diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,165 @@
+		   GNU LESSER GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
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diff --git a/Setup.hs b/Setup.hs
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+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/bidispec.cabal b/bidispec.cabal
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--- /dev/null
+++ b/bidispec.cabal
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+Name:                bidispec
+Version:             0.1
+Synopsis:            Specification of generators and parsers
+Description:
+ This is a generalisation of the idea behind the XML pickle/unpickle
+ functions of HXT.  It allows to simulatanously specify functions for
+ parsing and generating.
+License:             LGPL
+License-file:        LICENSE
+Category:            Data
+Author:              David Leuschner
+Maintainer:          David Leuschner <leuschner@openfactis.org>
+Build-Type:          Simple
+Cabal-Version:       >= 1.2
+
+Library
+  Hs-Source-Dirs:    src
+  GHC-Options: -Wall
+  Build-Depends:     base >= 3 && < 4, bytestring, mtl
+  Exposed-Modules:   Data.BidiSpec
diff --git a/src/Data/BidiSpec.hs b/src/Data/BidiSpec.hs
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--- /dev/null
+++ b/src/Data/BidiSpec.hs
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+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE PatternGuards #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE FlexibleContexts #-}
+
+module Data.BidiSpec
+    (Spec, SpecGen, SpecParser(..)
+    ,mkSpec, parseBySpec, genBySpec, runSpecParser, rsGen, rsParse
+    ,spGet, spGets, spCheck, spFromMaybe, spFromEither
+    ,rsPair, rsTriple, rsQuadruple
+    ,rsWrap, rsWrapMaybe, rsWrapEither, rsWrapEither',rsCondSeq
+    ,rsChoice, rsAlt, rsTagSwitch, rsSwitch, rsCase, rsCaseConst
+    ,rsGetSet,rsCheckSet, rsLift, rsUnit, rsZero, rsWith
+    )
+where
+
+----------------------------------------
+-- STDLIB
+----------------------------------------
+import Control.Monad (MonadPlus(..), msum, guard, liftM)
+import Control.Monad.Error (MonadError(..), Error(..), ErrorT)
+import Control.Monad.Trans (MonadTrans, lift)
+import Control.Monad.Reader (MonadReader(..), ReaderT, runReaderT, ask, asks)
+
+import Data.Maybe (fromMaybe)
+import Data.List (find,elemIndex)
+
+-- ----------------------------------------------------------------------------
+--  SpecParser
+-- ----------------------------------------------------------------------------
+newtype SpecParser s e a
+    = SpecParser { unSpecParser :: ReaderT s (Either e) a }
+    deriving (Monad, MonadPlus)
+
+instance Error e => MonadError e (SpecParser s e) where
+    throwError e = SpecParser $ lift (throwError e)
+    catchError (SpecParser ra) f =
+        do s <- spGet
+           case runReaderT ra s of
+             Left e -> f e
+             Right x -> return x
+
+instance Error e => MonadReader s (SpecParser s e) where
+    ask = SpecParser ask
+    local mapSt (SpecParser cont) = SpecParser (local mapSt cont)
+
+runSpecParser :: SpecParser s e a -> s -> Either e a
+runSpecParser = runReaderT . unSpecParser
+
+spGet :: Error e => SpecParser s e s
+spGet = ask
+
+spGets :: Error e => (s -> a) -> SpecParser s e a
+spGets = asks
+
+spCheck :: Error e => (a -> Bool) -> (a -> e) -> a -> SpecParser s e ()
+spCheck check mkErr a = guard (check a) `mplus` (throwError (mkErr a))
+
+-- Not exported because the user should supply custom error messages!
