diff --git a/package.yaml b/package.yaml
index 97132ab..1f9d21f 100644
--- a/package.yaml
+++ b/package.yaml
@@ -50,6 +50,8 @@ tests:
     - -with-rtsopts=-N
     dependencies:
     - too-fast-too-free
+    - inspection-testing
+    - hspec
 
 benchmarks:
   too-fast-too-free-bench:
diff --git a/src/Data/OpenUnion.hs b/src/Data/OpenUnion.hs
index 089fd67..761bfb4 100644
--- a/src/Data/OpenUnion.hs
+++ b/src/Data/OpenUnion.hs
@@ -1,91 +1,142 @@
 {-# LANGUAGE AllowAmbiguousTypes   #-}
+{-# LANGUAGE ConstraintKinds       #-}
 {-# LANGUAGE DataKinds             #-}
+{-# LANGUAGE FlexibleContexts      #-}
 {-# LANGUAGE FlexibleInstances     #-}
 {-# LANGUAGE GADTs                 #-}
 {-# LANGUAGE KindSignatures        #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE PolyKinds             #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE RankNTypes            #-}
 {-# LANGUAGE ScopedTypeVariables   #-}
+{-# LANGUAGE TypeApplications      #-}
+{-# LANGUAGE TypeFamilies          #-}
 {-# LANGUAGE TypeOperators         #-}
 {-# LANGUAGE UndecidableInstances  #-}
+{-# OPTIONS_GHC -Wall              #-}
 
 module Data.OpenUnion where
 
 import Control.Monad.Discount.Effect
-import Unsafe.Coerce
+import Data.Typeable
+
+data Dict c where Dict :: c => Dict c
+
+
+data Nat = Z | S Nat
+  deriving Typeable
+
+data SNat :: Nat -> * where
+  SZ :: SNat 'Z
+  SS :: Typeable n => SNat n -> SNat ('S n)
+  deriving Typeable
+
+
+type family IndexOf (ts :: [k]) (n :: Nat) :: k where
+  IndexOf (k ': ks) 'Z = k
+  IndexOf (k ': ks) ('S n) = IndexOf ks n
+
+
+type family Found (ts :: [k]) (t :: k) :: Nat where
+  Found (t ': ts) t = 'Z
+  Found (u ': ts) t = 'S (Found ts t)
+
+
+class Typeable (Found r t) => Find (r :: [k]) (t :: k) where
+  finder :: SNat (Found r t)
+
+instance {-# OVERLAPPING #-} Find (t ': z) t where
+  finder = SZ
+  {-# INLINE finder #-}
+
+instance ( Find z t
+         , Found (_1 ': z) t ~ 'S (Found z t)
+         ) => Find (_1 ': z) t where
+  finder = SS $ finder @_ @z @t
+  {-# INLINE finder #-}
+
+
 
 
 data Union (r :: [(* -> *) -> * -> *]) (m :: * -> *) a where
-  Union :: Effect e => Word -> e m a -> Union r m a
+  Union
+      :: Effect (IndexOf r n)
+      => SNat n
+      -> IndexOf r n m a
+      -> Union r m a
+
+
+decomp :: Union (e ': r) m a -> Either (Union r m a) (e m a)
+decomp (Union p a) =
+  case p of
+    SZ   -> Right a
+    SS n -> Left $ Union n a
+{-# INLINE decomp #-}
 
 
 extract :: Union '[e] m a -> e m a
-extract (Union _ a) = unsafeCoerce a
+extract (Union SZ a) = a
+extract _ = error "impossible"
 {-# INLINE extract #-}
 
