diff --git a/Examples/tree.hs b/Examples/tree.hs
--- a/Examples/tree.hs
+++ b/Examples/tree.hs
@@ -1,41 +1,41 @@
-
-{-# LANGUAGE OverloadedStrings #-}
-
-import Text.LaTeX.Base
-import Text.LaTeX.Packages.Trees.Qtree
-import Text.LaTeX.Packages.Inputenc
-
-treeExample1 :: Tree String
-treeExample1 = Node (Just "Root") [Leaf "Leaf 1",Leaf "Leaf 2"]
-
-treeExample2 :: Tree Int
-treeExample2 = Node (Just 0) [Node Nothing [Leaf 1,Leaf 2] , Leaf 3]
-
-treeExample3 :: Tree LaTeX
-treeExample3 = Node (Just $ textbf $ textit "Bold and italic")
-                  [ Leaf $ textbf "Bold"
-                  , Leaf $ textit "Italic"
-                    ]
-
--- Main
-
-main :: IO ()
-main = renderFile "tree.tex" example
-
-example :: LaTeX
-example = thePreamble <> document theBody
-
-thePreamble :: LaTeX
-thePreamble =
-    documentclass [] article
- <> usepackage [] qtree
- <> usepackage [utf8] inputenc
- <> title "Examples with trees"
- <> author "Daniel Díaz"
-
-theBody :: LaTeX
-theBody =
-    maketitle
- <> tree fromString treeExample1
- <> rendertree treeExample2
+
+{-# LANGUAGE OverloadedStrings #-}
+
+import Text.LaTeX.Base
+import Text.LaTeX.Packages.Trees.Qtree
+import Text.LaTeX.Packages.Inputenc
+
+treeExample1 :: Tree String
+treeExample1 = Node (Just "Root") [Leaf "Leaf 1",Leaf "Leaf 2"]
+
+treeExample2 :: Tree Int
+treeExample2 = Node (Just 0) [Node Nothing [Leaf 1,Leaf 2] , Leaf 3]
+
+treeExample3 :: Tree LaTeX
+treeExample3 = Node (Just $ textbf $ textit "Bold and italic")
+                  [ Leaf $ textbf "Bold"
+                  , Leaf $ textit "Italic"
+                    ]
+
+-- Main
+
+main :: IO ()
+main = renderFile "tree.tex" example
+
+example :: LaTeX
+example = thePreamble <> document theBody
+
+thePreamble :: LaTeX
+thePreamble =
+    documentclass [] article
+ <> usepackage [] qtree
+ <> usepackage [utf8] inputenc
+ <> title "Examples with trees"
+ <> author "Daniel Díaz"
+
+theBody :: LaTeX
+theBody =
+    maketitle
+ <> tree fromString treeExample1
+ <> rendertree treeExample2
  <> tree id treeExample3
diff --git a/HaTeX.cabal b/HaTeX.cabal
--- a/HaTeX.cabal
+++ b/HaTeX.cabal
@@ -1,14 +1,13 @@
 Name: HaTeX
-Version: 3.3
+Version: 3.4
 Author: Daniel Díaz
 Category: Text
 Build-type: Simple
 License: BSD3
 License-file: license
-Maintainer: Daniel Díaz (danieldiaz <at> dhelta <dot> net)
+Maintainer: Daniel Díaz (dhelta `dot` diaz `at` gmail `dot` com)
 Stability: Experimental
-Homepage: http://dhelta.net/hprojects/HaTeX
-Bug-reports: danieldiaz <at> dhelta <dot> net
+Bug-reports: dhelta `dot` diaz `at` gmail `dot` com
 Synopsis: The Haskell LaTeX library.
 Description: As it is said: \"LaTeX is a high-quality typesetting system\". This library provides
              a bridge between that system and Haskell (i.e. it is a LaTeX DSL).
@@ -39,7 +38,7 @@
 Library
   Build-depends: base ==4.*
                , bytestring
-               , transformers ==0.3.*
+               , transformers >= 0.2.2 && < 0.4
                , text
                , parsec >=3.1.2 && <3.2
   Exposed-modules:
diff --git a/ReleaseNotes b/ReleaseNotes
--- a/ReleaseNotes
+++ b/ReleaseNotes
@@ -1,6 +1,19 @@
 HaTeX Release Notes
 -- From version 3.0.0
 
