diff --git a/src/PDF/Text.hs b/src/PDF/Text.hs
index a880b93..70b15b9 100644
--- a/src/PDF/Text.hs
+++ b/src/PDF/Text.hs
@@ -1,17 +1,19 @@
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE OverloadedStrings #-}
-module PDF.Text (
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE FlexibleContexts #-}
+module PDF.Text {-(
     pageContents
-  ) where
+  )-} where
 
 import Control.Applicative ((<|>))
 import Control.Monad (foldM)
 import Control.Monad.Reader (ReaderT, runReaderT, asks)
-import Control.Monad.State (get, put)
-import Data.Attoparsec.ByteString.Char8 (choice, count, sepBy)
+import Control.Monad.State (MonadState, evalStateT, get, modify, put)
+import Data.Attoparsec.ByteString.Char8 (choice, sepBy)
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS (drop, null, take)
-import Data.ByteString.Char8 (unpack)
+import Data.ByteString.Char8 (pack, unpack)
 import Data.Map ((!))
 import qualified Data.Map as Map (lookup, toList)
 import PDF.CMap (CMappers, CMap, CRange(..), emptyCMap)
@@ -19,64 +21,103 @@ import PDF.Object (
       DirectObject(..), StringObject(..)
     , array, blank, name, regular, stringObject, toByteString
   )
-import PDF.Parser (MonadParser, (<?>), Parser, evalParser, string, takeAll)
+import PDF.Parser (MonadParser, (<?>), Parser, evalParser, string, takeAll1)
 
 data StateOperator =
   Cm | W | J | J_ | M | D | Ri | I | Gs -- general graphic state
-  deriving (Bounded, Enum, Show)
+  deriving (Bounded, Enum)
 data TextOperator =
     Td | TD | Tm | Tstar -- text positioning
   | TJ | Tj | Quote | DQuote -- text showing
   | Tc | Tw | Tz | TL | Tf | Tr | Ts -- text state
-  deriving (Bounded, Enum, Show)
+  deriving (Bounded, Enum)
 data Argument = Raw ByteString | Typed DirectObject deriving Show
 type Call a = (a, [Argument])
 
-stateOperator :: MonadParser m => StateOperator -> m (Call StateOperator)
-stateOperator Cm = (,) Cm <$> count 6 argument <* string "cm"
-stateOperator W = (,) W <$> count 1 argument <* string "w"
-stateOperator J = (,) J <$> count 1 argument <* string "J"
-stateOperator J_ = (,) J_ <$> count 1 argument <* string "j"
-stateOperator M = (,) M <$> count 1 argument <* string "M"
-stateOperator D = (,) D <$> count 2 argument <* string "d"
-stateOperator Ri = (,) Ri <$> count 1 argument <* string "ri"
-stateOperator I = (,) I <$> count 1 argument <* string "i"
-stateOperator Gs = (,) Gs <$> count 1 nameArg <* string "gs"
+instance Show StateOperator where
+  show Cm = "cm"
+  show W = "w"
+  show J = "J"
+  show J_ = "j"
+  show M = "M"
+  show D = "d"
+  show Ri = "ri"
+  show I = "i"
+  show Gs = "gs"
 
-{-
-textOperator :: MonadParser m => TextOperator -> m (Call TextOperator)
-textOperator Td = (,) Td <$> count 2 argument <* string "Td"
-textOperator TD = (,) TD <$> count 2 argument <* string "TD"
-textOperator Tm = (,) Tm <$> count 6 argument <* string "Tm"
-textOperator Tstar = (,) Td <$> return [] <* string "T*"
-textOperator TJ = (,) TJ <$> count 1 arrayArg <* string "TJ"
-textOperator Tj = (,) Tj <$> count 1 stringArg <* string "Tj"
-textOperator Quote = (,) Quote <$> count 1 stringArg <* string "'"
-textOperator DQuote = (,) DQuote <$> count 1 stringArg <* string "\""
-textOperator Tc = (,) Tc <$> count 1 argument <* string "Tc"
-textOperator Tw = (,) Tw <$> count 1 argument <* string "Tw"
-textOperator Tz = (,) Tz <$> count 1 argument <* string "Tz"
-textOperator TL = (,) TL <$> count 1 argument <* string "TL"
-textOperator Tf = (,) Tf <$> sequence [nameArg, argument] <* string "Tf"
-textOperator Tr = (,) Tr <$> count 1 argument <* string "Tr"
-textOperator Ts = (,) Ts <$> count 1 argument <* string "Ts"
--}
+instance Show TextOperator where
+  show Td = "Td"
+  show TD = "TD"
+  show Tm = "Tm"
+  show Tstar = "T*"
+  show TJ = "TJ"
+  show Tj = "Tj"
+  show Quote = "'"
+  show DQuote = "\""
+  show Tc = "Tc"
+  show Tw = "Tw"
+  show Tz = "Tz"
+  show TL = "TL"
+  show Tf = "Tf"
+  show Tr = "Tr"
+  show Ts = "Ts"
 