 
-absurdU :: Union '[] m a -> b
-absurdU = error "absurd, empty union"
-
-
-unsafeInj :: Effect e => Word -> e m a -> Union r m a
-unsafeInj w = Union w
-{-# INLINE unsafeInj #-}
-
-
-unsafePrj :: Word -> Union r m a -> Maybe (t m a)
-unsafePrj n (Union n' x)
-  | n == n'   = Just (unsafeCoerce x)
-  | otherwise = Nothing
-{-# INLINE unsafePrj #-}
-
-newtype P t r = P {unP :: Word}
-
-
-class FindElem (t :: k) (r :: [k]) where
-  elemNo :: P t r
-
-
-instance FindElem t (t ': r) where
-  elemNo = P 0
-  {-# INLINE elemNo #-}
-
-
-instance {-# OVERLAPPABLE #-} FindElem t r => FindElem t (t' ': r) where
-  elemNo = P $ 1 + unP (elemNo :: P t r)
-  {-# INLINE elemNo #-}
-
-
-class (FindElem e r, Effect e)
-      => Member (e :: (* -> *) -> * -> *)
-                (r :: [(* -> *) -> * -> *]) where
-  inj :: Monad m => e m a -> Union r m a
-  prj :: Union r m a -> Maybe (e m a)
-
-
-instance (Effect t, FindElem t r) => Member t r where
-  inj = unsafeInj $ unP (elemNo :: P t r)
-  {-# INLINE inj #-}
-
-  prj = unsafePrj $ unP (elemNo :: P t r)
-  {-# INLINE prj #-}
-
-
-decomp :: Union (t ': r) m a -> Either (Union r m a) (e m a)
-decomp (Union 0 a) = Right $ unsafeCoerce a
-decomp (Union n a) = Left  $ Union (n - 1) a
-{-# INLINE [2] decomp #-}
-
-
 weaken :: Union r m a -> Union (e ': r) m a
-weaken (Union n a) = Union (n + 1) a
+weaken (Union n a) =
+  case induceTypeable n of
+    Dict -> Union (SS n) a
+{-# INLINE weaken #-}
+
+
+inj
+    :: forall r e a m
+     . ( Functor m
+       , Find r e
+       , Effect e
+       , e ~ IndexOf r (Found r e)
+       )
+    => e m a
+    -> Union r m a
+inj e = Union (finder @_ @r @e) e
+{-# INLINE inj #-}
+
+
+induceTypeable :: SNat n -> Dict (Typeable n)
+induceTypeable SZ = Dict
+induceTypeable (SS _) = Dict
+{-# INLINE induceTypeable #-}
+
+
+type Member e r = (Find r e, e ~ IndexOf r (Found r e), Effect e)
+
+
+
+prj
+    :: forall r e a m
+     . ( Find r e
+       , e ~ IndexOf r (Found r e)
+       )
+    => Union r m a
+    -> Maybe (e m a)
+prj (Union (s :: SNat n) a) =
+  case induceTypeable s of
+    Dict ->
+      case eqT @n @(Found r e) of
+        Just Refl -> Just a
+        Nothing -> Nothing
+{-# INLINE prj #-}
 
 
 instance Effect (Union r) where
   weave s f (Union w e) = Union w $ weave s f e
   {-# INLINE weave #-}
 
-instance Functor m => Functor (Union r m) where
-  fmap f (Union w t) = Union w $ fmap f t
+
+instance (Functor m) => Functor (Union r m) where
+  fmap f (Union w t) = Union w $ fmap' f t
+    where
+      fmap' :: (Functor m, Effect f) => (a -> b) -> f m a -> f m b
+      fmap' = fmap
+      {-# INLINE fmap' #-}
   {-# INLINE fmap #-}
 
diff --git a/test/Spec.hs b/test/Spec.hs
index cd4753f..bdd08ed 100644
--- a/test/Spec.hs
+++ b/test/Spec.hs
@@ -1,2 +1,26 @@
+{-# LANGUAGE DataKinds       #-}
+{-# LANGUAGE TemplateHaskell #-}
+
+import Test.Inspection
+import Control.Monad.Discount
+import Data.OpenUnion
+import TRYAGAIN hiding (main)
+
+
 main :: IO ()
-main = putStrLn "Test suite not yet implemented"
+main = pure ()
+
+go :: Eff '[State Int] Int
+go = do
+  n <- send (Get id)
+  if n <= 0
+     then pure n
+     else do
+       send $ Put (n-1) ()
+       go
+
+countDown :: Int -> Int
+countDown start = fst $ run $ runState start go
+
+inspect $ 'countDown `hasNoType` ''SNat
+