diff --git a/oi.cabal b/oi.cabal
--- a/oi.cabal
+++ b/oi.cabal
@@ -1,5 +1,5 @@
 Name:			oi
-Version:		0.2.1.0
+Version:		0.3.1
 Category:		Data
 Synopsis:		Library for purely functional lazy interactions with the outer world.
 Description:		This package implements a data structure and operations on it for writing interactive program with no imperative flavor of IO monads.
@@ -15,12 +15,12 @@
 Cabal-Version:		>= 1.14
 
 Data-dir:		sample
-Data-files:		Makefile catsSIO.hs catsLIO.hs catsIO.hs catsOI.hs echo.hs recdircs.hs talk.hs
+Data-files:		Makefile cats.hs cats2.hs echo.hs morec.hs recdircs.hs talk.hs
 
 Library
   Default-Language:         Haskell2010
   Hs-Source-Dirs:	    src/
-  Build-Depends:	    base >= 4.5 && < 5, parallel, comonad >= 3, filepath
+  Build-Depends:	    base >= 4.5 && < 5, parallel, comonad >= 3, filepath, directory, SafeSemaphore
   GHC-Options:		    -Wall -O0
   Exposed-modules:	    Data.OI
                            ,Data.OI.Internal
@@ -28,6 +28,7 @@
                            ,Data.OI.Force
                            ,Data.OI.IFun
                            ,Data.OI.IO
+                           ,Data.OI.Merge
                            ,Data.OI.Resource
                            ,Data.OI.System
 
diff --git a/sample/Makefile b/sample/Makefile
--- a/sample/Makefile
+++ b/sample/Makefile
@@ -1,4 +1,4 @@
-EXECS = echo recdircs talk catsSIO catsIO catsLIO catsOI
+EXECS = echo recdircs talk cats cats2 morec
 
 all	: $(EXECS)
 
@@ -11,16 +11,13 @@
 talk	: talk.hs
 	ghc --make -O0 -threaded $< -o $@
 
-catsIO	: catsIO.hs
-	ghc --make -O0 -threaded $< -o $@
-
-catsSIO	: catsSIO.hs
+cats	: cats.hs
 	ghc --make -O0 -threaded $< -o $@
 
