diff --git a/CHANGELOG.md b/CHANGELOG.md
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--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,11 @@
+# Changelog for `higher-order-open-union`
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to the
+[Haskell Package Versioning Policy](https://pvp.haskell.org/).
+
+## Unreleased
+
+## 0.1.0.0 - YYYY-MM-DD
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,26 @@
+Copyright 2025 Yoshikuni Jujo
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1.  Redistributions of source code must retain the above copyright notice, this
+    list of conditions and the following disclaimer.
+
+2.  Redistributions in binary form must reproduce the above copyright notice,
+    this list of conditions and the following disclaimer in the documentation
+    and/or other materials provided with the distribution.
+
+3.  Neither the name of the copyright holder nor the names of its contributors
+    may be used to endorse or promote products derived from this software
+    without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/README.md b/README.md
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--- /dev/null
+++ b/README.md
@@ -0,0 +1,7 @@
+# higher-order-open-union
+
+This package used by package yaftee.
+
+```Haskell
+type Eff effs = Data.HigherFunctor.H (Control.HigherOpenUnion.U effs)
+```
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/app/Main.hs b/app/Main.hs
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--- /dev/null
+++ b/app/Main.hs
@@ -0,0 +1,6 @@
+module Main (main) where
+
+import Lib
+
+main :: IO ()
+main = someFunc
diff --git a/higher-order-open-union.cabal b/higher-order-open-union.cabal
new file mode 100644
--- /dev/null
+++ b/higher-order-open-union.cabal
@@ -0,0 +1,76 @@
+cabal-version: 2.2
+
+-- This file has been generated from package.yaml by hpack version 0.38.1.
+--
+-- see: https://github.com/sol/hpack
+
+name:           higher-order-open-union
+version:        0.1.0.0
+synopsis:       This package is used by package yaftee
+description:    Please see the README on GitHub at <https://github.com/YoshikuniJujo/higher-order-open-union#readme>
+category:       Control
+homepage:       https://github.com/YoshikuniJujo/higher-order-open-union#readme
+bug-reports:    https://github.com/YoshikuniJujo/higher-order-open-union/issues
+author:         Yoshikuni Jujo
+maintainer:     yoshikuni.jujo@gmail.com
+copyright:      (c) 2025 Yoshikuni Jujo
+license:        BSD-3-Clause
+license-file:   LICENSE
+build-type:     Simple
+extra-source-files:
+    README.md
+extra-doc-files:
+    CHANGELOG.md
+
+source-repository head
+  type: git
+  location: https://github.com/YoshikuniJujo/higher-order-open-union
+
+library
+  exposed-modules:
+      Control.HigherOpenUnion
+      Data.HigherFunctor
+      Data.TypeElem
+      Lib
+  other-modules:
+      Paths_higher_order_open_union
+  autogen-modules:
+      Paths_higher_order_open_union
+  hs-source-dirs:
+      src
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints
+  build-depends:
+      base >=4.7 && <5
+    , freer-base-classes ==0.1.*
+  default-language: Haskell2010
+
+executable higher-order-open-union-exe
+  main-is: Main.hs
+  other-modules:
+      Paths_higher_order_open_union
+  autogen-modules:
+      Paths_higher_order_open_union
+  hs-source-dirs:
+      app
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , freer-base-classes ==0.1.*
+    , higher-order-open-union
+  default-language: Haskell2010
+
+test-suite higher-order-open-union-test
+  type: exitcode-stdio-1.0
+  main-is: Spec.hs
+  other-modules:
+      Paths_higher_order_open_union
+  autogen-modules:
+      Paths_higher_order_open_union
+  hs-source-dirs:
+      test
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , freer-base-classes ==0.1.*
+    , higher-order-open-union
+  default-language: Haskell2010
diff --git a/src/Control/HigherOpenUnion.hs b/src/Control/HigherOpenUnion.hs
new file mode 100644
--- /dev/null
+++ b/src/Control/HigherOpenUnion.hs
@@ -0,0 +1,102 @@
+{-# LANGUAGE ImportQualifiedPost #-}
+{-# LANGUAGE ScopedTypeVariables, TypeApplications #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE KindSignatures, TypeOperators #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# OPTIONS_GHC -Wall -fno-warn-tabs #-}
+
+module Control.HigherOpenUnion (
+
+	-- * TYPES
+
+	U, HT, Member, Base, FromFirst(..),
+
+	-- * INJECTION
+
+	inj, injBase, injh,
+
+	-- * PROJECTION
+
+	prj, decomp, extract, extracth,
+
+	-- * WEAKEN
+
+	weaken,
+
+	-- * NON DET AND FAIL
+
+	NonDet(..), Fail(..)
