DOKK Library

GNU LibreJS for version 7.21

Authors Loic J. Duros Yuchen Pei

License GFDL-1.3-or-later

                                     for version 7.21, 21 July 2022

Loic J. Duros (
Yuchen Pei (
This manual is for GNU LibreJS (version 7.21, 21 July 2022), a GNU IceCat extension to
detect and block nonfree nontrivial JavaScript on webpages.
Copyright c 2011 2012 2014 2015 Loic J. Duros, 2022 Yuchen Pei
      Permission is granted to copy, distribute and/or modify this document under
      the terms of the GNU Free Documentation License, Version 1.3 or any later
      version published by the Free Software Foundation; with no Invariant Sections,
      with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license
      is included in the section entitled “GNU Free Documentation License”.

Table of Contents

1    Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2    Disclaimer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3    Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4    How to Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
    4.1   LibreJS in action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
    4.2   Script Blacklist/whitelist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
    4.3   Complaint Feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
    4.4   Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

5    JavaScript Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

6    Free Licenses Detection . . . . . . . . . . . . . . . . . . . . . . . . . 7

7    Setting Your JavaScript Free . . . . . . . . . . . . . . . . . . . 8
    7.1   License tags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
    7.2   Undetected Free Licenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
    7.3   Known limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8    LibreJS Development Notes . . . . . . . . . . . . . . . . . . . 15
    8.1   Dependencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    8.2   Building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    8.3   Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    8.4   Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    8.5   Headless testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    8.6   Headless compliance check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
    8.7   Adding new whitelisted libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
    8.8   Releasing a new version. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Appendix A                       Installation Requirements . . . . . . . . . 17
    A.1    Mozilla Browser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Appendix B                       LibreJS Internals. . . . . . . . . . . . . . . . . . . 18

Appendix C                       GNU Free Documentation License . . 19

1 Overview
GNU LibreJS —an add-on for GNU IceCat and Mozilla Firefox— detects and blocks nonfree
nontrivial JavaScript while allowing its execution on pages containing code that is either
trivial and/or free.
    Many websites run nontrivial JavaScript on your computer. Some use it for complex
tasks; many use it gratuitously for minor jobs that could be done easily with plain HTML.
Sometimes this JavaScript code is malicious. Either way, the JavaScript code is often
nonfree. For explanation of the issue, see "The JavaScript Trap"(
    If you care about freedom in your computing, and don’t wish to let all and sundry make
you run nonfree programs, now you can prevent it by using LibreJS.

2 Disclaimer
• LibreJS is not a security tool. Its goal is to detect nonfree nontrivial JavaScript, and
  it currently does not detect whether free or trivial code is malicious or not. Other free
  Mozilla extensions and add-ons may be available for this purpose.
• LibreJS is always a work in progress. If you find a bug, please report it to

3 Installation
You can install LibreJS directly using a generated librejs.xpi file, or by building it from
source (See Chapter 8 [LibreJS Development Notes], page 15).
    You can also download it from or https://, but due to Mozilla’s review process the download
isn’t always up to date.

4 How to Use

4.1 LibreJS in action
After installing the add-on, you will see the LibreJS widget in the add-on bar at the top
right of the browser window. After loading a page, left-click on the widget to view the
deactivated JavaScript code from the page (both on page and external) and, if applicable,
the scripts that were accepted.

4.2 Script Blacklist/whitelist
Scripts may be blacklisted or whitelisted through the pop-up menu box.
   Whitelisted scripts will always be accepted and blacklisted scripts will always get re-
   It is important to note that this feature recognizes which scripts are blacklisted and
whitelisted based on hash. This means that even a slight difference in a script’s code will
cause it to be recognized as a separate script.
   Sometimes, JavaScript will be dynamically generated so that it is different every time
a website is loaded. These types of scripts cannot be whitelisted or blacklisted since they
cannot be recognized.
   LibreJS has a default whitelist of scripts that are known to be free but may not declare
their license in a format that LibreJS can understand.

4.3 Complaint Feature
It is very important to complain when a site has nonfree JavaScript code, especially if it
won’t work without that code. LibreJS makes it easy to complain by heuristically finding
where to send the complaint.
    When nonfree/nontrivial code is detected in a page, LibreJS attempts to find a relevant
contact link or email for the website you are visiting. In order to do so, it will attempt to
visit a few links from the current page (for instance, a link labeled “contact” on the same
domain as the current page, . . . )
   LibreJS detects contact pages, email addresses that are likely to be owned by the main-
tainer of the site, Twitter and links, and phone numbers.
   When you complain to the website for their nonfree nontrivial JavaScript, provide them
with the link to the JavaScript Trap essay so that they can get more information on what
the issue is and how they can solve it on their own site.
   LibreJS includes a default subject line and body for the complaint email, with a link
to the JavaScript Trap essay. This can be configured in the LibreJS add-on preferences in
your web browser.