-catsLIO	: catsLIO.hs
+cats2	: cats2.hs
 	ghc --make -O0 -threaded $< -o $@
 
-catsOI	: catsOI.hs
+morec	: morec.hs
 	ghc --make -O0 -threaded $< -o $@
 
 clean	:
diff --git a/sample/cats.hs b/sample/cats.hs
new file mode 100644
--- /dev/null
+++ b/sample/cats.hs
@@ -0,0 +1,38 @@
+{-# LANGUAGE TypeOperators #-}
+module Main where
+
+import Data.OI hiding (openFile)
+import System.IO (IOMode (..), Handle)
+import qualified System.IO as IO
+import Prelude hiding (readFile, writeFile)
+
+main :: IO ()
+main = runInteraction pmain
+
+pmain 
+  :: (([String],Either ((Handle, String), [(Handle, String)]) [(Handle, String)]), (Handle, ()))
+  :-> ()
+pmain = args 
+    |/| choiceOIOn (const $ lines . readFile "files" |/| zipWithOI readFile) 
+                   (zipWithOI readFile)
+                   null
+    |/| writeFile "output.txt" . concatMap textproc
+
+textproc :: String -> String
+textproc = id -- unlines . take 1 . lines
+
+readFile :: FilePath -> (Handle, String) :-> String
+readFile f      = openFile f ReadMode   |/| hGetContents
+
+writeFile :: FilePath -> String -> (Handle, ()) :-> ()
+writeFile f cs  = openFile f WriteMode  |/| flip hPutStr cs
+
+openFile :: FilePath -> IOMode -> Handle :-> Handle
+openFile     = (iooi .) . IO.openFile
+
+hGetContents :: Handle -> String :-> String
+hGetContents = iooi . IO.hGetContents
+
+hPutStr :: Handle -> String -> () :-> ()
+hPutStr      = (iooi .) . IO.hPutStr
+
diff --git a/sample/cats2.hs b/sample/cats2.hs
new file mode 100644
--- /dev/null
+++ b/sample/cats2.hs
@@ -0,0 +1,37 @@
+{-# LANGUAGE TypeOperators #-}
+module Main where
+
+import Control.Applicative ((<*>))
+import Data.OI hiding (openFile)
+import System.IO (IOMode (..), Handle)
+import qualified System.IO as IO
+import Prelude hiding (readFile, writeFile)
+
+main :: IO ()
+main = runInteraction pmain
+
+pmain 
+  :: (([String],Either ((Handle, String), [(Handle, String)]) [(Handle, String)]), (Handle, ()))
+  :-> ()
+pmain = args 
+    |/| ((choiceOI (lines . readFile "files" |/| zipWithOI readFile) . zipWithOI readFile) <*> null)
+    |/| writeFile "output.txt" . concatMap textproc
+
+textproc :: String -> String
+textproc = id -- unlines . take 1 . lines
+
+readFile :: FilePath -> (Handle, String) :-> String
+readFile f      = openFile f ReadMode   |/| hGetContents
+
+writeFile :: FilePath -> String -> (Handle, ()) :-> ()
+writeFile f cs  = openFile f WriteMode  |/| flip hPutStr cs
+
+openFile :: FilePath -> IOMode -> Handle :-> Handle
+openFile     = (iooi .) . IO.openFile
+
+hGetContents :: Handle -> String :-> String
+hGetContents = iooi . IO.hGetContents
+
+hPutStr :: Handle -> String -> () :-> ()
+hPutStr      = (iooi .) . IO.hPutStr
+
diff --git a/sample/catsIO.hs b/sample/catsIO.hs
deleted file mode 100644
--- a/sample/catsIO.hs
+++ /dev/null
@@ -1,35 +0,0 @@
-module Main where
-
-import System.Environment
-import System.IO hiding (readFile, writeFile)
-import Prelude hiding (readFile, writeFile)
-
-{-
- If "files" contains much many files, "./catsIO" throws a resource-exhaused exception."
--}
-
-main :: IO ()
-main = do
-  args     <- getArgs
-  contents <- if not $ null args 
-                then do
-                  mapM readFile args
-                else do 
-                  files <- readFile "files"
-                  mapM readFile (lines files)
-  writeFile "output.txt" (concatMap textproc contents)
-
-textproc :: String -> String
-textproc = unlines . take 1 . lines
-
-readFile :: String -> IO String
-readFile f = do
-  h  <- openFile f ReadMode
-  cs <- hGetContents h
-  return cs
-
-writeFile :: String -> String -> IO ()
-writeFile f cs = do
-  h  <- openFile f WriteMode
-  r  <- hPutStr h cs
-  return r
diff --git a/sample/catsLIO.hs b/sample/catsLIO.hs