+>>> 3.4
+
+- Num instance for LaTeXT.
+- New re-exports: liftIO.
+- Relaxed transformers package dependency to: >= 0.2.2 && < 0.4
+- Changed infix rule of (=:) and (/=:).
+- More documentation notes.
+- New functions: hfill, vfill.
+- The internal representation of LaTeXT has changed to prevent
+  a set of errors.
+- Changes in Text.LaTeX.
+- Several minor fixes.
+
 >>> 3.3
 
 - New functions 'protectString' and 'protectText'.
diff --git a/Text/LaTeX.hs b/Text/LaTeX.hs
--- a/Text/LaTeX.hs
+++ b/Text/LaTeX.hs
@@ -1,13 +1,15 @@
 
--- | This module is a re-export of the main modules.
---   Importing this will you give access to almost all
---   functionality of the library. However, it is recommended
---   to import the @Base@ module and, then, import only
---   packages you actually need.
+-- | This module is a re-export of the Base module.
+--   You may find it shorter to import.
+--
+--   Historically, this module also exported the Packages
+--   module. But, since it's more common to import the Base
+--   module and, then, only the packages you need (instead
+--   of all of them), this module has been upgraded supporting
+--   it.
 module Text.LaTeX
- ( module Text.LaTeX.Base
- , module Text.LaTeX.Packages
+ ( -- * Base module
+   module Text.LaTeX.Base
    ) where
 
-import Text.LaTeX.Base
-import Text.LaTeX.Packages
+import Text.LaTeX.Base
diff --git a/Text/LaTeX/Base.hs b/Text/LaTeX/Base.hs
--- a/Text/LaTeX/Base.hs
+++ b/Text/LaTeX/Base.hs
@@ -17,7 +17,9 @@
 * The "Text.LaTeX.Base.Commands" module, which exports the LaTeX standard commands
   and environments.
 
-Here is also defined a 'Num' instance for 'LaTeX'.
+* The "Text.LaTeX.Base.Writer" module, to work with the monad interface of the library.
+
+Here are also defined 'Num' and 'Fractional' instances for both 'LaTeX' and 'LaTeXT'.
 -}
 module Text.LaTeX.Base
  ( -- * @LaTeX@ datatype
@@ -39,7 +41,8 @@
 #endif
    ) where
 
-import Text.LaTeX.Base.Syntax (LaTeX (..),(<>),protectString,protectText)
+import Text.LaTeX.Base.Syntax (LaTeX (..), TeXArg (..),(<>),protectString,protectText)
+import Text.LaTeX.Base.Class
 import Text.LaTeX.Base.Render
 import Text.LaTeX.Base.Types
 import Text.LaTeX.Base.Commands
@@ -47,15 +50,44 @@
 --
 import Data.Monoid
 
