diff --git a/Data/SequentialIndex.hs b/Data/SequentialIndex.hs
new file mode 100644
--- /dev/null
+++ b/Data/SequentialIndex.hs
@@ -0,0 +1,87 @@
+module Data.SequentialIndex
+(
+  SequentialIndex
+, mantissa
+, exponent
+, zero
+, one
+, sequentialIndex
+, between
+, toByteString
+, fromByteString
+)
+where
+
+import           Data.Bits
+import           Prelude         hiding (exponent)
+import qualified Data.ByteString as B
+
+-- must always be in normalised form!
+data SequentialIndex 
+    = SI !Integer !Int
+    deriving (Eq)
+
+mantissa :: SequentialIndex -> Integer
+mantissa (SI m _) = m
+
+exponent :: SequentialIndex -> Int
+exponent (SI _ e) = e
+
+zero :: SequentialIndex
+zero = SI 0 0
+
+one :: SequentialIndex
+one = SI 1 0
+
+commonBase :: SequentialIndex -> SequentialIndex -> (Integer, Integer, Int)
+commonBase (SI m1 e1) (SI m2 e2) = (m1', m2', e)
+    where e = max e1 e2
+          m1' = m1 `shift` (e - e1)
+          m2' = m2 `shift` (e - e2)
+
+sequentialIndex :: Integer -> Int -> SequentialIndex
+sequentialIndex 0 _ = zero
+sequentialIndex mx ex
+    = case () of
+        _ | v < zero  -> error "Invalid SequentialIndex: below zero"
+          | v > one   -> error "Invalid SequentialIndex: beyond one"
+          | otherwise -> v
+    where v = until (\(SI m _) -> m `testBit` 0) 
+                    (\(SI m e) -> SI (m `shiftR` 1) (e - 1)) 
+                    (SI mx ex)
+
+instance Bounded SequentialIndex where
+    minBound = zero
+    maxBound = one
+
+instance Ord SequentialIndex where
+    a `compare` b = a' `compare` b'
+        where (a', b', _) = commonBase a b
+
+instance Show SequentialIndex where
+    show (SI m e) = case map sbit bits of
+                      []      -> "0.0"
+                      [d1]    -> d1 : ".0"
+                      (d1:ds) -> d1 : '.' : ds
+        where bits = map (testBit m) [e, e - 1 .. 0]
+              
+              sbit False = '0'
+              sbit True  = '1'
+
+between :: SequentialIndex -> SequentialIndex -> SequentialIndex
+between a b = sequentialIndex (m1 + m2) (e + 1)
+    where (m1, m2, e) = commonBase a b
+
+toByteString :: SequentialIndex -> B.ByteString
+toByteString (SI m e) = B.unfoldr step m'
+    where e' = (e `div` 8) * 8 + 7
+          m' = m `shift` (e' - e)
+
+          step 0 = Nothing
+          step v = let (q, r) = v `divMod` 256
+                   in Just (fromInteger r, q)
+
+fromByteString :: B.ByteString -> SequentialIndex
+fromByteString bs = sequentialIndex m (max 0 $ e - 1)
+    where (m, e) = B.foldr step (0, 0) bs
+          step w (mx, ex) = (mx `shiftL` 8 + toInteger w, ex + 8)
diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright (c)2011, Aristid Breitkreuz
+
+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.
+
+    * Neither the name of Aristid Breitkreuz nor the names of other
+      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 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/README b/README
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--- /dev/null
+++ b/README
@@ -0,0 +1,4 @@
+Sequential index numbers between 0.0 and 1.0 that allow arbitrarily inserting new numbers in between. They can possibly used for disk-based and other special containers, where adding a new element without changing the indexes of the other elements is important.
+
+Conceptually, SequentialIndex denotes a path to an element in an imaginary binary tree.
+
diff --git a/Setup.hs b/Setup.hs
new file mode 100644
--- /dev/null
+++ b/Setup.hs
@@ -0,0 +1,2 @@
+import Distribution.Simple
+main = defaultMain
diff --git a/sequential-index.cabal b/sequential-index.cabal
new file mode 100644
--- /dev/null
+++ b/sequential-index.cabal
@@ -0,0 +1,67 @@
+-- sequential-index.cabal auto-generated by cabal init. For additional
+-- options, see
+-- http://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.html#pkg-descr.
+-- The name of the package.
+Name:                sequential-index
+
+-- The package version. See the Haskell package versioning policy
+-- (http://www.haskell.org/haskellwiki/Package_versioning_policy) for
+-- standards guiding when and how versions should be incremented.
+Version:             0.0
+
+-- A short (one-line) description of the package.
+Synopsis:            Sequential index numbers between 0.0 and 1.0 that allow arbitrarily inserting new numbers in between (for special containers).
+
+-- A longer description of the package.
+Description:         Sequential index numbers between 0.0 and 1.0 that allow arbitrarily inserting new numbers in between. They can possibly used for disk-based and other special containers, where adding a new element without changing the indexes of the other elements is important.
+                     
+                     Conceptually, SequentialIndex denotes a path to an element in an imaginary binary tree.
+
+-- The license under which the package is released.
+License:             BSD3
+
+-- The file containing the license text.
+License-file:        LICENSE
+
+-- The package author(s).
+Author:              Aristid Breitkreuz
+
+-- An email address to which users can send suggestions, bug reports,
+-- and patches.
+Maintainer:          aristidb@googlemail.com
+
+-- A copyright notice.
+Copyright:           Copyright (C) 2011 Aristid Breitkreuz
+
+Homepage:            https://github.com/aristidb/sequential-index
+
+Category:            Data
+
+Build-type:          Simple
+
+-- Extra files to be distributed with the package, such as examples or
+-- a README.
+Extra-source-files:  README
+
+-- Constraint on the version of Cabal needed to build this package.
+Cabal-version:       >=1.2
+
+
+Library
+  -- Modules exported by the library.
+  Exposed-modules:     Data.SequentialIndex
+
+  -- GHC Options.
+  GHC-Options:         -Wall
+  
+  -- Packages needed in order to build this package.
+  Build-depends:
+                       base >=4.2.0.0 && <5,
+                       bytestring >=0.9.1.5 && <0.10
+  
+  -- Modules not exported by this package.
+  -- Other-modules:       
+  
+  -- Extra tools (e.g. alex, hsc2hs, ...) needed to build the source.
+  -- Build-tools:         
+