rest)
     , schedDepState = nextDepState
     }
-  nextDepState = let ds = schedDepState s in maybe ds (updateDepState ds) prior
+  nextDepState = let ds = schedDepState s in maybe ds (uncurry $ updateDepState ds) prior
 
   -- Pick a new thread to run, not considering bounds. Choose the
   -- current thread if available and it hasn't just yielded, otherwise
@@ -537,11 +514,7 @@ data DepState = DepState
   -- the masking state is assumed to be @Unmasked@. This nicely
   -- provides compatibility with dpor-0.1, where the thread IDs are
   -- not available.
-  }
-
-instance NFData DepState where
-  -- Cheats: 'MaskingState' has no 'NFData' instance.
-  rnf ds = rnf (depCRState ds, M.keys (depMaskState ds))
+  } deriving (Eq, Show)
 
 -- | Initial dependency state.
 initialDepState :: DepState
@@ -549,8 +522,8 @@ initialDepState = DepState M.empty M.empty
 
 -- | Update the 'CRef' buffer state with the action that has just
 -- happened.
-updateDepState :: DepState -> (ThreadId, ThreadAction) -> DepState
-updateDepState depstate (tid, act) = DepState
+updateDepState :: DepState -> ThreadId -> ThreadAction -> DepState
+updateDepState depstate tid act = DepState
   { depCRState   = updateCRState       act $ depCRState   depstate
   , depMaskState = updateMaskState tid act $ depMaskState depstate
   }
@@ -698,3 +671,17 @@ toDotFiltered check showTid showAct = digraph . go "L" where
 -- | Internal errors.
 err :: String -> String -> a
 err func msg = error (func ++ ": (internal error) " ++ msg)
+
+-- | A combination of 'partition' and 'concat'.
+concatPartition :: (a -> Bool) -> [[a]] -> ([a], [a])
+{-# INLINE concatPartition #-}
+-- note: `foldr (flip (foldr select))` is slow, as is `foldl (foldl
+-- select))`, and `foldl'` variants. The sweet spot seems to be `foldl
+-- (foldr select)` for some reason I don't really understand.
+concatPartition p = foldl (foldr select) ([], []) where
+  -- Lazy pattern matching, got this trick from the 'partition'
+  -- implementation. This reduces allocation fairly significantly; I
+  -- do not know why.
+  select a ~(ts, fs)
+    | p a       = (a:ts, fs)
+    | otherwise = (ts, a:fs)
diff --git a/dejafu/dejafu.cabal b/dejafu/dejafu.cabal
index 92093ee..e32550c 100755
--- a/dejafu/dejafu.cabal
+++ b/dejafu/dejafu.cabal
@@ -96,7 +96,6 @@ library
   build-depends:       base              >=4.8  && <5
                      , concurrency       >=1.0  && <1.1
                      , containers        >=0.5  && <0.6
-                     , deepseq           >=1.3  && <1.5
                      , exceptions        >=0.7  && <0.9
                      , monad-loops       >=0.4  && <0.5
                      , mtl               >=2.2  && <2.3