4.4 Options
            You can manage LibreJS’s preferences either from the extension’s entry in your
            browser’s Add-ons Manager page (about:addons) or by clicking the LibreJS
Chapter 4: How to Use                                                                   5

            toolbar icon and then the "Settings..." button on the top right of the popup.
            This will open a panel containing a whitelist/blacklist manager and a section
            to configure your complaints messages to site owners.
           LibreJS lets you whitelist or blacklist domain names and subdomains, to bypass
           the regular JavaScript checks. This might be useful, for example, if you are
           running your own code in a local web server, or if you don’t want to waste
           computing resources on script origins you already know you can’t trust. librejs
           provides a lists manager UI to handle both the lists on the top of its Options
Complaint email subject
           Configure the default subject used in complaint emails.
Complaint email body
           Configure the default body used in complaint emails.

5 JavaScript Detection
LibreJS considers a very strict subset of JavaScript to be acceptable for use in non-free
scripts. This is meant to maximimize compatibility with websites that haven’t tried to be
LibreJS compatible.
    We consider modification of the document non-trivial. There isn’t much that javascript
could do that we would consider trivial, for anything else a free software license would be
    The criterion is as follows:
    For each function definition:
  • It must call only primitives.
  • The number of conditionals and loops must be at most 3.
  • It does not declare an array more than 50 elements long.
  • It must not call itself
  For the rest of the script, outside of function definitions:
 • It must call only primitives and functions defined above in the page.
 • The number of conditionals and loops must be at most 3.
  "function" means anything executable that gets a name, including methods.
  Allowed primitives exclude:
 • eval()
 • ajax
 • calling methods with the square bracket notation
 • altering the dom
 • most other items found as methods of the ‘.window‘ object.

6 Free Licenses Detection
The machine readable format for license declarations that LibreJS uses has changed in the
most recent version. This was necessary in order to not break the asynchronous JS loading
model that browsers use. Scripts are now evaluated independent of eachother and strictly
as they arrive.

7 Setting Your JavaScript Free
The first step is releasing your JavaScript under a free license. If you are already using a
free library, or you’re not using any third-party libraries, it might only take a few minutes.
    On your website, take a look at your HTML source. You can identify distinct pieces of
JavaScript that might be free and some other that are nonfree.
    This might be the case with an analytics tracker, social media widgets, and code that
runs ads. Removing these pieces of code from your site is required to have the rest accepted
as free. There are often alternatives to nonfree libraries or to third-party services:
  • If you have used nonfree third-party code as the base to write your own code, try to
     find a free alternative.
  • If you’re using a third-party service such as an analytics service, replace it with a free
     alternative like Matomo.
  • If you can’t find free JavaScript that has already been developed, write it yourself!
     Who knows, your own solution might be the start of a brilliant project!

7.1 License tags
LibreJS will allow non-trivial scripts to run as long as they use a free license.
   In order for the license of a script to be recognized by LibreJS, it must be declared using
a machine-readable license format.
   This format is the same for both remote in-line scripts.
   "// @license [magnet link] [identifier]" [Script here] "// @license-end"
   "Identifier" is a name of a license from the following list and the magnet link in the
canonicalUrl field is that license’s exact corresponding magnet link.
  ’AGPL-3.0’: {
     licenseName: ’GNU AFFERO GENERAL PUBLIC LICENSE version 3’,
     identifier: ’AGPL-3.0’,
     canonicalUrl: [


  ’Apache-2.0’: {
     licenseName: ’Apache License, Version 2.0’,
     identifier: ’Apache-2.0’,
     canonicalUrl: [
Chapter 7: Setting Your JavaScript Free                                    9

  ’Artistic-2.0’: {
     licenseName: "Artistic License 2.0",
     identifier: ’Artistic-2.0’,
     canonicalUrl: [

  ’BSD-2-Clause’: {
     licenseName: "BSD 2-Clause License",
     identifier: ’BSD-2-Clause’,
     canonicalUrl: [

  ’BSD-3-Clause’: {
     licenseName: "BSD 3-Clause License",
     identifier: ’BSD-3-Clause’,
     canonicalUrl: [

  ’BSL-1.0’: {
     licenseName: ’Boost Software License 1.0’,
     identifier: ’BSL-1.0’,
     canonicalUrl: [

  ’CC-BY-1.0’: {
     licenseName: ’Creative Commons Attribution 1.0 Generic’,
     identifier: ’CC-BY-1.0’,
     canonicalUrl: [’’],