--- Num instance for LaTeX
+-- Num and Fractional instances for LaTeX and LaTeXT
 
--- | Methods 'abs' and 'signum' are undefined. Don't use them!
+-- | Careful! Method 'signum' is undefined. Don't use it!
 instance Num LaTeX where
  (+) = TeXOp "+"
  (-) = TeXOp "-"
  (*) = (<>)
  negate = (TeXEmpty -)
  fromInteger = rendertex
+ abs x = between x "|" "|"
  -- Non-defined methods
- abs _    = error "Cannot use \"abs\" Num method with a LaTeX value."
  signum _ = error "Cannot use \"signum\" Num method with a LaTeX value."
+
+-- | Division uses the LaTeX @frac@ command.
+instance Fractional LaTeX where
+ p / q = TeXComm "frac" [FixArg p, FixArg q]
+ fromRational = rendertex . (fromRational :: Rational -> Float)
+
+-- | Warning: this instance only exist for the 'Num' instance.
+instance Monad m => Eq (LaTeXT m a) where
+ _ == _ = error "Cannot use \"(==)\" Eq method with a LaTeXT value."
+
+-- | Warning: this instance only exist for the 'Num' instance.
+instance Monad m => Show (LaTeXT m a) where
+ show _ = error "Cannot use \"show\" Show method with a LaTeXT value."
+
+-- | Careful! Method 'signum' is undefined. Don't use it!
+instance Monad m => Num (LaTeXT m a) where
+ (+) = liftOp (+)
+ (-) = liftOp (-)
+ (*) = (>>)
+ negate = (mempty -)
+ fromInteger = fromLaTeX . fromInteger
+ abs = liftL abs
+ -- Non-defined methods
+ signum _ = error "Cannot use \"signum\" Num method with a LaTeXT value."
+
+-- | Division uses the LaTeX @frac@ command.
+instance Monad m => Fractional (LaTeXT m a) where
+ (/) = liftOp (/)
+ fromRational = fromLaTeX . fromRational
diff --git a/Text/LaTeX/Base/Class.hs b/Text/LaTeX/Base/Class.hs
--- a/Text/LaTeX/Base/Class.hs
+++ b/Text/LaTeX/Base/Class.hs
@@ -1,4 +1,7 @@
 
+-- | Definition of the 'LaTeXC' class, used to combine the classic applicative and
+--   the latter monadic interfaces of /HaTeX 3/. The user can define new instances
+--   as well, adding flexibility to the way /HaTeX/ is used.
 module Text.LaTeX.Base.Class (
    LaTeXC (..)
  , Monoid (..)
@@ -6,6 +9,7 @@
  , fromLaTeX
  , liftL
  , liftL2
+ , liftL3
  , comm0
  , commS
  , braces
@@ -16,33 +20,54 @@
 import Data.Monoid
 import Data.String
 
+-- | This is the class of 'LaTeX' code generators. It has 'Monoid' and 'IsString' as
+--   superclasses.
 class (Monoid l,IsString l) => LaTeXC l where
+ -- | This method must take a function that combines a list of 'LaTeX' values into a new one,
+ --   and creates a function that combines @l@-typed values. The combining function can be
+ --   seen as a function with 0 or more 'LaTeX' arguments with a 'LaTeX' value as output.
  liftListL :: ([LaTeX] -> LaTeX) -> [l] -> l
 
+-- | This instance just sets @liftListL = id@.
 instance LaTeXC LaTeX where
  liftListL = id
 
 -- COMBINATORS
 
+-- | Map a 'LaTeX' value to its equivalent in any 'LaTeXC' instance.
 fromLaTeX :: LaTeXC l => LaTeX -> l
 fromLaTeX l = liftListL (\_ -> l) []
 
+-- | Lift a inner function of 'LaTeX' values into any 'LaTeXC' instance.
 liftL :: LaTeXC l => (LaTeX -> LaTeX) -> l -> l
 liftL f x = liftListL (\[x] -> f x) [x]
 
+-- | Variant of 'liftL' with a two arguments function.
 liftL2 :: LaTeXC l => (LaTeX -> LaTeX -> LaTeX) -> l -> l -> l
 liftL2 f x y = liftListL (\[x,y] -> f x y) [x,y]
 
--- | A simple (without arguments) command generator,
---   given the name of the command.
+-- | Variant of 'liftL' with a three arguments function.
+liftL3 :: LaTeXC l => (LaTeX -> LaTeX -> LaTeX -> LaTeX) -> l -> l -> l -> l
+liftL3 f x y z = liftListL (\[x,y,z] -> f x y z) [x,y,z]
+
+-- | A simple (without arguments) and handy command generator
+--   using the name of the command.
 --
 -- > comm0 str = fromLaTeX $ TeXComm str []
 --
 comm0 :: LaTeXC l => String -> l
 comm0 str = fromLaTeX $ TeXComm str []
 
