[catagits/Catalyst-Runtime.git] / lib / Catalyst / Manual / Tutorial / Authentication.pod

=head1 NAME

Catalyst::Manual::Tutorial::Authentication - Catalyst Tutorial - Part 4: Authentication


=head1 OVERVIEW

This is B<Part 4 of 9> for the Catalyst tutorial.

L<Tutorial Overview|Catalyst::Manual::Tutorial>

=over 4

=item 1

L<Introduction|Catalyst::Manual::Tutorial::Intro>

=item 2

L<Catalyst Basics|Catalyst::Manual::Tutorial::CatalystBasics>

=item 3

L<Basic CRUD|Catalyst::Manual::Tutorial_BasicCRUD>

=item 4

B<Authentication>

=item 5

L<Authorization|Catalyst::Manual::Tutorial::Authorization>

=item 6

L<Debugging|Catalyst::Manual::Tutorial::Debugging>

=item 7

L<Testing|Catalyst::Manual::Tutorial::Testing>

=item 8

L<AdvancedCRUD|Catalyst::Manual::Tutorial::AdvancedCRUD>

=item 9

L<Appendices|Catalyst::Manual::Tutorial::Appendicies>

=back


=head1 DESCRIPTION

Now that we finally have a simple yet functional application, we can
focus on providing authentication (with authorization coming next in
Part 5).

This part of the tutorial is divided into two main sections: 1) basic,
cleartext authentication and 2) hash-based authentication.

B<TIP>: Note that all of the code for this part of the tutorial can be
pulled from the Catalyst Subversion repository in one step with the
following command:

    svn checkout http://dev.catalyst.perl.org/repos/Catalyst/trunk/examples/Tutorial@###
    IMPORTANT: Does not work yet.  Will be completed for final version.


=head1 BASIC AUTHENTICATION

This section explores how to add authentication logic to a Catalyst
application.


=head2 Add Users and Roles to the Database

First, we add both user and role information to the database (we will
add the role information here although it will not be used until the
authorization section, Part 5).  Create a new SQL script file by opening
C<myapp02.sql> in your editor and insert:

    --
    -- Add users and roles tables, along with a many-to-many join table
    --
    CREATE TABLE users (
            id            INTEGER PRIMARY KEY,
            username      TEXT,
            password      TEXT,
            email_address TEXT,
            first_name    TEXT,
            last_name     TEXT,
            active        INTEGER
    );
    CREATE TABLE roles (
            id   INTEGER PRIMARY KEY,
            role TEXT
    );
    CREATE TABLE user_roles (
            user_id INTEGER,
            role_id INTEGER,
            PRIMARY KEY (user_id, role_id)
    );
    --
    -- Load up some initial test data
    --
    INSERT INTO users VALUES (1, 'test01', 'mypass', 't01@na.com', 'Joe',  'Blow', 1);
    INSERT INTO users VALUES (2, 'test02', 'mypass', 't02@na.com', 'Jane', 'Doe',  1);
    INSERT INTO users VALUES (3, 'test03', 'mypass', 't03@na.com', 'No',   'Go',   0);
    INSERT INTO roles VALUES (1, 'user');
    INSERT INTO roles VALUES (2, 'admin');
    INSERT INTO user_roles VALUES (1, 1);
    INSERT INTO user_roles VALUES (1, 2);
    INSERT INTO user_roles VALUES (2, 1);
    INSERT INTO user_roles VALUES (3, 1);

Then load this into the C<myapp.db> database with the following command:

    $ sqlite3 myapp.db < myapp02.sql


=head2 Add User and Role Information to DBIC Schema

This step adds DBIC-based classes for the user-related database tables
(the role information will not be used until Part 5):

Edit C<lib/MyAppDB.pm> and update the contents to match (only the
C<MyAppDB =E<gt> [qw/Book BookAuthor Author User UserRole Role/]> line
has changed):

    package MyAppDB;
    
    =head1 NAME 
    
    MyAppDB -- DBIC Schema Class
    
    =cut
    
    # Our schema needs to inherit from 'DBIx::Class::Schema'
    use base qw/DBIx::Class::Schema/;
    
    # Need to load the DB Model classes here.
    # You can use this syntax if you want:
    #    __PACKAGE__->load_classes(qw/Book BookAuthor Author User UserRole Role/);
    # Also, if you simply want to load all of the classes in a directory
    # of the same name as your schema class (as we do here) you can use:
    #    __PACKAGE__->load_classes(qw//);
    # But the variation below is more flexible in that it can be used to 
    # load from multiple namespaces.
    __PACKAGE__->load_classes({
        MyAppDB => [qw/Book BookAuthor Author User UserRole Role/]
    });
    
    1;


=head2 Create New "Result Source Objects"

Create the following three files with the content shown below.