deleted file mode 100644
--- a/sample/catsLIO.hs
+++ /dev/null
@@ -1,45 +0,0 @@
-module Main where
-
-import System.Environment
-import System.IO hiding (readFile, writeFile)
-import System.IO.Unsafe
-import Prelude hiding (readFile, writeFile)
-
-main :: IO ()
-main = do
-  args     <- getArgs
-  contents <- if not $ null args 
-                then do
-                  sequenceLz $ map readFile args
-                else do 
-                  files <- readFile "files"
-                  sequenceLz $ map readFile (lines files)
-  writeFile "output.txt" (concatMap textproc contents)
-
-textproc :: String -> String
-textproc = id -- unlines . take 1 . lines
-
-lazy :: IO a -> IO a
-lazy = unsafeInterleaveIO
-
-mapLzM :: (a -> IO b) -> [a] -> IO [b]
-mapLzM = (sequenceLz .) . map
-
-sequenceLz :: [IO a] -> IO [a]
-sequenceLz []     = lazy $ return []
-sequenceLz (i:is) = lazy $ do
-  r  <- lazy i
-  rs <- sequenceLz is
-  return (r:rs)
-
-readFile :: String -> IO String
-readFile f = do
-  h  <- openFile f ReadMode
-  cs <- hGetContents h
-  return cs
-
-writeFile :: String -> String -> IO ()
-writeFile f cs = do
-  h  <- openFile f WriteMode
-  r  <- hPutStr h cs
-  return r
diff --git a/sample/catsOI.hs b/sample/catsOI.hs
deleted file mode 100644
--- a/sample/catsOI.hs
+++ /dev/null
@@ -1,38 +0,0 @@
-{-# LANGUAGE TypeOperators #-}
-module Main where
-
-import Data.OI hiding (openFile)
-import System.IO (IOMode (..), Handle)
-import qualified System.IO as IO
-import Prelude hiding (readFile, writeFile)
-
-main :: IO ()
-main = runInteraction pmain
-
-pmain 
-  :: (([String],Either ((Handle, String), [(Handle, String)]) [(Handle, String)]), (Handle, ()))
-  :-> ()
-pmain = args 
-    |/| choiceOIOn (const $ lines . readFile "files" |/| zipWithOI readFile) 
-                   (zipWithOI readFile)
-                   null
-    |/| writeFile "output.txt" . concatMap textproc
-
-textproc :: String -> String
-textproc = id -- unlines . take 1 . lines
-
-readFile :: FilePath -> (Handle, String) :-> String
-readFile f      = openFile f ReadMode   |/| hGetContents
-
-writeFile :: FilePath -> String -> (Handle, ()) :-> ()
-writeFile f cs  = openFile f WriteMode  |/| flip hPutStr cs
-
-openFile :: FilePath -> IOMode -> Handle :-> Handle
-openFile     = (iooi .) . IO.openFile
-
-hGetContents :: Handle -> String :-> String
-hGetContents = iooi . IO.hGetContents
-
-hPutStr :: Handle -> String -> () :-> ()
-hPutStr      = (iooi .) . IO.hPutStr
-
diff --git a/sample/catsSIO.hs b/sample/catsSIO.hs
deleted file mode 100644
--- a/sample/catsSIO.hs
+++ /dev/null
@@ -1,40 +0,0 @@
-module Main where
-
-import System.Environment
-import qualified System.IO as IO
-import System.IO.Strict hiding (readFile, writeFile)
-import Prelude hiding (readFile, writeFile)
-
-{-
- If the amount of size of files contained in "files" is too much, "./catsSIO" throws a out-of-memory exception."
--}
-
-main :: IO ()
-main = do
-  args     <- getArgs
-  main' args
-
-main' :: [String] -> IO ()
-main' args = run $ do
-  contents <- if not $ null args 
-                then do
-                  mapM readFile args
-                else do 
-                  files <- readFile "files"
-                  mapM readFile (lines files)
-  writeFile "output.txt" (concatMap textproc contents)
-
-textproc :: String -> String
-textproc = unlines . take 1 . lines
-
-readFile :: String -> SIO String
-readFile f = do
-  h  <- openFile f IO.ReadMode
-  cs <- hGetContents h
-  return cs
-
-writeFile :: String -> String -> SIO ()
-writeFile f cs = do
-  h  <- openFile f IO.WriteMode
-  r  <- hPutStr h cs
-  return r
diff --git a/sample/echo.hs b/sample/echo.hs
--- a/sample/echo.hs
+++ b/sample/echo.hs
@@ -3,39 +3,50 @@
 
