diff --git a/Data/Constraint.hs b/Data/Constraint.hs
--- a/Data/Constraint.hs
+++ b/Data/Constraint.hs
@@ -1,10 +1,13 @@
-module Data.Constraint where
+module Data.Constraint (Constraint, Dict (..), withDict, (:-) (..), (\\), unmapDict) where
 
 import Prelude hiding (Functor, (.), id)
 
 import Control.Categorical.Functor
 import Control.Category
+import Data.Semigroup
 
+import GHC.Exts (Constraint)
+
 data Dict a where Dict :: a => Dict a
 
 deriving instance Eq (Dict a)
@@ -36,6 +39,9 @@
     Sub c . Sub b = Sub (withDict b c)
 
 instance Functor (:-) (->) Dict where map (Sub a) Dict = withDict a Dict
+
+(\\) :: (b => c) -> (a :- b) -> (a => c)
+f \\ Sub Dict = f
 
 unmapDict :: (Dict a -> Dict b) -> a :- b
 unmapDict f = Sub (f Dict)
diff --git a/Data/Constraint/Compose.hs b/Data/Constraint/Compose.hs
new file mode 100644
--- /dev/null
+++ b/Data/Constraint/Compose.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE UndecidableSuperClasses #-}
+
+module Data.Constraint.Compose (Compose, decompose) where
+
+import Data.Constraint
+
+class c (f a) => Compose (c :: β -> Constraint) (f :: α -> β) (a :: α)
+instance c (f a) => Compose c f a
+
+decompose :: Compose c f a :- c (f a)
+decompose = Sub Dict
diff --git a/constraint.cabal b/constraint.cabal
--- a/constraint.cabal
+++ b/constraint.cabal
@@ -1,5 +1,5 @@
 name:                constraint
-version:             0.1.0.0
+version:             0.1.0.1
 synopsis:            Reified constraints
 -- description:         
 license:             BSD3
@@ -13,9 +13,10 @@
 
 library
   exposed-modules:     Data.Constraint
+                     , Data.Constraint.Compose
   -- other-modules:       
   -- other-extensions:    
-  build-depends:       base >=4.10 && <4.12
+  build-depends:       base >=4.10 && <5
                      , category >=0.1.2
   -- hs-source-dirs:      
   default-language:    Haskell2010
@@ -24,5 +25,6 @@
                      , PolyKinds
                      , ConstraintKinds
                      , MultiParamTypeClasses
+                     , FlexibleInstances
                      , GADTs
                      , StandaloneDeriving