diff --git a/Control/Monad/SearchTree.hs b/Control/Monad/SearchTree.hs
--- a/Control/Monad/SearchTree.hs
+++ b/Control/Monad/SearchTree.hs
@@ -2,7 +2,7 @@
 -- |
 -- Module      : Control.Monad.SearchTree
 -- Copyright   : Sebastian Fischer
--- License     : PublicDomain
+-- License     : BSD3
 -- 
 -- Maintainer  : Sebastian Fischer (sebf@informatik.uni-kiel.de)
 -- Stability   : experimental
@@ -20,6 +20,7 @@
 module Control.Monad.SearchTree ( SearchTree(..), Search, searchTree ) where
 
 import Control.Monad
+import Control.Applicative
 
 -- | 
 -- The type @SearchTree a@ represents non-deterministic computations
@@ -27,8 +28,20 @@
 data SearchTree a = None | One a | Choice (SearchTree a) (SearchTree a)
  deriving Show
 
-instance Monad SearchTree
- where
+instance Functor SearchTree where
+  fmap _ None         = None
+  fmap f (One x)      = One (f x)
+  fmap f (Choice s t) = Choice (fmap f s) (fmap f t)
+
+instance Applicative SearchTree where
+  pure  = return
+  (<*>) = ap
+
+instance Alternative SearchTree where
+  empty = mzero
+  (<|>) = mplus
+
+instance Monad SearchTree where
   return = One
 
   None       >>= _ = None
@@ -37,12 +50,10 @@
 
   fail _ = None
 
-instance MonadPlus SearchTree
- where
+instance MonadPlus SearchTree where
   mzero = None
   mplus = Choice
 
-
 -- |
 -- Another search monad based on continuations that produce search
 -- trees.
@@ -57,15 +68,22 @@
 searchTree :: Search a -> SearchTree a
 searchTree a = search a One
 
-instance Monad Search
- where
+instance Functor Search where
+  fmap f a = Search (\k -> search a (k . f))
+
+instance Applicative Search where
+  pure  = return
+  (<*>) = ap
+
+instance Alternative Search where
+  empty = mzero
+  (<|>) = mplus
+
+instance Monad Search where
   return x = Search ($x)
   a >>= f  = Search (\k -> search a (\x -> search (f x) k))
   fail _   = mzero
 
-instance MonadPlus Search
- where
-  mzero       = Search (const None)
-  a `mplus` b = Search (\k -> search a k `Choice` search b k)
-
-
+instance MonadPlus Search where
+  mzero       = Search (const mzero)
+  a `mplus` b = Search (\k -> search a k `mplus` search b k)
diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,15 +1,32 @@
-ALL PUBLIC DOMAIN MATERIAL IS OFFERED AS-IS. NO REPRESENTATIONS OR
-WARRANTIES OF ANY KIND ARE MADE CONCERNING THE MATERIALS, EXPRESS,
-IMPLIED, STATUTORY OR OTHERWISE, INCLUDING, WITHOUT LIMITATION,
-WARRANTIES OF TITLE, MERCHANTIBILITY, FITNESS FOR A PARTICULAR
-PURPOSE, NONINFRINGEMENT, OR THE ABSENCE OF LATENT OR OTHER DEFECTS,
-ACCURACY, OR THE PRESENCE OF ABSENCE OF ERRORS, WHETHER OR NOT
-DISCOVERABLE.
+Copyright (c) 2010, Thomas Wilke, Frank Huch, Sebastian Fischer
 
-IN NO EVENT WILL THE AUTHOR(S), PUBLISHER(S), OR PRESENTER(S) OF ANY
-PUBLIC DOMAIN MATERIAL BE LIABLE TO YOU ON ANY LEGAL THEORY FOR ANY
-SPECIAL, INCIDENTAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES
-ARISING OUT OF THIS LICENSE OR THE USE OF THE WORK, EVEN IF THE
-AUTHOR(S), PUBLISHER(S), OR PRESENTER(S) HAVE BEEN ADVISED OF THE
-POSSIBILITY OF SUCH DAMAGES.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+ 1. Redistributions of source code must retain the above copyright
+    notice, this list of conditions and the following disclaimer.
+
+ 2. Redistributions in binary form must reproduce the above copyright
+    notice, this list of conditions and the following disclaimer in
+    the documentation and/or other materials provided with the
+    distribution.
+
+ 3. Neither the name of the author nor the names of his contributors
+    may be used to endorse or promote products derived from this
+    software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR
+CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
diff --git a/tree-monad.cabal b/tree-monad.cabal
--- a/tree-monad.cabal
+++ b/tree-monad.cabal
@@ -1,5 +1,5 @@
 Name:          tree-monad
-Version:       0.2.1
+Version:       0.3
 Cabal-Version: >= 1.6
 Synopsis:      Non-Determinism Monad for Tree Search
 Description:   
@@ -9,7 +9,7 @@
   constructors represent mzero, return, and mplus.
 
 Category:      Control, Monads
-License:       PublicDomain
+License:       BSD3
 License-File:  LICENSE
 Author:        Sebastian Fischer
 Maintainer:    sebf@informatik.uni-kiel.de
@@ -21,7 +21,7 @@
 Extra-Source-Files: README
 
 Library
-  Build-Depends:    base
+  Build-Depends:    base >= 3 && < 5
   Exposed-Modules:  Control.Monad.SearchTree
   Ghc-Options:      -Wall
   Extensions:       Rank2Types