 import Data.OI hiding (isEOF)
 
-import Data.Char
+import Data.Maybe
 import System.IO
 
+import qualified System.Environment as Sys (getArgs)
+
 main :: IO ()
-main = runInteraction $ pmain1
+main =  runInteraction pmain
 
-pmain0 = forceSeq . mapOI echo
-pmain1 = forceSeq . echos
+pmain :: ([String], Either [(Maybe Char, Maybe ())] ([Maybe Char], [Maybe ()])) :-> ()
+pmain = null . getArgs |/| choiceOI pmain0 pmain1
 
-eof :: Int
-eof = -1
+pmain0 :: [(Maybe Char, Maybe ())] :-> ()
+pmain0 = forceSeq . takeWhile isJust . mapOI echochar
 
-getc :: Int :-> Int
+pmain1 :: ([Maybe Char], [Maybe ()]) :-> ()
+pmain1 = forceSeq . echostring
+
+getc :: Maybe Char :-> Maybe Char
 getc = iooi getchar
 
-putc :: Char -> () :-> ()
-putc = iooi . putChar
+putc :: Maybe Char -> Maybe () :-> Maybe ()
+putc = iooi . putchar
 
-putc' :: Int -> () :-> ()
-putc' (-1) = const ()
-putc' c    = iooi (putChar (chr c))
+getchar :: IO (Maybe Char)
+getchar = choice (return Nothing) (return . Just =<< getChar) =<< isEOF 
 
-gets :: [Int] :-> String
-gets = map chr . takeWhile (eof /=) . mapOI getc
+putchar :: Maybe Char -> IO (Maybe ())
+putchar = maybe (return Nothing)
+                ((return . Just =<<) . putChar)
 
-puts :: String -> [()] :-> [()]
-puts = zipWithOI putc
+gets :: [Maybe Char] :-> String
+gets = map fromJust . takeWhile isJust . mapOI getc
 
-getchar :: IO Int
-getchar = choice (return (-1)) (return . ord =<< getChar) =<< isEOF 
+puts :: String -> [Maybe ()] :-> [Maybe ()]
+puts = zipWithOI putc . map Just
 
-puts' :: String -> OI [()] -> ()
-puts' = seqsOI . map putc
+echochar :: (Maybe Char, Maybe ()) :-> Maybe ()
+echochar  = getc |/| putc
 
-echo  = getc |/| putc'
-echos = gets |/| puts
+echostring :: ([Maybe Char], [Maybe ()]) :-> [Maybe ()]
+echostring = gets |/| puts
+
+getArgs :: [String] :-> [String]
+getArgs = iooi Sys.getArgs
+
+choice :: a -> a -> Bool -> a
+choice t f c = if c then t else f
diff --git a/sample/morec.hs b/sample/morec.hs
new file mode 100644
--- /dev/null
+++ b/sample/morec.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE TypeOperators #-}
+module Main where
+
+import Data.OI
+import System.IO
+
+main :: IO ()
+main = do 
+  hSetBuffering stdin NoBuffering
+  runInteraction morec
+  Prelude.putStrLn ""
+
+(|?|) :: (a :-> c)
+      -> (b :-> c)
+      -> Bool
+      -> (Either a b :-> c)
+(|?|) = choiceOI
+
+getc :: Char :-> Char
+getc  = iooi getChar
+
+putc :: Char -> (() :-> ())
+putc  = iooi . putChar
+
+echoc :: (Char,()) :-> ()
+echoc = getc |/| putc
+
+morec :: (Char, Either (Char, ()) ()) :-> ()
+morec = ('e' ==) . getc |/| (echoc |?| putc 'q')
diff --git a/sample/recdircs.hs b/sample/recdircs.hs
--- a/sample/recdircs.hs
+++ b/sample/recdircs.hs
@@ -16,6 +16,7 @@
 pmain :: (([String], [(Bool, IOResult [FilePath])]), ()) :-> ()
 pmain = (((args >>> head) |/| getDirCsRec) >>> snd) |/| putStrOI . unlines
 