C<lib/MyAppDB/User.pm>:

    package MyAppDB::User;
    
    use base qw/DBIx::Class/;
    
    # Load required DBIC stuff
    __PACKAGE__->load_components(qw/PK::Auto Core/);
    # Set the table name
    __PACKAGE__->table('users');
    # Set columns in table
    __PACKAGE__->add_columns(qw/id username password email_address first_name last_name/);
    # Set the primary key for the table
    __PACKAGE__->set_primary_key('id');
    
    #
    # Set relationships:
    #
    
    # has_many():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *foreign* table
    __PACKAGE__->has_many(map_user_role => 'MyAppDB::UserRole', 'user_id');
    
    
    =head1 NAME
    
    MyAppDB::User - A model object representing a person with access to the system.
    
    =head1 DESCRIPTION
    
    This is an object that represents a row in the 'users' table of your application
    database.  It uses DBIx::Class (aka, DBIC) to do ORM.
    
    For Catalyst, this is designed to be used through MyApp::Model::MyAppDB.
    Offline utilities may wish to use this class directly.
    
    =cut
    
    1;


C<lib/MyAppDB/Role.pm>:

    package MyAppDB::Role;
    
    use base qw/DBIx::Class/;
    
    # Load required DBIC stuff
    __PACKAGE__->load_components(qw/PK::Auto Core/);
    # Set the table name
    __PACKAGE__->table('roles');
    # Set columns in table
    __PACKAGE__->add_columns(qw/id role/);
    # Set the primary key for the table
    __PACKAGE__->set_primary_key('id');
    
    #
    # Set relationships:
    #
    
    # has_many():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *foreign* table
    __PACKAGE__->has_many(map_user_role => 'MyAppDB::UserRole', 'role_id');
    
    
    =head1 NAME
    
    MyAppDB::Role - A model object representing a class of access permissions to 
    the system.
    
    =head1 DESCRIPTION
    
    This is an object that represents a row in the 'roles' table of your 
    application database.  It uses DBIx::Class (aka, DBIC) to do ORM.
    
    For Catalyst, this is designed to be used through MyApp::Model::MyAppDB.
    "Offline" utilities may wish to use this class directly.
    
    =cut
    
    1;


C<lib/MyAppDB/UserRole.pm>:

    package MyAppDB::UserRole;
    
    use base qw/DBIx::Class/;
    
    # Load required DBIC stuff
    __PACKAGE__->load_components(qw/PK::Auto Core/);
    # Set the table name
    __PACKAGE__->table('user_roles');
    # Set columns in table
    __PACKAGE__->add_columns(qw/user_id role_id/);
    # Set the primary key for the table
    __PACKAGE__->set_primary_key(qw/user_id role_id/);
    
    #
    # Set relationships:
    #
    
    # belongs_to():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *this* table
    __PACKAGE__->belongs_to(user => 'MyAppDB::User', 'user_id');
    
    # belongs_to():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *this* table
    __PACKAGE__->belongs_to(role => 'MyAppDB::Role', 'role_id');
    
    
    =head1 NAME
    
    MyAppDB::UserRole - A model object representing the JOIN between Users and Roles.
    
    =head1 DESCRIPTION
    
    This is an object that represents a row in the 'user_roles' table of your application
    database.  It uses DBIx::Class (aka, DBIC) to do ORM.
    
    You probably won't need to use this class directly -- it will be automatically
    used by DBIC where joins are needed.
    
    For Catalyst, this is designed to be used through MyApp::Model::MyAppDB.
    Offline utilities may wish to use this class directly.
    
    =cut
    
    1;

The code for these three result source classes is obviously very familiar to the C<Book>, C<Author>, and C<BookAuthor> classes created in Part 2.