+
+	) where
+
+import Control.Monad.Freer.NonDetable qualified as NonDetable
+import Control.Monad.Freer.Failable qualified as Failable
+import Data.Kind
+import Data.TypeElem
+import Unsafe.Coerce
+
+-- * TYPES
+
+data U (hs :: [HT]) (f :: Type -> Type -> Type -> Type) i o a =
+	forall h . U Word (h f i o a)
+
+type HT = (Type -> Type -> Type -> Type) -> Type -> Type -> Type -> Type
+
+data FromFirst t (f :: Type -> Type -> Type -> Type) i o a
+	= forall x . FromFirst (t x) (x -> a)
+
+-- * INJECTION
+
+inj :: forall t hs (f :: Type -> Type -> Type -> Type) i o a .
+	Member (FromFirst t) hs => t a -> U hs f i o a
+inj = U (unP (elemNo :: P (FromFirst t) hs)) . (`FromFirst` id)
+
+injBase :: forall t hs (f :: Type -> Type -> Type -> Type) i o a .
+	Base (FromFirst t) hs => t a -> U hs f i o a
+injBase = U (unP (elemNoBase :: P (FromFirst t) hs)) . (`FromFirst` id)
+
+injh :: forall h hs f i o a . Member h hs => h f i o a -> U hs f i o a
+injh = U $ unP (elemNo :: P h hs)
+
+-- * PROJECTION
+
+prj :: forall h hs f i o a . Member h hs => U hs f i o a -> Maybe (h f i o a)
+prj (U i hx)
+	| i == unP (elemNo :: P h hs) = Just $ unsafeCoerce hx
+	| otherwise = Nothing
+
+decomp :: U (h ': hs) f i o a -> Either (U hs f i o a) (h f i o a)
+decomp (U 0 hx) = Right $ unsafeCoerce hx
+decomp (U i hx) = Left $ U (i - 1) hx
+
+extract :: Functor t => U '[FromFirst t] f i o a -> t a
+extract (U _ hx) = case (unsafeCoerce hx) of FromFirst tx k -> k <$> tx
+
+extracth :: U '[h] f i o a -> h f i o a
+extracth (U _ hx) = unsafeCoerce hx
+
+-- * WEAKEN
+
+weaken :: U hs f i o a -> U (any ': hs) f i o a
+weaken (U n a) = U (n + 1) a
+
+-- * NON DET AND FAIL
+
+instance Member (FromFirst NonDet) effs => NonDetable.N (U effs f i o) where
+	mz = inj (NonDetable.mz @NonDet); mp = inj (NonDetable.mp @NonDet)
+
+data NonDet a where MZero :: NonDet a; MPlus :: NonDet Bool
+
+deriving instance Show (NonDet a)
+
+instance NonDetable.N NonDet where mz = MZero; mp = MPlus
+
+instance Member Fail effs => Failable.F (U effs f i o) where
+	fail = injh . Fail
+
+data Fail (f :: Type -> Type -> Type -> Type) i o a where
+	Fail :: forall f a i o . String -> Fail f i o a
+	FailCatch :: forall f a i o .
