diff --git a/Data/Number/FixedFunctions.hs b/Data/Number/FixedFunctions.hs
--- a/Data/Number/FixedFunctions.hs
+++ b/Data/Number/FixedFunctions.hs
@@ -360,8 +360,8 @@
 	-- Inverse hyperbolic tangent with approximation eps
 	--
 	
-	| x >= 1     = 1%0
-	| x <= -1    = -1%0
+--	| x >= 1     = 1%0
+--	| x <= -1    = -1%0
 	| otherwise  = (1%2) * (log eps ((1 + x) / (1 - x)))
 	
 asinh :: Rational -> Rational -> Rational
@@ -369,8 +369,8 @@
 	--
 	-- Inverse hyperbolic sine
 	--
-	| x == 1%0  =  1%0
-	| x == -1%0 = -1%0
+--	| x == 1%0  =  1%0
+--	| x == -1%0 = -1%0
 	| otherwise  = log eps (x + (sqrt eps (x^2 + 1)))
 	
 acosh :: Rational -> Rational -> Rational
@@ -378,8 +378,8 @@
 	--
 	-- Inverse hyperbolic cosine
 	--
-	| x == 1%0 = 1%0
-	| x < 1     = 1%0
+--	| x == 1%0 = 1%0
+--	| x < 1     = 1%0
 	| otherwise = log eps (x + (sqrt eps (x^2 - 1)))
 		    		      
 ---------------------------------------------------------------------
diff --git a/Data/Number/Natural.hs b/Data/Number/Natural.hs
new file mode 100644
--- /dev/null
+++ b/Data/Number/Natural.hs
@@ -0,0 +1,54 @@
+module Data.Number.Natural(Natural) where
+
+data Natural = Z | S Natural
+
+instance Show Natural where
+    showsPrec p n = showsPrec p (toInteger n)
+
+instance Eq Natural where
+    x == y  =  x `compare` y == EQ
+
+instance Ord Natural where
+    Z   `compare` Z    =  EQ
+    Z   `compare` S _  =  LT
+    S _ `compare` Z    =  GT
+    S x `compare` S y  =  x `compare` y
+
+instance Num Natural where
+    Z   + y  =  y
+    S x + y  =  S (x + y)
+
+    x   - Z    =  x
+    Z   - S _  =  error "Natural: (-)"
+    S x - S y  =  x - y
+
+    Z   * y  =  Z
+    S x * y  =  y + x * y
+
+    abs x = x
+    signum Z = Z
+    signum (S _) = S Z
+
+    fromInteger x | x < 0 = error "Natural: fromInteger"
+    fromInteger 0 = Z
+    fromInteger x = S (fromInteger (x-1))
+
+instance Integral Natural where
+    -- Not the most efficient version, but efficiency isn't the point of this module. :)
+    quotRem x y =
+        if x < y then
+	    (0, x)
+        else
+	    let (q, r) = quotRem (x-y) y
+	    in  (q+1, r)
+    div = quot
+    mod = rem
+    toInteger Z = 0
+    toInteger (S x) = 1 + toInteger x
+
+instance Real Natural where
+    toRational = toRational . toInteger
+
+instance Enum Natural where
+    toEnum = fromIntegral
+    fromEnum = fromIntegral
diff --git a/numbers.cabal b/numbers.cabal
--- a/numbers.cabal
+++ b/numbers.cabal
@@ -1,5 +1,5 @@
 Name:		numbers
-Version:	2007.9.23
+Version:	2007.9.24
 License:	BSD3
 Author:		Lennart Augustsson
 Maintainer:	Lennart Augustsson
@@ -9,9 +9,11 @@
 		different numbers: (computable) real numbers,
 		arbitrary precision fixed numbers,
 		arbitrary precision floating point numbers,
-		differentiable numbers, symbolic numbers.
+		differentiable numbers, symbolic numbers,
+		natural numbers, interval arithmetic.
 Build-Depends:	base
 Exposed-modules:	Data.Number.Symbolic Data.Number.Dif
 			Data.Number.CReal Data.Number.Fixed
 			Data.Number.Interval Data.Number.BigFloat
+			Data.Number.Natural
 Other-modules:	Data.Number.Vectorspace Data.Number.FixedFunctions