diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -1,6 +1,9 @@
 LeanCheck
 =========
 
+[![Build Status][build-status]][build-log]
+[![Hackage version][hackage-version]][leancheck-on-hackage]
+
 LeanCheck is a simple enumerative [property-based testing] library.  Properties
 are defined as Haskell functions returning a boolean value which should be
 `True` for all possible choices of argument values.    LeanCheck applies
@@ -20,6 +23,7 @@
 
 To install the latest LeanCheck version from Hackage, just run:
 
+	$ cabal update
 	$ cabal install leancheck
 
 
@@ -154,3 +158,7 @@
 [SmallCheck]: https://hackage.haskell.org/package/smallcheck
 [QuickCheck]: https://hackage.haskell.org/package/QuickCheck
 
+[build-status]: https://travis-ci.org/rudymatela/leancheck.svg?branch=master
+[build-log]:    https://travis-ci.org/rudymatela/leancheck
+[hackage-version]: https://img.shields.io/hackage/v/leancheck.svg
+[leancheck-on-hackage]: https://hackage.haskell.org/package/leancheck
diff --git a/TODO.md b/TODO.md
--- a/TODO.md
+++ b/TODO.md
@@ -29,13 +29,17 @@
 * on data-invariant.md, write missing section;
 
 
-v0.6.2
+v0.6.3
 ------
 
-* add copyright notices on modules using Haddock markup
+* add `classify` function to measure distribution of data:
+  something like:
 
+    classifyBy :: (a -> b) -> [a] -> [(b,a)]
+	countsBy :: (a -> b) -> [a] -> [(b,Int)]
 
-v0.6.3
+
+v0.6.4
 ------
 
 * implement stub `Test.LeanCheck.Function.*` modules;
diff --git a/leancheck.cabal b/leancheck.cabal
--- a/leancheck.cabal
+++ b/leancheck.cabal
@@ -1,4 +1,4 @@
--- LeanCheck
+-- Cabal file for LeanCheck
 --
 -- Template Haskell dependency is optional.  To deactivate it:
 -- 1. In this file, comment out:
@@ -11,7 +11,7 @@
 -- this cabal file too complicated.  -- Rudy
 
 name:                leancheck
-version:             0.6.1
+version:             0.6.2
 synopsis:            Cholesterol-free property-based testing
 description:
   LeanCheck is a simple enumerative property-based testing library.
@@ -50,7 +50,7 @@
 source-repository this
   type:            git
   location:        https://github.com/rudymatela/leancheck
-  tag:             v0.6.1
+  tag:             v0.6.2
 