+
 putStrOI :: String -> () :-> ()
 putStrOI = iooi . putStr
 
@@ -41,3 +42,6 @@
     (False, fps) -> (os,fps)
     (True,  fps) -> second concat $ mapAccumL (flip getDirCsRec) os fps
   _           -> error $ "getDirCs: perhaps `"++ fp ++"' is not found"
+
+choice :: a -> a -> Bool -> a
+choice t f c = if c then t else f
diff --git a/sample/talk.hs b/sample/talk.hs
--- a/sample/talk.hs
+++ b/sample/talk.hs
@@ -4,7 +4,6 @@
 
 import Data.OI
 
-import Control.Concurrent
 import Control.Parallel
 import System.Environment
 
@@ -33,5 +32,3 @@
 
 showscr :: FilePath -> [String] -> () :-> ()
 showscr s = iooi . writeFile s . unlines
-
-mergeOI = (iooi .) . mergeIO
diff --git a/src/Data/OI.hs b/src/Data/OI.hs
--- a/src/Data/OI.hs
+++ b/src/Data/OI.hs
@@ -13,6 +13,7 @@
   ,module Data.OI.Combinator
   ,module Data.OI.IFun
   ,module Data.OI.IO
+  ,module Data.OI.Merge
   ,module Data.OI.Resource
   ,module Data.OI.System
   ,module Data.OI.Force
@@ -23,5 +24,6 @@
 import Data.OI.Force
 import Data.OI.IFun
 import Data.OI.IO
+import Data.OI.Merge
 import Data.OI.Resource
 import Data.OI.System
diff --git a/src/Data/OI/Combinator.hs b/src/Data/OI/Combinator.hs
--- a/src/Data/OI/Combinator.hs
+++ b/src/Data/OI/Combinator.hs
@@ -6,15 +6,11 @@
 -- Maintainer  : nobsun@sampou.org
 -- Stability   : experimental
 --
-{-# LANGUAGE TypeOperators
-            ,BangPatterns
-  #-}
+{-# LANGUAGE TypeOperators #-}
 module Data.OI.Combinator
   (
-   -- * Utility functions
-   choice
    -- * Interaction Combinators
-  ,(|:|)
+   (|:|)
   ,(|>|),(|/|)
   ,(|><|)
    -- * Iteration
@@ -35,12 +31,6 @@
 import Data.OI.Internal
 import Data.OI.Force
 
-(|>) :: (a -> b) -> (b -> c) -> (a -> c)
-(|>) = flip (.)
-
-choice :: a -> a -> Bool -> a
-choice t f c = if c then t else f
-
 -- | Connect two interactions into an interaction
 
 infixl 3 |:|
@@ -63,6 +53,7 @@
   (a,b) -> (c,d) where (q,c) = f a p; (p,d) = g b q
 
 -- | Iteration
+
 foldOI :: (a :-> (b -> b)) -> b -> ([a] :-> b)
 foldOI op z xxs = case deList xxs of
   Just (x,xs) -> x `op` foldOI op z xs
@@ -104,6 +95,7 @@
 choiceOIOn f g p x = choiceOI (f x) (g x) (p x)
 
 -- | Sequencing
+
 seqsOI :: [a :-> b] -> ([a] :-> ())
 seqsOI s os =  forceSeq $ zipWithOI ($) s os
 
diff --git a/src/Data/OI/Internal.hs b/src/Data/OI/Internal.hs
--- a/src/Data/OI/Internal.hs
+++ b/src/Data/OI/Internal.hs
@@ -19,6 +19,9 @@
   ,(:->)
   -- * Primitive operators on OI
   ,(??)
+  ,(##)
+  -- * Assign operator
+  ,(=:)
   -- * Splitters against `OI' datatype
   ,dePair
   ,deList
@@ -39,14 +42,14 @@
   where
 
 import Control.Applicative
-import Control.Category
+-- import Control.Category
 import Control.Comonad
 import Control.Exception
-import Control.Monad
+-- import Control.Monad
 import Control.Concurrent
 import Control.Parallel
 import System.IO.Unsafe
-import Prelude hiding ((.),id,catch)
+-- import Prelude hiding ((.),id,catch)
 