+-- | Like 'comm0' but using 'commS'.
+--
+-- > commS = fromLaTeX . TeXCommS
+--
 commS :: LaTeXC l => String -> l
 commS = fromLaTeX . TeXCommS
 
+-- | A lifted version of the 'TeXBraces' constructor.
+--
+-- > braces = liftL TeXBraces
+--
 braces :: LaTeXC l => l -> l
 braces = liftL TeXBraces
diff --git a/Text/LaTeX/Base/Commands.hs b/Text/LaTeX/Base/Commands.hs
--- a/Text/LaTeX/Base/Commands.hs
+++ b/Text/LaTeX/Base/Commands.hs
@@ -109,6 +109,8 @@
  , hspace
  , hspace_
  , vspace
+ , hfill
+ , vfill
  , stretch
  , smallskip
  , bigskip
@@ -177,7 +179,7 @@
  , makebox
  , raisebox
  , rule
-   -- * Cross references
+   -- ** Cross references
  , caption
  , label
  , ref
@@ -360,7 +362,7 @@
 abstract :: LaTeXC l => l -> l
 abstract = liftL $ TeXEnv "abstract" []
 
-cite :: LaTeX -> LaTeX
+cite :: LaTeXC l => l -> l
 cite = liftL $ \l -> TeXComm "cite" [FixArg l]
 
 -- Document class
@@ -665,6 +667,14 @@
 
 vspace :: LaTeXC l => Measure -> l
 vspace m = fromLaTeX $ TeXComm "vspace" [FixArg $ rendertex m]
+
+-- | Fill out all available horizontal space.
+hfill :: LaTeXC l => l
+hfill = comm0 "hfill"
+
+-- | Fill out all available vertical space.
+vfill :: LaTeXC l => l
+vfill = comm0 "vfill"
 
 protect :: LaTeXC l => l -> l
 protect l = commS "protect" <> l
diff --git a/Text/LaTeX/Base/Render.hs b/Text/LaTeX/Base/Render.hs
--- a/Text/LaTeX/Base/Render.hs
+++ b/Text/LaTeX/Base/Render.hs
@@ -1,6 +1,10 @@
 
 {-# LANGUAGE OverloadedStrings #-}
 
+-- | The final purpose of this module is to render a Text value
+--   from a 'LaTeX' value. The interface is abstracted via a typeclass
+--   so you can cast to 'Text' other types as well. Also, some other
+--   handy 'Text'-related functions are defined.
 module Text.LaTeX.Base.Render
  ( -- * Re-exports
    Text
diff --git a/Text/LaTeX/Base/Writer.hs b/Text/LaTeX/Base/Writer.hs
--- a/Text/LaTeX/Base/Writer.hs
+++ b/Text/LaTeX/Base/Writer.hs
@@ -42,9 +42,12 @@
  , rendertexM
  , liftFun
  , liftOp
+   -- * Errors
+ , throwError
  , merror
    -- * Re-export
  , lift
+ , liftIO
    ) where
 
 import Control.Monad.Trans.Writer
@@ -64,19 +67,38 @@
 import Control.Monad (liftM)
 
 newtype LaTeXT m a =
- LaTeXT { unwrapLaTeXT :: WriterT LaTeX m a }
-   deriving (Functor,Applicative,Monad,MonadIO)
+  LaTeXT { unwrapLaTeXT :: WriterT LaTeX m (a,Maybe String) }
 
+instance Functor m => Functor (LaTeXT m) where
+ fmap f (LaTeXT c) = LaTeXT $ fmap (first f) c
+
+type LaTeXT_ m = LaTeXT m ()
+
+pairNoth :: a -> (a,Maybe b)
+pairNoth x = (x,Nothing)
+
 instance MonadTrans LaTeXT where
- lift = LaTeXT . lift
+ lift = LaTeXT . liftM pairNoth . lift
 
