diff --git a/experiments/Experiment4.hs b/experiments/Experiment4.hs
new file mode 100644
index 0000000..5a6254b
--- /dev/null
+++ b/experiments/Experiment4.hs
@@ -0,0 +1,227 @@
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE LambdaCase #-}
+{-# OPTIONS_GHC -Wall #-}
+
+import Data.Map.Strict (Map)
+-- import Data.Set (Set)
+import Control.Applicative
+import Control.Monad
+import Data.Map.Strict qualified as Map
+-- import GHC.Base (Any)
+-- import Debug.Trace
+import Test.Tasty.Bench.Fit
+import Data.Text qualified as Text
+import Data.Text (Text)
+import Data.Some
+import Data.Functor.Identity
+import Data.GADT.Show
+import Data.Bifunctor
+import Unsafe.Coerce (unsafeCoerce)
+import Data.GADT.Compare
+import Data.Type.Equality
+
+-- Blog topic: static type safety breaks modularity
+
+data RHS f a = Pure a | T Char (RHS f a) | forall x. NT (f x) (x -> RHS f a) | Or (RHS f a) (RHS f a) | Fail
+deriving instance Functor (RHS f)
+
+instance Applicative (RHS f) where
+  pure = Pure
+  (<*>) = ap
+
+instance Alternative (RHS f) where
+  (<|>) = Or
+  empty = Fail
+
+instance Monad (RHS f) where
+  Pure x >>= k = k x
+  T c k >>= k' = T c (k >>= k')
+  NT f k >>= k' = NT f (k >=> k')
+  Or p q >>= k = Or (p >>= k) (q >>= k)
+  Fail >>= _ = Fail
+
+data CFG f a = CFG (f a) (forall x. f x -> RHS f x)
+
+data T3 a b c = T3 !a !b !c deriving Show
+
+data Comm f = Comm !(Some f) !Int deriving (Eq, Ord, Show)
+
+newtype Cont f a = Cont { getCont :: Text -> Descr -> a -> Command f }
+instance Show (Cont f a) where
+  show _ = "<Cont>"
+
+data Descr = Descr Slot !Int !Int
+data Slot = Slot -- String [Symbol] [Symbol]
+  deriving Show
+
+newtype Rel a b = Rel (Map a [b]) deriving Show
+
+rel :: Ord a => Rel a b -> a -> [b]
+rel (Rel m) x = Map.findWithDefault [] x m
+
+relMay :: Ord a => Rel a b -> a -> Maybe [b]
+relMay (Rel m) x = Map.lookup x m
+
+initRel :: Ord a => a -> Rel a b -> Rel a b
+initRel x (Rel m) = Rel (Map.insertWith (++) x [] m)
+
+addRel :: Ord a => a -> b -> Rel a b -> Rel a b
+addRel x y (Rel m) = Rel (Map.insertWith (++) x [y] m)
+
+instance GShow Identity where
+  gshowsPrec _ _ = showString "<Identity>"
+instance GShow (Cont f) where
+  gshowsPrec _ _ = showString "<Cont>"
+
+-- newtype U = U (Set Descr)
+newtype G f = G { getG :: Rel (Comm f) (Slot, Int, Some (Cont f)) } deriving Show
+newtype P f = P { getP :: Rel (Comm f) (Int, Some Identity) } deriving Show
+
+newtype Command f = Command { getCommand :: forall x. T3 (G f) (P f) (Maybe x) -> T3 (G f) (P f) (Maybe x) }
+
+newtype M f a = M { getM :: Text -> Descr -> Cont f a -> Command f }
+
+extents :: GCompare f => f a -> M f (Maybe [(Int, a)])
+extents nt = M $ \inp dsc@(Descr _ _ i) (Cont k) ->
+  Command $ \(T3 g p b) -> -- trace ("extents " ++ show (nt, i)) $
+  getCommand
+    (k inp dsc (
+      let res = relMay (getP p) (Comm (Some nt) i)
+      in fmap (map (second (\(Some (Identity x)) -> unsafeCoerce x))) res))
+    (T3 g (P (initRel (Comm (Some nt) i) (getP p))) b)
+
+addExtent :: GCompare f => f a -> a -> M f ()
+addExtent nt x = M $ \inp dsc@(Descr _ l i) (Cont k) ->