 library
   exposed-modules: Test.LeanCheck
diff --git a/src/Test/LeanCheck.hs b/src/Test/LeanCheck.hs
--- a/src/Test/LeanCheck.hs
+++ b/src/Test/LeanCheck.hs
@@ -1,6 +1,12 @@
 {-# OPTIONS_HADDOCK prune #-}
--- | LeanCheck is a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
 --
+-- LeanCheck is a simple enumerative property-based testing library.
+--
 -- A __property__ is a function returning a 'Bool' that should be 'True' for
 -- all possible choices of arguments.  Properties can be viewed as a
 -- parameterized unit tests.
@@ -15,7 +21,7 @@
 --
 -- For example:
 --
--- > holds $ \xs -> length (sort xs) == length (xs::[Int])
+-- > holds 1000 $ \xs -> length (sort xs) == length (xs::[Int])
 --
 --
 -- To get the smallest 'counterExample' by testing up to a thousand values,
@@ -26,8 +32,7 @@
 --
 -- Arguments of properties should be instances of the 'Listable' typeclass.
 -- 'Listable' instances are provided for the most common Haskell types.
--- New instances are easily defined
--- (see 'Listable' for more info).
+-- New instances are easily defined (see 'Listable' for more info).
 module Test.LeanCheck
   (
   -- * Checking and testing
diff --git a/src/Test/LeanCheck/Basic.hs b/src/Test/LeanCheck/Basic.hs
--- a/src/Test/LeanCheck/Basic.hs
+++ b/src/Test/LeanCheck/Basic.hs
@@ -1,5 +1,11 @@
--- | This module is part of LeanCheck,
---   a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck.Basic
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
 --
 -- This module exports "Test.LeanCheck.Core" along with:
 --
diff --git a/src/Test/LeanCheck/Core.hs b/src/Test/LeanCheck/Core.hs
--- a/src/Test/LeanCheck/Core.hs
+++ b/src/Test/LeanCheck/Core.hs
@@ -1,4 +1,10 @@
--- | LeanCheck is a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck.Core
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- LeanCheck is a simple enumerative property-based testing library.
 --
 -- This is the core module of the library, with the most basic definitions.  If
 -- you are looking just to use the library, import and see "Test.LeanCheck".
diff --git a/src/Test/LeanCheck/Derive.hs b/src/Test/LeanCheck/Derive.hs
--- a/src/Test/LeanCheck/Derive.hs
+++ b/src/Test/LeanCheck/Derive.hs
@@ -1,5 +1,12 @@
 {-# LANGUAGE TemplateHaskell, CPP #-}
--- | LeanCheck is a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck.Derive
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
 --
 -- This is an experimental module for deriving 'Listable' instances.
 --
diff --git a/src/Test/LeanCheck/Error.hs b/src/Test/LeanCheck/Error.hs
--- a/src/Test/LeanCheck/Error.hs
+++ b/src/Test/LeanCheck/Error.hs
@@ -1,4 +1,11 @@
--- | LeanCheck is a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck.Error
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
 --
 -- This module re-exports "Test.LeanCheck" but some test functions have been
 -- specialized to catch errors (see the explicit export list below).
diff --git a/src/Test/LeanCheck/Function.hs b/src/Test/LeanCheck/Function.hs
--- a/src/Test/LeanCheck/Function.hs
+++ b/src/Test/LeanCheck/Function.hs
@@ -1,4 +1,11 @@
--- | LeanCheck is a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck.Function
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
 --
 -- This module exports 'Listable' and 'Show' function typeclass instances.
 -- These can be useful for testing higher-order properties --- properties that
diff --git a/src/Test/LeanCheck/Function/CoListable.hs b/src/Test/LeanCheck/Function/CoListable.hs
--- a/src/Test/LeanCheck/Function/CoListable.hs
+++ b/src/Test/LeanCheck/Function/CoListable.hs
@@ -1,6 +1,14 @@
--- | This module is part of 'Test.LeanCheck'.
--- It exports a 'Listable' instance for function enumeration
--- by means of a 'CoListable' typeclass.
+-- |
+-- Module      : Test.LeanCheck.CoListable
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- This module exports a 'Listable' instance for function enumeration by means
+-- of a 'CoListable' typeclass.
 --
 -- This module /does not currently work/, it it just a sketch and a stub.
 module Test.LeanCheck.Function.CoListable
diff --git a/src/Test/LeanCheck/Function/Eq.hs b/src/Test/LeanCheck/Function/Eq.hs
--- a/src/Test/LeanCheck/Function/Eq.hs
+++ b/src/Test/LeanCheck/Function/Eq.hs
@@ -1,3 +1,12 @@
+-- |
+-- Module      : Test.LeanCheck.Function.Eq
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
 -- A toy Eq instance for Functions.
 module Test.LeanCheck.Function.Eq () where
 
diff --git a/src/Test/LeanCheck/Function/ListsOfPairs.hs b/src/Test/LeanCheck/Function/ListsOfPairs.hs
--- a/src/Test/LeanCheck/Function/ListsOfPairs.hs
+++ b/src/Test/LeanCheck/Function/ListsOfPairs.hs
@@ -1,6 +1,14 @@
--- | This module is part of 'Test.LeanCheck'.
---   It exports a 'Listable' instance for function enumeration
---   via lists of pairs.
+-- |
+-- Module      : Test.LeanCheck.Function.ListsOfPairs
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- This module exports a 'Listable' instance for function enumeration
+-- via lists of pairs.
 --
 -- This module considers functions as a finite list of exceptional input-output
 -- cases to a default value (list of pairs of arguments and results).
diff --git a/src/Test/LeanCheck/Function/Periodic.hs b/src/Test/LeanCheck/Function/Periodic.hs
--- a/src/Test/LeanCheck/Function/Periodic.hs
+++ b/src/Test/LeanCheck/Function/Periodic.hs
@@ -1,5 +1,14 @@
--- | This module is part of 'Test.LeanCheck'.
--- It exports a 'Listable' instance for enumeration of periodic functions.
+-- |
+-- Module      : Test.LeanCheck.Function.Periodic
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- This module exports a 'Listable' instance for enumeration of periodic
+-- functions.
 --
 -- This module /barely works/ and is just a sketch.
 module Test.LeanCheck.Function.Periodic
diff --git a/src/Test/LeanCheck/Function/Show.hs b/src/Test/LeanCheck/Function/Show.hs
--- a/src/Test/LeanCheck/Function/Show.hs
+++ b/src/Test/LeanCheck/Function/Show.hs
@@ -1,4 +1,13 @@
--- | A 'Show' instance for functions.
+-- |
+-- Module      : Test.LeanCheck.Function.Show
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- A 'Show' instance for functions.
 module Test.LeanCheck.Function.Show () where
 
