diff --git a/explicit-exception.cabal b/explicit-exception.cabal
--- a/explicit-exception.cabal
+++ b/explicit-exception.cabal
@@ -1,5 +1,5 @@
 Name:             explicit-exception
-Version:          0.1.1
+Version:          0.1.2
 License:          BSD3
 License-File:     LICENSE
 Author:           Henning Thielemann <haskell@henning-thielemann.de>
@@ -46,7 +46,7 @@
 Source-Repository this
   type:     darcs
   location: http://code.haskell.org/explicit-exception/
-  tag:      0.1.1
+  tag:      0.1.2
 
 Library
   Build-Depends: base >= 2, transformers >=0.0 && <0.2
diff --git a/src/Control/Monad/Exception/Asynchronous.hs b/src/Control/Monad/Exception/Asynchronous.hs
--- a/src/Control/Monad/Exception/Asynchronous.hs
+++ b/src/Control/Monad/Exception/Asynchronous.hs
@@ -202,6 +202,8 @@
 and the second stream is not touched.
 If the first stream can be read successfully,
 the second one is appended until stops.
+
+'append' is less strict than the 'Monoid' method 'mappend' instance.
 -}
 append ::
    Monoid a =>
@@ -225,9 +227,15 @@
    Exceptional (mplus ea eb) a
 
 
+{- |
+'mappend' must be strict in order to fulfill the Monoid laws
+@mappend mempty a = a@ and @mappend a mempty = a@ for @a=undefined@.
+-}
 instance Monoid a => Monoid (Exceptional e a) where
    mempty = pure mempty
-   mappend = append
+--   mappend = append
+   mappend (Exceptional ea a) (Exceptional eb b) =
+      Exceptional (mplus ea eb) (mappend a (maybe b (const mempty) ea))
 
 
 {- | construct Exceptional constructor lazily -}
@@ -240,6 +248,11 @@
 mapExceptional :: (e0 -> e1) -> (a -> b) -> Exceptional e0 a -> Exceptional e1 b
 mapExceptional f g ~(Exceptional e a) = Exceptional (fmap f e) (g a)
 
+{-
+fmap (f.g) = fmap f . fmap g
+
+
+-}
 instance Functor (Exceptional e) where
    fmap f ~(Exceptional e a) = Exceptional e (f a)