+  Command $ \(T3 g p b) -> -- trace ("addExtent " ++ show (nt, l, i)) $
+  getCommand (k inp dsc ()) (T3 g (P (addRel (Comm (Some nt) l) (i, Some (Identity x)) (getP p))) b)
+
+resume :: GCompare f => f a -> a -> M f a
+resume nt x = M $ \inp (Descr Slot l r) _ ->
+  Command $ \(T3 g p b) ->
+    let cnts = rel (getG g) (Comm (Some nt) l) in -- trace ("resume " ++ show (nt, l, cnts)) $
+    foldr (\(s, l', Some (Cont k)) go -> go . getCommand (unsafeCoerce k inp (Descr s l' r) x))
+      id cnts (T3 g p b)
+
+addCont :: GCompare f => f a -> M f c -> M f c
+addCont nt m = M $ \inp dsc@(Descr s l i) k ->
+  Command $ \(T3 g p b) -> -- trace ("addCont " ++ show (nt, i)) $
+    getCommand (getM m inp dsc k) (T3 (G (addRel (Comm (Some nt) i) (s, l, Some k) (getG g))) p b)
+
+match :: Char -> M f ()
+match c = M $ \inp (Descr (Slot {- nt alpha beta -}) l i) (Cont k) ->
+  case Text.uncons inp of
+    Just (x,inp') | c == x -> k inp' (Descr (Slot {- nt alpha beta -}) l (i + 1)) ()
+    _ -> Command id
+
+skip :: Int -> M f ()
+skip r = M $ \inp (Descr s l i) (Cont k) -> k (Text.drop (r - i) inp) (Descr s l r) ()
+
+descend :: M f ()
+descend = M $ \inp (Descr Slot _ i) (Cont k) -> k inp (Descr Slot i i) ()
+
+-- traceI :: String -> M ()
+-- traceI msg = M $ \inp dsc@(Descr _ _ i) k -> trace (show i ++ ": " ++ msg) getCont k inp dsc ()
+
+instance Functor (M f) where
+  fmap f (M p) = M $ \inp dsc (Cont k) ->
+    p inp dsc $ Cont $ \inp' dsc' x ->
+      k inp' dsc' (f x)
+instance Applicative (M f) where
+  pure x = M $ \inp dsc (Cont k) -> k inp dsc x
+  (<*>) = ap
+instance Alternative (M f) where
+  empty = M $ \_ _ _ -> Command id
+  M p <|> M q = M $ \inp dsc k -> Command (getCommand (q inp dsc k) . getCommand (p inp dsc k))
+instance Monad (M f) where
+  M p >>= k = M $ \inp dsc k' ->
+    p inp dsc $ Cont $ \inp' dsc' x ->
+      getM (k x) inp' dsc' k'
+
+parseCFG :: forall f a. GCompare f => CFG f a -> Text -> T3 (G f) (P f) (Maybe a)
+parseCFG (CFG nt0 prods) inp0 =
+  getCommand
+    (getM (parseRHS (NT nt0 pure)) inp0 (Descr Slot 0 0) final)
+    (T3 (G (Rel mempty)) (P (Rel mempty)) Nothing) where
+
+  final :: Cont f a
+  final = Cont $ \inp _ x -> Command $ \(T3 p g b) -> (T3 p g (b <|> unsafeCoerce x <$ guard (Text.null inp)))
+
+  parseRHS :: RHS f x -> M f x
+  parseRHS (Pure x) = pure x
+  parseRHS (T c k) = parseT c *> parseRHS k
+  parseRHS (NT f k) = parseNT f >>= parseRHS . k
+  parseRHS (Or p q) = parseRHS p <|> parseRHS q
+  parseRHS Fail = empty
+
+  parseNT :: f x -> M f x
+  parseNT nt = addCont nt $
+    extents nt >>= \case
+      Nothing -> do
+        descend
+        x <- parseRHS (prods nt)
+        addExtent nt x
+        resume nt x
+      Just rs -> asum (map (\(r, x) -> x <$ skip r) rs)
+
+  parseT :: Char -> M f ()
+  parseT = match
+
+t :: Char -> RHS f ()
+t c = T c (pure ())
+
+nt :: f a -> RHS f a
+nt f = NT f pure
+
+data E a where E :: E Int
+
+deriving instance Eq (E a)
+deriving instance Ord (E a)
+instance GEq E where
+  geq E E = Just Refl
+instance GCompare E where
