diff --git a/Control/Monad/Operational/Class.hs b/Control/Monad/Operational/Class.hs
--- a/Control/Monad/Operational/Class.hs
+++ b/Control/Monad/Operational/Class.hs
@@ -1,80 +1,82 @@
-{-# LANGUAGE FunctionalDependencies #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE UndecidableInstances #-}
-{-# LANGUAGE FlexibleInstances #-}
-{-# LANGUAGE TypeOperators #-}
-{-# LANGUAGE CPP #-}
------------------------------------------------------------------------------
--- |
--- Module      :  Control.Monad.Operational.Class
--- Copyright   :  (C) 2012-2013 Fumiaki Kinoshita
--- License     :  BSD-style (see the file LICENSE)
---
--- Maintainer  :  Fumiaki Kinsohita <fumiexcel@gmail.com>
--- Stability   :  experimental
--- Portability :  non-portable
---
--- A class for operational monads
-----------------------------------------------------------------------------
-module Control.Monad.Operational.Class ((:!)(..)) where
-
-import Control.Monad.Trans.Reader
-import qualified Control.Monad.Trans.State.Strict as Strict
-import qualified Control.Monad.Trans.State.Lazy as Lazy
-import qualified Control.Monad.Trans.Writer.Strict as Strict
-import qualified Control.Monad.Trans.Writer.Lazy as Lazy
-import qualified Control.Monad.Trans.RWS.Strict as Strict
-import qualified Control.Monad.Trans.RWS.Lazy as Lazy
-import Control.Monad.Trans.Cont
-import Control.Monad.Trans.Maybe
-import Control.Monad.Trans.List
-import Control.Monad.Trans.Error
-import Control.Monad.Trans.Except
-import Control.Monad.Trans.Identity
-import Control.Monad.Trans.Class
-import Data.Monoid
-
-class Monad m => t :! m | m -> t where
-  -- | Construct an operational action from a single imperative.
-  singleton :: t a -> m a
-
-instance (t :! m) => t :! ReaderT e m where
-  singleton = lift . singleton
-
-instance (t :! m) => t :! Lazy.StateT s m where
-  singleton = lift . singleton
-
-instance (t :! m) => t :! Strict.StateT s m where
-  singleton = lift . singleton
-
-instance (t :! m) => t :! ContT r m where
-  singleton = lift . singleton
-
-instance (t :! m, Monoid w) => t :! Lazy.WriterT w m where
-  singleton = lift . singleton
-
-instance (t :! m, Monoid w) => t :! Strict.WriterT w m where
-  singleton = lift . singleton
-
-instance (t :! m, Monoid w) => t :! Strict.RWST r w s m where
-  singleton = lift . singleton
-
-instance (t :! m, Monoid w) => t :! Lazy.RWST r w s m where
-  singleton = lift . singleton
-
-instance (t :! m) => t :! MaybeT m where
-  singleton = lift . singleton
-
-instance (t :! m) => t :! IdentityT m where
-  singleton = lift . singleton
-
-instance (t :! m) => t :! ListT m where
-  singleton = lift . singleton
-
-instance (t :! m, Error e) => t :! ErrorT e m where
-  singleton = lift . singleton
-
-#if MIN_VERSION_transformers(0,4,0)
-instance (t :! m) => t :! ExceptT e m where
-  singleton = lift . singleton
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE CPP #-}
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Monad.Operational.Class
+-- Copyright   :  (C) 2012-2013 Fumiaki Kinoshita
+-- License     :  BSD-style (see the file LICENSE)
+--
+-- Maintainer  :  Fumiaki Kinsohita <fumiexcel@gmail.com>
+-- Stability   :  experimental
+-- Portability :  non-portable
+--
+-- A class for operational monads
+----------------------------------------------------------------------------
+module Control.Monad.Operational.Class ((:!)(..)) where
+
+import Control.Monad.Trans.Reader
+import qualified Control.Monad.Trans.State.Strict as Strict
+import qualified Control.Monad.Trans.State.Lazy as Lazy
+import qualified Control.Monad.Trans.Writer.Strict as Strict
+import qualified Control.Monad.Trans.Writer.Lazy as Lazy
+import qualified Control.Monad.Trans.RWS.Strict as Strict
+import qualified Control.Monad.Trans.RWS.Lazy as Lazy
+import Control.Monad.Trans.Cont
+import Control.Monad.Trans.Maybe
+import Control.Monad.Trans.List
+import Control.Monad.Trans.Error
+#if MIN_VERSION_transformers(0,4,0)
+import Control.Monad.Trans.Except
+#endif
+import Control.Monad.Trans.Identity
+import Control.Monad.Trans.Class
+import Data.Monoid
+
+class Monad m => t :! m | m -> t where
+  -- | Construct an operational action from a single imperative.
+  singleton :: t a -> m a
+
+instance (t :! m) => t :! ReaderT e m where
+  singleton = lift . singleton
+
+instance (t :! m) => t :! Lazy.StateT s m where
+  singleton = lift . singleton
+
+instance (t :! m) => t :! Strict.StateT s m where
+  singleton = lift . singleton
+
+instance (t :! m) => t :! ContT r m where
+  singleton = lift . singleton
+
+instance (t :! m, Monoid w) => t :! Lazy.WriterT w m where
+  singleton = lift . singleton
+
+instance (t :! m, Monoid w) => t :! Strict.WriterT w m where
+  singleton = lift . singleton
+
+instance (t :! m, Monoid w) => t :! Strict.RWST r w s m where
+  singleton = lift . singleton
+
+instance (t :! m, Monoid w) => t :! Lazy.RWST r w s m where
+  singleton = lift . singleton
+
+instance (t :! m) => t :! MaybeT m where
+  singleton = lift . singleton
+
+instance (t :! m) => t :! IdentityT m where
+  singleton = lift . singleton
+
+instance (t :! m) => t :! ListT m where
+  singleton = lift . singleton
+
+instance (t :! m, Error e) => t :! ErrorT e m where
+  singleton = lift . singleton
+
+#if MIN_VERSION_transformers(0,4,0)
+instance (t :! m) => t :! ExceptT e m where
+  singleton = lift . singleton
 #endif
diff --git a/minioperational.cabal b/minioperational.cabal
--- a/minioperational.cabal
+++ b/minioperational.cabal
@@ -1,5 +1,5 @@
 name:                minioperational
-version:             0.4.5
+version:             0.4.6
 synopsis:            fast and simple operational monad
 description:         This package provides tiny implementation of operational monad.
 homepage:            https://github.com/fumieval/minioperational