+		f i o a -> (String -> f i o a) -> Fail f i o a
diff --git a/src/Data/HigherFunctor.hs b/src/Data/HigherFunctor.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/HigherFunctor.hs
@@ -0,0 +1,70 @@
+{-# LANGUAGE ImportQualifiedPost #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE DefaultSignatures #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE FlexibleContexts, FlexibleInstances, UndecidableInstances #-}
+{-# OPTIONS_GHC -Wall -fno-warn-tabs #-}
+
+module Data.HigherFunctor (Loose(..), TightI(..), TightO(..), Tight(..)) where
+
+import Prelude hiding (map)
+import Control.HigherOpenUnion
+
+class Loose h where
+	map :: (forall i o x . f i o x -> g i o (t x)) ->
+		(forall x . x -> t x) -> h f i' o' x' -> h g i' o' (t x')
+	default map :: Tight h => (forall i o x . f i o x -> g i o (t x)) ->
+		(forall x . x -> t x) -> h f i' o' x' -> h g i' o' (t x')
+	map = mapT
+
+instance Loose (FromFirst t)
+instance Loose (U '[])
+
+instance (Loose h, Loose (U hs)) => Loose (U (h ': hs)) where
+	map f g u = case decomp u of
+		Left u' -> weaken $ map f g u'
+		Right h -> injh $ map f g h
+
+class TightI h where
+	mapI :: (forall o x . f i o x -> g i' o (t x)) ->
+		(forall x . x -> t x) -> h f i o' x' -> h g i' o' (t x')
+	default mapI :: Tight h => (forall o x . f i o x -> g i' o (t x)) ->
+		(forall x . x -> t x) -> h f i o' x' -> h g i' o' (t x')
+	mapI = mapT
+
+instance TightI (FromFirst t)
+instance TightI (U '[])
+
+instance (TightI h, TightI (U hs)) => TightI (U (h ': hs)) where
+	mapI f g u = case decomp u of
+		Left u' -> weaken $ mapI f g u'
+		Right h -> injh $ mapI f g h
+
+class TightO h where
+	mapO :: (forall i x . f i o x -> g i o' (t x)) ->
+		(forall x . x -> t x) -> h f i' o x' -> h g i' o' (t x')
+	default mapO :: Tight h => (forall i x . f i o x -> g i o' (t x)) ->
+		(forall x . x -> t x) -> h f i' o x' -> h g i' o' (t x')
+	mapO = mapT
+
+instance TightO (FromFirst t)
+instance TightO (U '[])
+
+instance (TightO h, TightO (U hs)) => TightO (U (h ': hs)) where
+	mapO f g u = case decomp u of
+		Left u' -> weaken $ mapO f g u'
+		Right h -> injh $ mapO f g h
+
+class Tight h where
+	mapT :: (f i o x -> g i' o' y) -> (x -> y) -> h f i o x -> h g i' o' y
+
+instance Tight (FromFirst t) where
+	mapT _ g (FromFirst x k) = FromFirst x (g . k)
+
+instance Tight (U '[]) where mapT _ _ _ = error "bad"
+
+instance (Tight h, Tight (U hs)) => Tight (U (h ': hs)) where
+	mapT f g u = case decomp u of
+		Left u' -> weaken $ mapT f g u'
+		Right h -> injh $ mapT f g h
diff --git a/src/Data/TypeElem.hs b/src/Data/TypeElem.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/TypeElem.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE DataKinds, PolyKinds #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# OPTIONS_GHC -Wall -fno-warn-tabs #-}
+
+module Data.TypeElem (P, unP, Member, elemNo, Base, elemNoBase) where
+
+newtype P t ts = P { unP :: Word } deriving Show
+
+class Member (t :: k) (ts :: [k]) where elemNo :: P t ts
+
+instance Member t (t ': ts) where elemNo = P 0
+
+instance {-# OVERLAPPABLE #-} Member t ts => Member t (_t' ': ts) where
+	elemNo = P $ 1 + unP (elemNo :: P t ts)
+
+class Base (t :: k) (ts :: [k]) where elemNoBase :: P t ts
+
+instance Base t '[t] where elemNoBase = P 0
+
+instance {-# OVERLAPPABLE #-} Base t ts => Base t (_t' ': ts) where
+	elemNoBase = P $ 1 + unP (elemNoBase :: P t ts)
diff --git a/src/Lib.hs b/src/Lib.hs
new file mode 100644
--- /dev/null
+++ b/src/Lib.hs
@@ -0,0 +1,6 @@
+module Lib
+    ( someFunc
+    ) where
+
+someFunc :: IO ()
+someFunc = putStrLn "someFunc"
diff --git a/test/Spec.hs b/test/Spec.hs
new file mode 100644
--- /dev/null
+++ b/test/Spec.hs
@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "Test suite not yet implemented"