+instance Monad m => Monad (LaTeXT m) where
+ return = lift . return
+ (LaTeXT c) >>= f = LaTeXT $ do 
+  (a,_) <- c
+  let LaTeXT c' = f a
+  c'
+
+instance MonadIO m => MonadIO (LaTeXT m) where
+ liftIO = lift . liftIO
+
 instance Monad m => LaTeXC (LaTeXT m a) where
  liftListL f xs = mapM extractLaTeX_ xs >>= merror "liftListL" . textell . f
 
-type LaTeXT_ m = LaTeXT m ()
-
-runLaTeXT :: LaTeXT m a -> m (a,LaTeX)
-runLaTeXT = runWriterT . unwrapLaTeXT
+runLaTeXT :: Monad m => LaTeXT m a -> m (Either String a,LaTeX)
+runLaTeXT (LaTeXT c) = runWriterT c >>= (
+  \((a,m),l) -> case m of
+             Nothing  -> return (Right a ,l)
+             Just err -> return (Left err,l)
+       )
 
 -- | This is the usual way to run the 'LaTeXT' monad
 --   and obtain a 'LaTeX' value.
@@ -91,7 +113,9 @@
 -- | This function run a 'LaTeXT' computation,
 --   lifting the result again in the monad.
 extractLaTeX :: Monad m => LaTeXT m a -> LaTeXT m (a,LaTeX)
-extractLaTeX = LaTeXT . lift . runLaTeXT
+extractLaTeX (LaTeXT c) = LaTeXT $ do
+ ((a,m),l) <- lift $ runWriterT c
+ return ((a,l),m)
 
 extractLaTeX_ :: Monad m => LaTeXT m a -> LaTeXT m LaTeX
 extractLaTeX_ = liftM snd . extractLaTeX
@@ -99,20 +123,18 @@
 -- | With 'textell' you can append 'LaTeX' values to the
 --   state of the 'LaTeXT' monad.
 textell :: Monad m => LaTeX -> LaTeXT m ()
-textell = LaTeXT . tell
+textell = LaTeXT . liftM pairNoth . tell
 
 -- | Lift a function over 'LaTeX' values to a function
 --   acting over the state of a 'LaTeXT' computation.
 liftFun :: Monad m
         => (LaTeX -> LaTeX)
         -> (LaTeXT m a -> LaTeXT m a)
-liftFun f ml = do
- (a,l') <- lift $ do
-            (a,l) <- runLaTeXT ml
-            let l' = f l
-            return (a,l')
- textell l'
- return a
+liftFun f (LaTeXT c) = LaTeXT $ do
+ (p,l) <- lift $ runWriterT c
+ tell $ f l
+ return p
+ 
 
 -- | Lift an operator over 'LaTeX' values to an operator
 --   acting over the state of two 'LaTeXT' computations.
@@ -121,33 +143,36 @@
 -- /second argument of the lifted operator./
 liftOp :: Monad m
        => (LaTeX -> LaTeX -> LaTeX)
-       -> (LaTeXT m a -> LaTeXT m a -> LaTeXT m a)
-liftOp op ml1 ml2 = do
- (a,l') <- lift $ do
-            (_,l1) <- runLaTeXT ml1
-            (a,l2) <- runLaTeXT ml2
-            let l' = l1 `op` l2
-            return (a,l')
- textell l'
- return a
+       -> (LaTeXT m a -> LaTeXT m b -> LaTeXT m b)
+liftOp op (LaTeXT c) (LaTeXT c') = LaTeXT $ do
+ (_,l)  <- lift $ runWriterT c
+ (p,l') <- lift $ runWriterT c'
+ tell $ l `op` l'
+ return p
 
 -- | Just like 'rendertex', but with 'LaTeXT' output.
 --
 -- > rendertexM = textell . rendertex
-rendertexM :: (Render a, Monad m) => a -> LaTeXT_ m
+rendertexM :: (Render a, Monad m) => a -> LaTeXT m ()
 rendertexM = textell . rendertex
 