+  gcompare E E = GEQ
+deriving instance Show (E a)
+
+example :: CFG E Int
+example = CFG E $ \E -> nt E *> t '+' *> nt E <|> 0 <$ t 'a'
+
+-- >>> parseCFG example "a+a+a+a+a+a"
+-- (G {getG = Rel (fromList [(Comm "E" 0,[(Slot,0,<Cont>),(Slot,0,<Cont>)]),(Comm "E" 2,[(Slot,2,<Cont>),(Slot,0,<Cont>)]),(Comm "E" 4,[(Slot,2,<Cont>),(Slot,4,<Cont>),(Slot,0,<Cont>)]),(Comm "E" 6,[(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,4,<Cont>),(Slot,6,<Cont>),(Slot,0,<Cont>)]),(Comm "E" 8,[(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,4,<Cont>),(Slot,6,<Cont>),(Slot,8,<Cont>),(Slot,0,<Cont>)]),(Comm "E" 10,[(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,0,<Cont>),(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,0,<Cont>),(Slot,2,<Cont>),(Slot,0,<Cont>),(Slot,4,<Cont>),(Slot,0,<Cont>),(Slot,0,<Cont>),(Slot,4,<Cont>),(Slot,0,<Cont>),(Slot,6,<Cont>),(Slot,8,<Cont>),(Slot,10,<Cont>),(Slot,0,<Cont>)])])},P {getP = Rel (fromList [(Comm "E" 0,[11,11,11,11,9,11,11,11,11,9,11,11,11,9,7,11,11,9,11,11,11,11,9,11,11,9,7,5,11,11,11,9,11,11,11,9,7,11,11,9,11,11,11,11,9,11,11,11,9,7,11,11,9,11,11,11,9,11,11,11,9,7,5,3,1]),(Comm "E" 2,[11,11,11,11,9,11,11,11,9,11,11,9,7,11,11,11,9,11,11,9,7,5,3]),(Comm "E" 4,[11,11,11,9,11,11,9,7,5]),(Comm "E" 6,[11,11,9,7]),(Comm "E" 8,[11,9]),(Comm "E" 10,[11])])},True)
+
+data N a where N :: N Int
+deriving instance Eq (N a)
+deriving instance Ord (N a)
+deriving instance Show (N a)
+instance GEq N where
+  geq N N = Just Refl
+instance GCompare N where
+  gcompare N N = GEQ
+instance GShow N where
+  gshowsPrec _ N = showString "N"
+
+example3 :: CFG N Int
+example3 = CFG N $ \N -> (+ 1) <$ t 'a' <*> nt N <|> pure 0
+
+example4 :: CFG N Int
+example4 = CFG N $ \N -> (+ 1) <$> nt N <* t 'a' <|> pure 0
+
+-- >>> parseCFG example3 (Text.pack "aaaa")
+-- T3 (G {getG = Rel (fromList [(Comm (Some N) 0,[(Slot,0,Some <Cont>)]),(Comm (Some N) 1,[(Slot,0,Some <Cont>)]),(Comm (Some N) 2,[(Slot,1,Some <Cont>)]),(Comm (Some N) 3,[(Slot,2,Some <Cont>)]),(Comm (Some N) 4,[(Slot,3,Some <Cont>)])])})
+--    (P {getP = Rel (fromList [(Comm (Some N) 0,[(0,Some <Identity>),(1,Some <Identity>),(2,Some <Identity>),(3,Some <Identity>),(4,Some <Identity>)]),(Comm (Some N) 1,[(1,Some <Identity>),(2,Some <Identity>),(3,Some <Identity>),(4,Some <Identity>)]),(Comm (Some N) 2,[(2,Some <Identity>),(3,Some <Identity>),(4,Some <Identity>)]),(Comm (Some N) 3,[(3,Some <Identity>),(4,Some <Identity>)]),(Comm (Some N) 4,[(4,Some <Identity>)])])})
+--    (Just 4)
+
+-- >>> parseCFG example4 (Text.pack "aaaa")
+-- T3 (G {getG = Rel (fromList [(Comm (Some N) 0,[(Slot,0,Some <Cont>),(Slot,0,Some <Cont>)])])})
+--    (P {getP = Rel (fromList [(Comm (Some N) 0,[(4,Some <Identity>),(3,Some <Identity>),(2,Some <Identity>),(1,Some <Identity>),(0,Some <Identity>)])])})
+--    (Just 4)
+
+main :: IO ()
+-- main = print (parseCFG example "a+a+a")
+
+main = do
+  result <-
+    fits $
+      mkFitConfig
+        (\n -> (\(T3 _ _ b) -> b) $ parseCFG example4 (Text.replicate (fromIntegral n) (Text.pack "a")))
+        (1000, 1000000)
+  mapM_ print result