=head2 Sanity-Check Reload of Development Server

We aren't ready to try out the authentication just yet; we only want to do a quick check to be sure our model loads correctly.  Press C<Ctrl-C> to kill the previous server instance (if it's still running) and restart it:

    $ script/myapp_server.pl

Look for the three new model objects in the startup debug output:

    ...
     .-------------------------------------------------------------------+----------.
    | Class                                                             | Type     |
    +-------------------------------------------------------------------+----------+
    | MyApp::Controller::Books                                          | instance |
    | MyApp::Controller::Root                                           | instance |
    | MyApp::Model::MyAppDB                                             | instance |
    | MyApp::Model::MyAppDB::Author                                     | class    |
    | MyApp::Model::MyAppDB::Book                                       | class    |
    | MyApp::Model::MyAppDB::BookAuthor                                 | class    |
    | MyApp::Model::MyAppDB::Role                                       | class    |
    | MyApp::Model::MyAppDB::User                                       | class    |
    | MyApp::Model::MyAppDB::UserRole                                   | class    |
    | MyApp::View::TT                                                   | instance |
    '-------------------------------------------------------------------+----------'
    ...

Again, notice that your "result source" classes have been "re-loaded" by Catalyst under C<MyApp::Model>.


=head2 Include Authentication and Session Plugins

Edit C<lib/MyApp.pm> and update it as follows (everything below C<StackTrace> is new):

    use Catalyst qw/
            -Debug
            ConfigLoader
            Static::Simple
            
            StackTrace
            
            Authentication
            Authentication::Store::DBIC
            Authentication::Credential::Password
            
            Session
            Session::Store::FastMmap
            Session::State::Cookie
            /;

The three C<Authentication> plugins work together to support
Authentication while the C<Session> plugins are required to maintain
state across multiple HTTP requests.  Note that there are several
options for L<Session::Store|Catalyst::Plugin::Session::Store> 
(L<Session::Store::FastMmap|Catalyst::Plugin::Session::Store::FastMmap>
is generally a good choice if you are on Unix; try
L<Cache::FileCache|Catalyst::Plugin::Cache::FileCache> if you are on
Win32) -- consult L<Session::Store|Catalyst::Plugin::Session::Store> and
its subclasses for additional information.


=head2 Configure Authentication

Although C<__PACKAGE__-E<gt>config(name =E<gt> 'value');> is still
supported, newer Catalyst applications tend to place all configuration
information in C<myapp.yml> and automatically load this information into
C<MyApp-E<gt>config> using the 
L<ConfigLoader|Catalyst::Plugin::ConfigLoader> plugin.  Here, we need
to load several parameters that tell 
L<Catalyst::Plugin::Authentication|Catalyst::Plugin::Authentication>
where to locate information in your database.  To do this, edit the 
C<myapp.yml> YAML and update it to match:

    ---
    name: MyApp
    authentication:
        dbic:
            # Note this first definition would be the same as setting
            # __PACKAGE__->config->{authentication}->{dbic}->{user_class} = 'MyAppDB::User'
            # in lib/MyApp.pm (IOW, each hash key becomes a "name:" in the YAML file).
            #
            # This is the model object created by Catalyst::Model::DBIC from your
            # schema (you created 'MyAppDB::User' but as the Catalyst startup
            # debug messages show, it was loaded as 'MyApp::Model::MyAppDB::User').
            # NOTE: Omit 'MyApp::Model' to avoid a component lookup issue in Catalyst 5.66
            user_class: MyAppDB::User
            # This is the name of the field in your 'users' table that contains the user's name
            user_field: username
            # This is the name of the field in your 'users' table that contains the password
            password_field: password
            # Other options can go here for hashed passwords

Inline comments in the code above explain how each field is being used.

B<TIP>: Although YAML uses a very simple and easy-to-ready format, it
does require the use of a consistent level of indenting.  Be sure you
line up everything on a given 'level' with the same number of indents.
Also, be sure not to use C<tab> characters (YAML does not support them
because they are handled inconsistently across editors).


