diff --git a/Data/Heap.hs b/Data/Heap.hs
--- a/Data/Heap.hs
+++ b/Data/Heap.hs
@@ -1,7 +1,7 @@
-{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
+{-# LANGUAGE EmptyDataDecls, FlexibleInstances, MultiParamTypeClasses #-}
 
 -- | 
--- An efficent implementation of min-, max- or custom-priority heaps
+-- A flexible implementation of min-, max- or custom-priority heaps
 -- based on the leftist-heaps from Chris Okasaki's book \"Purely Functional Data
 -- Structures\", Cambridge University Press, 1998, chapter 3.1.
 --
@@ -33,7 +33,6 @@
 
 import Data.List (foldl')
 import Data.Monoid
-import Data.Ord
 import Prelude hiding (head, null)
 
 -- |
@@ -75,20 +74,25 @@
 -- a 'Heap'.
 class HeapPolicy p a where
 	-- |
-	-- Compare two elements, just like 'compare' of the 'Ord' class.
-	-- /The first parameter must be ignored by the implementation/.
-	heapCompare :: p -> a -> a -> Ordering
+	-- Compare two elements, just like 'compare' of the 'Ord' class,
+	-- so this function has to define a mathematical ordering.
+	-- When using a 'HeapPolicy' for a 'Heap', the minimal value
+	-- (defined by this order) will be the 'head' of the 'Heap'.
+	heapCompare :: p    -- ^ /Must not be used/.
+		-> a        -- ^ Must be compared to 3rd parameter.
+		-> a        -- ^ Must be compared to 2nd parameter.
+		-> Ordering -- ^ Result of the comparison.
 
 -- |
 -- Policy type for a 'MinHeap'.
-data MinPolicy = MinPolicy
+data MinPolicy
 
 instance (Ord a) => HeapPolicy MinPolicy a where
 	heapCompare = const compare
 
 -- |
 -- Policy type for a 'MaxHeap'
-data MaxPolicy = MaxPolicy
+data MaxPolicy
 
 instance (Ord a) => HeapPolicy MaxPolicy a where
 	heapCompare = const (flip compare)
@@ -119,7 +123,7 @@
 -- |
 -- /O(n)/. The number of elements in the 'Heap'.
 size :: (Num n) => Heap p a -> n
-size Empty = 0
+size Empty          = 0
 size (Tree _ _ a b) = 1 + size a + size b
 
 -- |
@@ -165,7 +169,7 @@
 	else makeT y l2 (union r2 heap1) -- heap into the right branch, it's shorter
 
 -- |
--- Combines a value x and two 'Heaps' to one 'Heap'. Therefore, x has to
+-- Combines a value @x@ and two 'Heaps' to one 'Heap'. Therefore, @x@ has to
 -- be less or equal the minima (depending on the 'HeapPolicy') of both
 -- 'Heap' parameters. /The precondition is not checked/.
 makeT :: a -> Heap p a -> Heap p a -> Heap p a
@@ -200,7 +204,7 @@
 
 -- |
 -- /O(n)/. Creates a 'Heap' from an ascending list. Note that the list
--- has to be ascending corresponding to the 'HeapPolicy', not to it's
+-- has to be ascending corresponding to the 'HeapPolicy', not to its
 -- 'Ord' instance declaration (if there is one).
 -- /The precondition is not checked/.
 fromAscList :: (HeapPolicy p a) => [a] -> Heap p a
@@ -213,7 +217,7 @@
 -- to the 'HeapPolicy').
 toAscList :: (HeapPolicy p a) => Heap p a -> [a]
 toAscList Empty            = []
-toAscList h@(Tree _ x a b) = x : mergeLists (toAscList a) (toAscList b)
+toAscList h@(Tree _ e a b) = e : mergeLists (toAscList a) (toAscList b)
 	where	mergeLists [] ys = ys
 		mergeLists xs [] = xs
 		mergeLists xs@(x:xs') ys@(y:ys') = if LT == heapCompare (policy h) x y
@@ -228,10 +232,10 @@
 check h@(Tree r x left right) = let
 		leftRank  = rank left
 		rightRank = rank right
-	in (isEmpty left || LT /= heapCompare (policy h) (head left) x)
-		&& (isEmpty right || LT /= heapCompare (policy h) (head right) x)
-		&& r == 1 + rightRank
-		&& leftRank >= rightRank
+	in (null left || LT /= heapCompare (policy h) (head left) x) -- heap property
+		&& (null right || LT /= heapCompare (policy h) (head right) x) -- dito
+		&& r == 1 + rightRank    -- rank = length of right spine
+		&& leftRank >= rightRank -- leftist property
 		&& check left
 		&& check right
 
diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,22 @@
+Copyright (c) 2008, Stephan Friedrichs
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * 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.
+
+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 COPYRIGHT OWNER 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/heap.cabal b/heap.cabal
--- a/heap.cabal
+++ b/heap.cabal
@@ -1,7 +1,8 @@
 Name:			heap
-Version:		0.1
+Version:		0.1.1
 Stability:		beta
 License:		BSD3
+License-File:		LICENSE
 Author:			Stephan Friedrichs
 Maintainer:		stephan[dot]friedrichs[at]tu-bs[dot]de
 Category:		Data
@@ -9,4 +10,4 @@
 Description:		A flexible Haskell heap implementation
 Build-Depends:		base
 Exposed-Modules:	Data.Heap
-ghc-options:		-O
+ghc-options:		-O -Wall