 -- | Datatype for intermediating interaction: 
 -- @OI@ has two states (programmer cannot distinguish), non-expressed and exressed.
@@ -81,6 +84,10 @@
 -- | Reference operator
 (##) :: a -> OI a
 (##) x = OI (unsafeNew x) x
+
+-- | Assign Operator
+(=:) :: a -> OI a -> a
+(=:) !x (OI var val) = put (return x) var `pseq` val
 
 -- | Decomposer for pair
 dePair :: OI (a,b) -> (OI a, OI b)
diff --git a/src/Data/OI/Merge.hs b/src/Data/OI/Merge.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/OI/Merge.hs
@@ -0,0 +1,87 @@
+{-# LANGUAGE TypeOperators #-}
+module Data.OI.Merge
+ ( -- * merge list
+  mergeOI
+ ,nmergeOI  
+ ) where
+
+import Data.OI.Internal
+
+import Control.Concurrent (forkIO)
+import Control.Concurrent.MVar (MVar,newEmptyMVar,newMVar,takeMVar,putMVar)
+import Control.Concurrent.SSem (SSem,new,signal,wait)
+import System.IO.Unsafe (unsafeInterleaveIO)
+
+mergeOI :: [a] -> [a] -> [a] :-> [a]
+mergeOI = (iooi .) . mergeIO
+
+nmergeOI :: [[a]] -> [a] :-> [a]
+nmergeOI =  iooi . nmergeIO
+
+-- The origin of following codes came from Control.Concurrent module.
+-- The original codes using Control.Concurrent.QSem were deprecated.
+-- The following codes are made by modifying the original code to use
+-- Control.Concurrent.SSem module (privided by SafeSemaphore).
+
+mergeIO :: [a] -> [a] -> IO [a]
+nmergeIO :: [[a]] -> IO [a]
+
+mergeIO ls rs
+ = newEmptyMVar                >>= \ tail_node ->
+   newMVar tail_node           >>= \ tail_list ->
+   new max_buff_size           >>= \ e ->
+   newMVar 2                   >>= \ branches_running ->
+   let
+    buff = (tail_list,e)
+   in
+    forkIO (suckIO branches_running buff ls) >>
+    forkIO (suckIO branches_running buff rs) >>
+    takeMVar tail_node  >>= \ val ->
+    signal e            >>
+    return val
+
+nmergeIO lss
+ = let
+    len = length lss
+   in
+    newEmptyMVar          >>= \ tail_node ->
+    newMVar tail_node     >>= \ tail_list ->
+    new     max_buff_size >>= \ e ->
+    newMVar len           >>= \ branches_running ->
+    let
+     buff = (tail_list,e)
+    in
+    mapIO (\ x -> forkIO (suckIO branches_running buff x)) lss >>
+    takeMVar tail_node  >>= \ val ->
+    signal e            >>
+    return val
+  where
+    mapIO f xs = sequence (map f xs)
+
+max_buff_size  ::  Int
+max_buff_size  =   1
+
+type Buffer a
+ = (MVar (MVar [a]), SSem)
+
+suckIO :: MVar Int -> Buffer a -> [a] -> IO ()
+suckIO branches_running buff@(tail_list,e) vs
+ = case vs of
+        [] -> takeMVar branches_running >>= \ val ->
+              if val == 1 then
+                 takeMVar tail_list     >>= \ node ->
+                 putMVar node []        >>
+                 putMVar tail_list node
+              else
+                 putMVar branches_running (val-1)
+        (x:xs) ->
+                wait e                           >>
+                takeMVar tail_list               >>= \ node ->
+                newEmptyMVar                     >>= \ next_node ->
+                unsafeInterleaveIO (
+                        takeMVar next_node  >>= \ y ->
+                        signal     e        >>
+                        return y)                >>= \ next_node_val ->
+                putMVar node (x:next_node_val)   >>
+                putMVar tail_list next_node      >>
+                suckIO branches_running buff xs