=head2 Add Login and Logout Controllers

Use the Catalyst create script to create two stub controller files:

    $ script/myapp_create.pl controller Login
    $ script/myapp_create.pl controller Logout

B<NOTE>: You could easily use a single controller here.  For example,
you could have a C<User> controller with both C<login> and C<logout>
actions.  Remember, Catalyst is designed to be very flexible, and leaves
such matters up to you, the designer and programmer.

Then open C<lib/MyApp/Controller/Login.pm> and add:

    =head2 base
    
    Login logic
    
    =cut
    
    sub base :Path :Args(0) {
        my ($self, $c) = @_;
    
        # Get the username and password from form
        my $username = $c->request->params->{username} || "";
        my $password = $c->request->params->{password} || "";
    
        # If the username and password values were found in form
        if ($username && $password) {
            # Attempt to log the user in
            if ($c->login($username, $password)) {
                # If successful, then let them use the application
                $c->response->redirect($c->uri_for('/books/list'));
                return;
            } else {
                # Set an error message
                $c->stash->{error_msg} = "Bad username or password.";
            }
        }
    
        # If either of above don't work out, send to the login page
        $c->stash->{template} = 'login.tt2';
    }

This controller fetches the C<username> and C<password> values from the
login form and attempts to perform a login.  If successful, it redirects
the user to the book list page.  If the login fails, the user will stay
at the login page but receive an error message.  If the C<username> and
C<password> values are not present in the form, the user will be taken
to the empty login form.

We are using C<sub base :Path :Args(0) {...}> here to specifically match 
the URL C</login>.  C<Path> actions (aka, "literal actions") create URI 
matches relative to the namespace of the controller where they are defined.  
Although C<Path> supports arguments that allow relative and absolute paths 
to be defined, here we use an empty C<Path> definition to match on just the 
name of the controller itself.  The method name, C<base>, is arbitrary.  
We make the match even more specific with the C<:Args(0)> action modifier 
-- this forces the match on I<only> C</login>, not C</login/somethingelse>.

Note that we could have used something like C<sub default :Private>; 
however, the use of C<default> actions is discouraged because it does
not receive path args as with other actions.  The recommended practice 
is to only use C<default> in C<MyApp::Controller::Root>.

Next, create a corresponding method in C<lib/MyApp/Controller/Logout.pm>:

    =head2 base
    
    Logout logic
    
    =cut
    
    sub base :Path :Args(0) {
        my ($self, $c) = @_;
    
        # Clear the user's state
        $c->logout;
    
        # Send the user to the starting point
        $c->response->redirect($c->uri_for('/'));
    }

Note that we are using the same C<sub base :Path :Args(0) {...}> style
of action as with the login logic.


=head2 Add a Login Form TT Template Page

Create a login form by opening C<root/src/login.tt2> and inserting:

    [% META title = 'Login' %]
    
    <!-- Login form -->
    <form method="post" action=" [% Catalyst.uri_for('/login') %] ">
      <table>
        <tr>
          <td>Username:</td>
          <td><input type="text" name="username" size="40" /></td>
        </tr>
        <tr>
          <td>Password:</td>
          <td><input type="password" name="password" size="40" /></td>
        </tr>
        <tr>
          <td colspan="2"><input type="submit" name="submit" value="Submit" /></td>
        </tr>
      </table>
    </form>


=head2 Add Valid User Check

We need something that provides enforcement for the authentication
mechanism -- a I<global> mechanism that prevents users who have not
passed authentication from reaching any pages except the login page.
This is generally done via an C<auto> action/method (prior to Catalyst
v5.66, this sort of thing would go in C<MyApp.pm>, but starting in
v5.66, the preferred location is C<lib/MyApp/Controller/Root.pm>).

Edit the existing C<lib/MyApp/Controller/Root.pm> class file and insert
the following method:

    =head2 auto
    
    Check if there is a user and, if not, forward to login page
    
    =cut
    
    # Note that 'auto' runs after 'begin' but before your actions and that
    # 'auto' "chain" (all from application path to most specific class are run)
    sub auto : Private {
        my ($self, $c) = @_;
    
        # Allow unauthenticated users to reach the login page
        if ($c->request->path =~ /login/) {
            return 1;
        }
    
