diff --git a/examples/FreeNum.hs b/examples/FreeNum.hs
new file mode 100644
--- /dev/null
+++ b/examples/FreeNum.hs
@@ -0,0 +1,32 @@
+{-# LANGUAGE FlexibleInstances #-}
+module FreeNum where
+
+import Data.Functor.Free
+
+import Control.Applicative
+
+instance Num (Free Num a) where
+  Free l + Free r = Free $ (+) <$> l <*> r
+  Free l * Free r = Free $ (*) <$> l <*> r
+  Free l - Free r = Free $ (-) <$> l <*> r
+  negate (Free f) = Free $ negate <$> f
+  abs (Free f)    = Free $ abs <$> f
+  signum (Free f) = Free $ signum <$> f
+  fromInteger i   = Free $ pure (fromInteger i)
+
+x, y :: Free Num String
+x = return "x"
+y = return "y"
+
+expr :: Free Num String
+expr = 1 + x * (3 - y)
+
+-- Monadic bind is variable substitution
+subst :: Free Num String -> Free Num a
+subst e = e >>= \v -> case v of 
+  "x" -> 10
+  _   -> 2
+
+-- Closed expressions can be evaluated to any other instance of Num
+result :: Int
+result = convertClosed (subst expr)
diff --git a/examples/NonEmptyList.hs b/examples/NonEmptyList.hs
--- a/examples/NonEmptyList.hs
+++ b/examples/NonEmptyList.hs
@@ -1,7 +1,4 @@
-{-# LANGUAGE
-    MultiParamTypeClasses
-  , FlexibleInstances
-  #-}
+{-# LANGUAGE FlexibleInstances #-}
 module NonEmptyList where
 
 import Data.Functor.Free
diff --git a/free-functors.cabal b/free-functors.cabal
--- a/free-functors.cabal
+++ b/free-functors.cabal
@@ -1,5 +1,5 @@
 name:                free-functors
-version:             0
+version:             0.1
 synopsis:            Provides free functors that are adjoint to functors that forget class constraints. 
 description:         A free functor is a left adjoint to a forgetful functor. It used to be the case
                      that the only category that was easy to work with in Haskell was Hask itself, so
@@ -42,7 +42,8 @@
     base >= 4.4 && < 5,
     constraints >= 0.3.2 && < 0.4,
     transformers >= 0.2.0.0 && < 0.4,
-    comonad >= 3.0 && < 3.1
+    comonad >= 3.0 && < 3.1,
+    void >= 0.4 && < 0.6
 
 source-repository head
   type:     git
diff --git a/src/Data/Functor/Free.hs b/src/Data/Functor/Free.hs
--- a/src/Data/Functor/Free.hs
+++ b/src/Data/Functor/Free.hs
@@ -32,6 +32,7 @@
 import Data.Functor.Compose
 import Data.Foldable
 import Data.Traversable
+import Data.Void
 
 
 -- | The free functor for constraint @c@.
@@ -88,3 +89,5 @@
 convert :: (c (f a), Applicative f) => Free c a -> f a
 convert = rightAdjunct pure
 
+convertClosed :: c x => Free c Void -> x
+convertClosed = rightAdjunct absurd
diff --git a/src/Data/Functor/HFree.hs b/src/Data/Functor/HFree.hs
--- a/src/Data/Functor/HFree.hs
+++ b/src/Data/Functor/HFree.hs
@@ -3,10 +3,6 @@
   , RankNTypes
   , TypeOperators
   , FlexibleInstances
-  , GADTs
-  , MultiParamTypeClasses
-  , UndecidableInstances
-  , ScopedTypeVariables
   #-}
 -----------------------------------------------------------------------------
 -- |
@@ -26,7 +22,6 @@
 -----------------------------------------------------------------------------
 module Data.Functor.HFree where
   
-import Control.Monad
 import Control.Applicative
 import Control.Monad.Trans.Class