 import Test.LeanCheck.Function.ShowFunction
diff --git a/src/Test/LeanCheck/Function/ShowFunction.hs b/src/Test/LeanCheck/Function/ShowFunction.hs
--- a/src/Test/LeanCheck/Function/ShowFunction.hs
+++ b/src/Test/LeanCheck/Function/ShowFunction.hs
@@ -1,5 +1,14 @@
--- | This module exports the 'ShowFunction' typeclass,
---   its instances and related functions.
+-- |
+-- Module      : Test.LeanCheck.Function.ShowFunction
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- This module exports the 'ShowFunction' typeclass,
+-- its instances and related functions.
 --
 -- Using this module, it is possible to implement
 -- a Show instance for functions:
diff --git a/src/Test/LeanCheck/IO.hs b/src/Test/LeanCheck/IO.hs
--- a/src/Test/LeanCheck/IO.hs
+++ b/src/Test/LeanCheck/IO.hs
@@ -1,4 +1,13 @@
--- | QuickCheck-like interface to 'Test.LeanCheck'
+-- |
+-- Module      : Test.LeanCheck.IO
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- QuickCheck-like interface to "Test.LeanCheck"
 {-# LANGUAGE CPP #-}
 module Test.LeanCheck.IO
   ( check
diff --git a/src/Test/LeanCheck/Invariants.hs b/src/Test/LeanCheck/Invariants.hs
--- a/src/Test/LeanCheck/Invariants.hs
+++ b/src/Test/LeanCheck/Invariants.hs
@@ -1,4 +1,13 @@
--- | Some invariants over Test.LeanCheck functions.
+-- |
+-- Module      : Test.LeanCheck.Invariants
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- Some invariants about Test.LeanCheck functions.
 --
 -- You should be importing this ONLY to test "Test.LeanCheck" itself.
 module Test.LeanCheck.Invariants
diff --git a/src/Test/LeanCheck/Tiers.hs b/src/Test/LeanCheck/Tiers.hs
--- a/src/Test/LeanCheck/Tiers.hs
+++ b/src/Test/LeanCheck/Tiers.hs
@@ -1,4 +1,10 @@
--- | LeanCheck is a simple enumerative property-based testing library.
+-- |
+-- Module      : Test.LeanCheck.Tiers
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- LeanCheck is a simple enumerative property-based testing library.
 --
 -- This module provides advanced functions for manipulating 'tiers'.
 -- Most definitions given here are exported by "Test.LeanCheck", except:
diff --git a/src/Test/LeanCheck/Utils.hs b/src/Test/LeanCheck/Utils.hs
--- a/src/Test/LeanCheck/Utils.hs
+++ b/src/Test/LeanCheck/Utils.hs
@@ -1,4 +1,13 @@
--- | Some utilities for property-based testing with 'Test.LeanCheck'.
+-- |
+-- Module      : Test.LeanCheck.Utils
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- Some utilities for property-based testing with 'Test.LeanCheck'.
 --
 -- Those utilities are general-purpose enough to be used with other
 -- property-based based testing libraries.  See each exported module for
diff --git a/src/Test/LeanCheck/Utils/Operators.hs b/src/Test/LeanCheck/Utils/Operators.hs
--- a/src/Test/LeanCheck/Utils/Operators.hs
+++ b/src/Test/LeanCheck/Utils/Operators.hs
@@ -1,3 +1,13 @@
+-- |
+-- Module      : Test.LeanCheck.Utils.Operators
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- Some operators for property-based testing.
 module Test.LeanCheck.Utils.Operators
   (
 --  (==>) -- already provided by Test.LeanCheck
diff --git a/src/Test/LeanCheck/Utils/TypeBinding.hs b/src/Test/LeanCheck/Utils/TypeBinding.hs
--- a/src/Test/LeanCheck/Utils/TypeBinding.hs
+++ b/src/Test/LeanCheck/Utils/TypeBinding.hs
@@ -1,4 +1,13 @@
--- | Infix operators for type binding using dummy first-class values.
+-- |
+-- Module      : Test.LeanCheck.Utils.TypeBinding
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- Infix operators for type binding using dummy first-class values.
 --
 -- Those are useful when property based testing to avoid repetition.
 -- Suppose:
diff --git a/src/Test/LeanCheck/Utils/Types.hs b/src/Test/LeanCheck/Utils/Types.hs
--- a/src/Test/LeanCheck/Utils/Types.hs
+++ b/src/Test/LeanCheck/Utils/Types.hs
@@ -1,4 +1,13 @@
--- | Types to aid in property-based testing.
+-- |
+-- Module      : Test.LeanCheck.Utils.Types
+-- Copyright   : (c) 2015-2017 Rudy Matela
+-- License     : 3-Clause BSD  (see the file LICENSE)
+-- Maintainer  : Rudy Matela <rudy@matela.com.br>
+--
+-- This module is part of LeanCheck,
+-- a simple enumerative property-based testing library.
+--
+-- Types to aid in property-based testing.
 module Test.LeanCheck.Utils.Types
   (
   -- * Integer types
diff --git a/tests/test-derive.hs b/tests/test-derive.hs
--- a/tests/test-derive.hs
+++ b/tests/test-derive.hs
@@ -1,3 +1,5 @@
+-- Copyright (c) 2015-2017 Rudy Matela.
+-- Distributed under the 3-Clause BSD licence (see the file LICENSE).
 {-# LANGUAGE TemplateHaskell, CPP #-}
 import Test.LeanCheck.Derive
 import Test.LeanCheck
diff --git a/tests/test-error.hs b/tests/test-error.hs
--- a/tests/test-error.hs
+++ b/tests/test-error.hs
@@ -1,3 +1,5 @@
+-- Copyright (c) 2015-2017 Rudy Matela.
+-- Distributed under the 3-Clause BSD licence (see the file LICENSE).
 import System.Exit (exitFailure)
 import Data.List (elemIndices,sort)
 