        # If a user doesn't exist, force login
        if (!$c->user_exists) {
            # Dump a log message to the development server debug output
            $c->log->debug('***Root::auto User not found, forwarding to /login');
            # Redirect the user to the login page
            $c->response->redirect($c->uri_for('/login'));
            # Return 0 to cancel 'post-auto' processing and prevent use of application
            return 0;
        }
    
        # User found, so return 1 to continue with processing after this 'auto'
        return 1;
    }

B<Note:> Catalyst provides a number of different types of actions, such
as C<Local>, C<Regex>, and C<Private>.  You should refer to
L<Catalyst::Manual::Intro> for a more detailed explanation, but the
following bullet points provide a quick introduction:

=over 4

=item *

The majority of application use C<Local> actions for items that respond
to user requests and C<Private> actions for those that do not directly
respond to user input.

=item *

There are five types of C<Private> actions: C<begin>, C<end>,
C<default>, C<index>, and C<auto>.

=item *

Unlike the other actions where only a single method is called for each 
request, I<every> auto action along the chain of namespaces will be 
called.

=back

By placing the authentication enforcement code inside the C<auto> method
of C<lib/MyApp/Controller/Root.pm> (or C<lib/MyApp.pm>), it will be
called for I<every> request that is received by the entire application.


=head2 Displaying Content Only to Authenticated Users

Let's say you want to provide some information on the login page that
changes depending on whether the user has authenticated yet.  To do
this, open C<root/src/login.tt2> in your editor and add the following
lines to the bottom of the file:

    <p>
    [%
       # This code illustrates how certain parts of the TT 
       # template will only be shown to users who have logged in
    %]
    [% IF Catalyst.user %]
        Please Note: You are already logged in as '[% Catalyst.user.username %]'.
        You can <a href="[% Catalyst.uri_for('/logout') %]">logout</a> here.
    [% ELSE %]
        You need to log in to use this application.
    [% END %]
    [%#
       Note that this whole block is a comment because the "#" appears
       immediate after the "[%" (with no spaces in between).  Although it 
       can be a handy way to temporarily "comment out" a whole block of 
       TT code, it's probably a little too subtle for use in "normal" 
       comments.
    %]

Although most of the code is comments, the middle few lines provide a
"you are already logged in" reminder if the user returns to the login
page after they have already authenticated.  For users who have not yet
authenticated, a "You need to log in..." message is displayed (note the
use of an IF-THEN-ELSE construct in TT).


=head2 Try Out Authentication

Press C<Ctrl-C> to kill the previous server instance (if it's still
running) and restart it:

    $ script/myapp_server.pl

B<IMPORTANT NOTE>: If you happen to be using Internet Explorer, you may
need to use the command C<script/myapp_server.pl -k> to enable the
keepalive feature in the development server.  Otherwise, the HTTP
redirect on successful login may not work correctly with IE (it seems to
work without -k if you are running the web browser and development
server on the same machine).  If you are using browser a browser other
than IE, it should work either way.  If you want to make keepalive the
default, you can edit C<script/myapp_server.pl> and change the
initialization value for C<$keepalive> to C<1>.  (You will need to do
this every time you create a new Catalyst application or rebuild the
C<myapp_server.pl> script.)

Now trying going to L<http://localhost:3000/books/list> and you should
be redirected to the login page, hitting Shift+Reload if necessary (the
"You are already logged in" message should I<not> appear -- if it does,
click the C<logout> button and try again). Note the C<***Root::auto User
not found...> debug message in the development server output.  Enter
username C<test01> and password C<mypass>, and you should be taken to
the Book List page.

Open C<root/src/books/list.tt2> and add the following lines to the
bottom:

    <p>
      <a href="[% Catalyst.uri_for('/login') %]">Login</a>
      <a href="[% Catalyst.uri_for('form_create') %]">Create</a>
    </p>

Reload your browser and you should now see a "Login" and "Create" links
at the bottom of the page (as mentioned earlier, you can update 
template files without reloading the development server).  Click this 
link to return to the login page.  This time you I<should> see the 
"You are already logged in" message.

Finally, click the C<You can logout here> link on the C</login> page.
You should stay at the login page, but the message should change to "You
need to log in to use this application."


=head1 USING PASSWORD HASHES

In this section we increase the security of our system by converting
from cleartext passwords to SHA-1 password hashes.