-textOperator :: MonadParser m => TextOperator -> m TextOperator
-textOperator textOp = string (show textOp) *> return textOp
+stateOperator :: TypeChecker StateOperator
+stateOperator (Cm, [Raw _, Raw _, Raw _, Raw _, Raw _, Raw _]) = True
+stateOperator (W, [Raw _]) = True
+stateOperator (J, [Raw _]) = True
+stateOperator (J_, [Raw _]) = True
+stateOperator (M, [Raw _]) = True
+stateOperator (D, [Raw _, Raw _]) = True
+stateOperator (Ri, [Raw _]) = True
+stateOperator (I, [Raw _]) = True
+stateOperator (Gs, [Typed (NameObject _)]) = True
+stateOperator _ = False
 
-textChunk :: MonadParser m => m (Either TextOperator Argument)
-textChunk =
-    choice $ Left . textOperator <$> [minBound .. maxBound]
-  <|> choice $ Right <$> [stringArg, nameArg, arrayArg, argument]
+textOperator :: TypeChecker TextOperator
+textOperator (Td, [Raw _, Raw _]) = True
+textOperator (TD, [Raw _, Raw _]) = True
+textOperator (Tm, [Raw _, Raw _, Raw _, Raw _, Raw _, Raw _]) = True
+textOperator (Tstar, []) = True
+textOperator (TJ, [Typed (Array _)]) = True
+textOperator (Tj, [Typed (StringObject _)]) = True
+textOperator (Quote, [Typed (StringObject _)]) = True
+textOperator (DQuote, [Typed (StringObject _)]) = True
+textOperator (Tc, [Raw _]) = True
+textOperator (Tw, [Raw _]) = True
+textOperator (Tz, [Raw _]) = True
+textOperator (TL, [Raw _]) = True
+textOperator (Tf, [Typed (NameObject _), Raw _]) = True
+textOperator (Tr, [Raw _]) = True
+textOperator (Ts, [Raw _]) = True
+textOperator _ = False
 
-aTextOperator :: MonadParser m => m (Call TextOperator)
+type ArgumentStackParser m = (MonadState [Argument] m, MonadParser m)
+type Operator a = (Bounded a, Enum a, Show a)
+type TypeChecker a = Call a -> Bool
 
-a :: (Bounded o, Enum o, MonadParser m) => (o -> m (Call o)) -> m (Call o)
-a parserGenerator = choice $ parserGenerator <$> [minBound .. maxBound]
+parseShowable :: (Show a, MonadParser m) => a -> m a
+parseShowable textOp = string (pack $ show textOp) *> return textOp
+
+callChunk :: (MonadParser m, Operator a) => m (Either a Argument)
+callChunk =
+    Left <$> choice (parseShowable <$> [minBound .. maxBound])
+  <|> Right <$> choice [stringArg, nameArg, arrayArg, argument]
+  <?> "call chunk"
+
+stackParser :: (ArgumentStackParser m, Operator a) => TypeChecker a -> m (Call a)
+stackParser typeChecker = either popCall push =<< callChunk
+  where
+    push arg = modify (arg:) >> stackParser typeChecker
+    popCall operator = do
+      call <- (,) operator . reverse <$> get
+      if typeChecker call then return call else fail (show call)
+
+a :: (Operator a, MonadParser m) => TypeChecker a -> m (Call a)
+a typeChecker = evalStateT (stackParser typeChecker) []
 
 argument :: MonadParser m => m Argument
-argument = Raw <$> takeAll regular <* blank
+argument = Raw <$> takeAll1 regular <* blank
 
 arrayArg :: MonadParser m => m Argument
 arrayArg = Typed . Array <$> array <* blank
@@ -107,7 +148,7 @@ text :: ParserWithFont [ByteString]
 text =
   string "BT" *> blank *> commands <* blank <* string "ET" <?> "Text operators"
   where
-    commands = concat <$> (aTextOperator >>= runOperator) `sepBy` blank
+    commands = concat <$> (a textOperator >>= runOperator) `sepBy` blank
 
 runOperator :: Call TextOperator -> ParserWithFont [ByteString]
 runOperator (Tf, [Typed (NameObject fontName), _]) =