+spFromMaybe :: Error e => e -> Maybe a -> SpecParser s e a
+spFromMaybe e Nothing = throwError e
+spFromMaybe _e (Just a) = return a
+
+spFromEither :: Error e => Either e a -> SpecParser s e a
+spFromEither (Left e) = throwError e
+spFromEither (Right a) = return a
+
+-- ----------------------------------------------------------------------------
+--  Spec generator
+-- ----------------------------------------------------------------------------
+
+type SpecGen tgt a = tgt -> a -> tgt
+
+-- ----------------------------------------------------------------------------
+--  Spec data type
+-- ----------------------------------------------------------------------------
+
+data Spec err src tgt a
+    = Spec
+    { rsGen ::  SpecGen tgt a
+    , rsParse :: SpecParser src err a }
+
+mkSpec :: SpecParser i e a -> SpecGen o a -> Spec e i o a
+mkSpec = flip Spec
+
+parseBySpec :: MonadError e m => Spec e i o a -> i -> m a
+parseBySpec sp i =
+    case runReaderT (unSpecParser (rsParse sp)) i of
+      Left err -> throwError err
+      Right res -> return res
+
+genBySpec :: Monad m => Spec e i o a -> o -> a -> m o
+genBySpec sp o = return . rsGen sp o
+
+
+-- ----------------------------------------------------------------------------
+--  Spec combinators
+-- ----------------------------------------------------------------------------
+
+rsPair :: Error e => Spec e i o a -> Spec e i o b -> Spec e i o (a,b)
+rsPair rsA rsB = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout (a,b) = rsGen rsA (rsGen rsB rout b) a
+          rsParseDef = do a <- rsParse rsA
+                          b <- rsParse rsB
+                          return (a, b)
+
+rsTriple :: Error e =>
+            Spec e i o a
+         -> Spec e i o b
+         -> Spec e i o c
+         -> Spec e i o (a,b,c)
+rsTriple rsA rsB rsC = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout (a,b,c) = rsGen rsC (rsGen rsB (rsGen rsA rout a) b) c
+          rsParseDef = do a <- rsParse rsA
+                          b <- rsParse rsB
+                          c <- rsParse rsC
+                          return (a, b, c)
+
+rsQuadruple :: Error e =>
+               Spec e i o a
+            -> Spec e i o b
+            -> Spec e i o c
+            -> Spec e i o d
+            -> Spec e i o (a,b,c,d)
+rsQuadruple rsA rsB rsC rsD = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout (a,b,c,d)
+              = rsGen rsD (rsGen rsC (rsGen rsB (rsGen rsA rout a) b) c) d
+          rsParseDef = do a <- rsParse rsA
+                          b <- rsParse rsB
+                          c <- rsParse rsC
+                          d <- rsParse rsD
+                          return (a, b, c, d)
+
+rsWrap :: Error e => (a -> b, b -> a) -> Spec e i o a -> Spec e i o b
+rsWrap (toB,toA) = rsWrapMaybe (error "BidiSpec: rsWrap") (return . toB, toA)
+
+rsWrapMaybe :: Error e =>
+               String                        -- error message for Maybe case
+            -> (a -> Maybe b, b -> a)        -- wrappers
+            -> Spec e i o a
+            -> Spec e i o b
+rsWrapMaybe msg (aToB, bToA) rsA = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout b = rsGen rsA rout (bToA b)
+          rsParseDef = rsParse rsA >>= parseA . aToB
+          parseA Nothing = fail (strMsg $ "rsWrapMaybe: " ++ msg)
+          parseA (Just b) = return b
+
+rsWrapEither :: Error e =>
+                (a -> Either e b, b -> a)     -- wrappers
+             -> Spec e i o a
+             -> Spec e i o b
+rsWrapEither (aToB, bToA) rsA = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout b = rsGen rsA rout (bToA b)