B<Note:> This section is optional.  You can skip it and the rest of the
tutorial will function normally.

Note that even with the techniques shown in this section, the browser
still transmits the passwords in cleartext to your application.  We are
just avoiding the I<storage> of cleartext passwords in the database by
using a SHA-1 hash. If you are concerned about cleartext passwords
between the browser and your application, consider using SSL/TLS, made
easy with the Catalyst plugin 
L<Catalyst::Plugin:RequireSSL|Catalyst::Plugin:RequireSSL>.


=head2 Get a SHA-1 Hash for the Password

Catalyst uses the C<Digest> module to support a variety of hashing
algorithms.  Here we will use SHA-1 (SHA = Secure Hash Algorithm).
First, we should compute the SHA-1 hash for the "mypass" password we are
using.  The following command-line Perl script provides a "quick and
dirty" way to do this:

    $ perl -MDigest::SHA -e 'print Digest::SHA::sha1_hex("mypass"), "\n"'
    e727d1464ae12436e899a726da5b2f11d8381b26
    $

B<Note:> You should probably modify this code for production use to
not read the password from the command line.  By having the script
prompt for the cleartext password, it avoids having the password linger
in forms such as your C<.bash_history> files (assuming you are using
BASH as your shell).  An example of such a script can be found in
Appendix 3.


=head2 Switch to SHA-1 Password Hashes in the Database

Next, we need to change the C<password> column of our C<users> table to
store this hash value vs. the existing cleartext password.  Open
C<myapp03.sql> in your editor and enter:

    --
    -- Convert passwords to SHA-1 hashes
    --
    UPDATE users SET password = 'e727d1464ae12436e899a726da5b2f11d8381b26' WHERE id = 1;
    UPDATE users SET password = 'e727d1464ae12436e899a726da5b2f11d8381b26' WHERE id = 2;
    UPDATE users SET password = 'e727d1464ae12436e899a726da5b2f11d8381b26' WHERE id = 3;

Then use the following command to update the SQLite database:

    $ sqlite3 myapp.db < myapp03.sql

B<Note:> We are using SHA-1 hashes here, but many other hashing
algorithms are supported.  See C<Digest> for more information.


=head2 Enable SHA-1 Hash Passwords in
C<Catalyst::Plugin::Authentication::Store::DBIC>

Edit C<myapp.yml> and update it to match (the C<password_type> and
C<password_hash_type> are new, everything else is the same):

    ---
    name: MyApp
    authentication:
        dbic:
            # Note this first definition would be the same as setting
            # __PACKAGE__->config->{authentication}->{dbic}->{user_class} = 'MyAppDB::User'
            # in lib/MyApp.pm (IOW, each hash key becomes a "name:" in the YAML file).
            #
            # This is the model object created by Catalyst::Model::DBIC from your
            # schema (you created 'MyAppDB::User' but as the Catalyst startup
            # debug messages show, it was loaded as 'MyApp::Model::MyAppDB::User').
            # NOTE: Omit 'MyApp::Model' to avoid a component lookup issue in Catalyst 5.66
            user_class: MyAppDB::User
            # This is the name of the field in your 'users' table that contains the user's name
            user_field: username
            # This is the name of the field in your 'users' table that contains the password
            password_field: password
            # Other options can go here for hashed passwords
            # Enabled hashed passwords
            password_type: hashed
            # Use the SHA-1 hashing algorithm
            password_hash_type: SHA-1


=head2 Try Out the Hashed Passwords

Press C<Ctrl-C> to kill the previous server instance (if it's still
running) and restart it:

    $ script/myapp_server.pl

You should now be able to go to L<http://localhost:3000/books/list> and
login as before.  When done, click the "Logout" link on the login page
(or point your browser at L<http://localhost:3000/logout>).


=head1 AUTHOR

Kennedy Clark, C<hkclark@gmail.com>

Please report any errors, issues or suggestions to the author.  The
most recent version of the Catlayst Tutorial can be found at
L<http://dev.catalyst.perl.org/repos/Catalyst/trunk/Catalyst-Runtime/lib/Catalyst/Manual/Tutorial/>.

Copyright 2006, Kennedy Clark, under Creative Commons License
(L<http://creativecommons.org/licenses/by-nc-sa/2.5/>).