diff --git a/tests/test-operators.hs b/tests/test-operators.hs
--- a/tests/test-operators.hs
+++ b/tests/test-operators.hs
@@ -1,3 +1,5 @@
+-- Copyright (c) 2015-2017 Rudy Matela.
+-- Distributed under the 3-Clause BSD licence (see the file LICENSE).
 import System.Exit (exitFailure)
 import Data.List (elemIndices,sort)
 import Test.LeanCheck
diff --git a/tests/test-tiers.hs b/tests/test-tiers.hs
--- a/tests/test-tiers.hs
+++ b/tests/test-tiers.hs
@@ -1,3 +1,5 @@
+-- Copyright (c) 2015-2017 Rudy Matela.
+-- Distributed under the 3-Clause BSD licence (see the file LICENSE).
 import System.Exit (exitFailure)
 import Data.List (elemIndices, sort, nub, delete)
 
diff --git a/tests/test-types.hs b/tests/test-types.hs
--- a/tests/test-types.hs
+++ b/tests/test-types.hs
@@ -1,3 +1,5 @@
+-- Copyright (c) 2015-2017 Rudy Matela.
+-- Distributed under the 3-Clause BSD licence (see the file LICENSE).
 import System.Exit (exitFailure)
 import Data.List (elemIndices,delete)
 import Test.LeanCheck.Utils.Types
diff --git a/tests/test.hs b/tests/test.hs
--- a/tests/test.hs
+++ b/tests/test.hs
@@ -1,3 +1,5 @@
+-- Copyright (c) 2015-2017 Rudy Matela.
+-- Distributed under the 3-Clause BSD licence (see the file LICENSE).
 import System.Exit (exitFailure)
 import Data.List (elemIndices)