+          rsParseDef = rsParse rsA >>= parseA . aToB
+          parseA (Left err) = throwError err
+          parseA (Right b) = return b
+
+rsWrapEither' :: (Show l, Error e) =>
+                 (a -> Either l b, b -> a)     -- wrappers
+              -> Spec e i o a
+              -> Spec e i o b
+rsWrapEither' (toB,toA) = rsWrapEither (mapLeft (strMsg . show) . toB, toA)
+    where mapLeft f (Left a) = Left (f a)
+          mapLeft _f (Right c) = Right c
+
+rsCondSeq :: Error e =>
+             Spec e i o b
+          -> (b -> a)
+          -> Spec e i o a
+          -> (a -> Spec e i o b)
+          -> Spec e i o b
+rsCondSeq pd f pa k = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout b = let a = f b
+                                pb = k a
+                            in rsGen pa (rsGen pb rout b) a
+          rsParseDef = do a <- rsParse pa
+                          rsParse (k a)
+                       `mplus` rsParse pd
+
+rsChoice :: Error e =>
+            Spec e i o b
+         -> Spec e i o a
+         -> (a -> Spec e i o b)
+         -> Spec e i o b
+rsChoice pb = rsCondSeq pb undefined
+
+rsAlt :: Error e => (a -> Int) -> [Spec e i o a] -> Spec e i o a
+rsAlt getIdx alts = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout a = rsGen (alts !! getIdx a) rout a
+          rsParseDef =
+              case alts of
+                [] -> rsParse rsZero
+                (x:xs) -> rsParse (rsChoice (rsAlt getIdx xs) x rsLift)
+
+data SpecCase e i o a = SpecCase { case_value :: a
+                                 , case_spec :: Spec e i o a }
+
+rsCase :: Error e => (a -> b, b -> a) -> Spec e i o a -> SpecCase e i o b
+rsCase wrapfuns@(aToB,_bToA) specA = SpecCase value spec
+    where value = aToB (error "rsCase: tagging function requires evaluation")
+          spec = rsWrap wrapfuns specA
+
+rsCaseConst :: Error e =>
+               a                               -- constant to match/generate
+            -> (Spec e i o b -> Spec e i o b)  -- continuation
+            -> SpecCase e i o a
+rsCaseConst a mkRs = rsCase (const a, const undefined) (mkRs (rsLift undef))
+    where undef = error "rsCaseConst: this value should have been ignored"
+
+rsSwitch :: (Error e, Show a) => [SpecCase e i o a] -> Spec e i o a
+rsSwitch = rsTagSwitch (takeWhile take . dropWhile (=='(') . show)
+    where take x = x /= ' ' && x /= ','
+
+rsTagSwitch :: (Error e, Eq t) => (a -> t) -> [SpecCase e i o a] -> Spec e i o a
+rsTagSwitch tag cases = mkSpec rsParseDef rsGenDef
+    where
+      rsParseDef = foldl mplus (fail noMatch) (map (rsParse . case_spec) cases)
+      rsGenDef rout a =
+          case fmap case_spec $ find ((tag a ==) . tag . case_value) cases of
+            Just spec -> rsGen spec rout a
+            Nothing -> error noMatch
+      noMatch = "rsSwitch: No case matched."
+
+rsGetSet :: Error e => (i -> a) -> (o -> a -> o) -> Spec e i o a
+rsGetSet get set = mkSpec rsParseDef rsGenDef
+    where rsGenDef = set
+          rsParseDef = spGet >>= return . get
+
+rsCheckSet :: Error e =>
+              SpecParser i e ()
+           -> (o -> o)
+           -> Spec e i o a
+           -> Spec e i o a
+rsCheckSet parser setfun rs = mkSpec rsParseDef rsGenDef
+    where rsGenDef rout a = rsGen rs (setfun rout) a
+          rsParseDef = parser >> rsParse rs
+
+rsLift :: Error e => a -> Spec e i o a
+rsLift x = mkSpec (return x) const
+
+rsZero :: Error e => Spec e i o a
+rsZero = mkSpec (fail "rsZero") const
+
+rsUnit :: Error e => Spec e i o ()
+rsUnit = rsLift ()
+
+rsWith :: (Spec e i o a -> Spec e i o b) -> Spec e i o a -> Spec e i o b
+rsWith = ($)