 -- Error throwing
 
+throwError :: Monad m => String -> LaTeXT m a
+throwError = LaTeXT . return . (error &&& Just)
+
+-- | Function 'merror' casts a value contained in a monad @m@ to the
+--   bottom value of another type. If you try to evaluate this value, you will
+--   get an error message with the 'String' passed as argument to 'merror'.
 merror :: Monad m => String -> LaTeXT m a -> LaTeXT m b
-merror = flip (>>) . return . error
+merror = flip (>>) . throwError
 
 -- Overloaded Strings
 
 -- | Be careful when using 'fromString' over a 'LaTeXT' value,
 --   the returned value of the computation is bottom (i.e. 'undefined').
 instance Monad m => IsString (LaTeXT m a) where
- fromString = (>> return undefined) . textell . fromString
+ fromString = merror "LaTeXT: fromString!" . textell . fromString
 
 -- | 'mappend' @=@ '>>'.
 instance Monad m => Monoid (LaTeXT m a) where
diff --git a/Text/LaTeX/Packages/AMSFonts.hs b/Text/LaTeX/Packages/AMSFonts.hs
--- a/Text/LaTeX/Packages/AMSFonts.hs
+++ b/Text/LaTeX/Packages/AMSFonts.hs
@@ -1,4 +1,5 @@
 
+-- | Module for the package @amsfonts@.
 module Text.LaTeX.Packages.AMSFonts
  ( -- * AMSFonts package
    amsfonts
diff --git a/Text/LaTeX/Packages/AMSMath.hs b/Text/LaTeX/Packages/AMSMath.hs
--- a/Text/LaTeX/Packages/AMSMath.hs
+++ b/Text/LaTeX/Packages/AMSMath.hs
@@ -129,10 +129,18 @@
  \l1 l2 ->
    (l1 <> commS "not") `op` l2
 
-infix 4 =: , /=:
+infixr 4 =: , /=:
 
-(=:),(/=:) :: LaTeXC l => l -> l -> l
+-- | Equal.
+--
+-- > infixr 4 =:
+(=:) :: LaTeXC l => l -> l -> l
 (=:)  = liftL2 $ TeXOp "="
+
+-- | Not equal.
+--
+-- > infixr 4 /=:
+(/=:) :: LaTeXC l => l -> l -> l
 (/=:) = notop (=:)
 
 -- | Greater.
@@ -184,7 +192,7 @@
 epsilon = comm0 "epsilon"
 
 varepsilon :: LaTeXC l => l
-varepsilon = comm0 "epsilon"
+varepsilon = comm0 "varepsilon"
 
 zeta :: LaTeXC l => l
 zeta = comm0 "zeta"
diff --git a/Text/LaTeX/Packages/Trees/Qtree.hs b/Text/LaTeX/Packages/Trees/Qtree.hs
--- a/Text/LaTeX/Packages/Trees/Qtree.hs
+++ b/Text/LaTeX/Packages/Trees/Qtree.hs
@@ -1,6 +1,9 @@
 
 {-# LANGUAGE OverloadedStrings #-}
 
+-- | Tree interface using the @qtree@ package.
+--   An example is of usage is provided in the /examples/ directory of
+--   the source distribution.
 module Text.LaTeX.Packages.Trees.Qtree (
     -- * Tree re-export
     module Text.LaTeX.Packages.Trees
@@ -18,6 +21,7 @@
 import Data.Monoid
 import Data.List (intersperse)
 
+-- | The 'qtree' package.
 qtree :: PackageName
 qtree = "qtree"
 
@@ -31,8 +35,11 @@
           , " ]"
             ]
 
+-- | Given a function to @LaTeX@ values, you can create a @LaTeX@ tree from a
+--   Haskell tree. The function specifies how to render the node values.
 tree :: LaTeXC l => (a -> l) -> Tree a -> l
 tree f t = commS "Tree" <> " " <> tree_ f t
 
+-- | This function works as 'tree', but use 'render' as rendering function.
 rendertree :: (Render a, LaTeXC l) => Tree a -> l
-rendertree = tree (raw . render)
+rendertree = tree (raw . protectText . render)