  ’CC-BY-2.0’: {
     licenseName: ’Creative Commons Attribution 2.0 Generic’,
     identifier: ’CC-BY-2.0’,
     canonicalUrl: [’’],
Chapter 7: Setting Your JavaScript Free                                     10

  ’CC-BY-2.5’: {
     licenseName: ’Creative Commons Attribution 2.5 Generic’,
     identifier: ’CC-BY-2.5’,
     canonicalUrl: [’’],

  ’CC-BY-3.0’: {
     licenseName: ’Creative Commons Attribution 3.0 Unported’,
     identifier: ’CC-BY-3.0’,
     canonicalUrl: [’’],

  ’CC-BY-4.0’: {
     licenseName: ’Creative Commons Attribution 4.0 International’,
     identifier: ’CC-BY-4.0’,
     canonicalUrl: [’’],

  ’CC-BY-SA-1.0’: {
     licenseName: ’Creative Commons Attribution-ShareAlike 1.0 Generic’,
     identifier: ’CC-BY-SA-1.0’,
     canonicalUrl: [’’],

  ’CC-BY-SA-2.0’: {
     licenseName: ’Creative Commons Attribution-ShareAlike 2.0 Generic’,
     identifier: ’CC-BY-SA-2.0’,
     canonicalUrl: [’’],

  ’CC-BY-SA-2.5’: {
     licenseName: ’Creative Commons Attribution-ShareAlike 2.5 Generic’,
     identifier: ’CC-BY-SA-2.5’,
     canonicalUrl: [’’],

  ’CC-BY-SA-3.0’: {
     licenseName: ’Creative Commons Attribution-ShareAlike 3.0 Unported’,
     identifier: ’CC-BY-SA-3.0’,
     canonicalUrl: [’’],

  ’CC-BY-SA-4.0’: {
    licenseName: ’Creative Commons Attribution-ShareAlike 4.0 International’,
    identifier: ’CC-BY-SA-4.0’,
    canonicalUrl: [’’],
Chapter 7: Setting Your JavaScript Free                                   11


  ’CC0-1.0’: {
     licenseName: ’Creative Commons CC0 1.0 Universal’,
     identifier: ’CC0-1.0’,
     canonicalUrl: [

  ’CPAL-1.0’: {
     licenseName: ’Common Public Attribution License Version 1.0 (CPAL)’,
     identifier: ’CPAL-1.0’,
     canonicalUrl: [

  ’EPL-1.0’: {
     licenseName: "Eclipse Public License Version 1.0",
     identifier: "EPL-1.0",
     canonicalUrl: [

  ’Expat’: {
     licenseName: ’Expat License (sometimes called MIT Licensed)’,
     identifier: ’Expat’,
     canonicalUrl: [

  ’FreeBSD’: {
     licenseName: "FreeBSD License",
     identifier: ’FreeBSD’,
     canonicalUrl: [
Chapter 7: Setting Your JavaScript Free                                   12

  ’GNU-All-Permissive’: {
     licenseName: ’GNU All-Permissive License’,
     identifier: ’GNU-All-Permissive’,
     canonicalUrl: [],

  ’GPL-2.0’: {
     licenseName: ’GNU General Public License (GPL) version 2’,
     identifier: ’GPL-2.0’,
     canonicalUrl: [

  ’GPL-3.0’: {
     licenseName: ’GNU General Public License (GPL) version 3’,
     identifier: ’GPL-3.0’,
     canonicalUrl: [

  ’ISC’: {
     licenseName: "The ISC License",
     identifier: ’ISC’,
     canonicalUrl: [

  ’jQueryTools’: {
     licenseName: "jQuery Tools",
     identifier: ’jQueryTools’,
     canonicalUrl: [],

  ’LGPL-2.1’: {
     licenseName: ’GNU Lesser General Public License, version 2.1’,
     identifier: ’LGPL-2.1’,
     canonicalUrl: [
Chapter 7: Setting Your JavaScript Free                                   13

  ’LGPL-3.0’: {
     licenseName: ’GNU Lesser General Public License, version 3’,
     identifier: ’LGPL-3.0’,
     canonicalUrl: [

  ’MPL-2.0’: {
     licenseName: ’Mozilla Public License Version 2.0’,
     identifier: ’MPL-2.0’,
     canonicalUrl: [

  ’PublicDomain’: {
     licenseName: ’Public Domain’,
     identifier: ’PublicDomain’,
     canonicalUrl: [

  ’Unlicense’: {
     licenseName: ’Unlicense’,
     identifier: ’Unlicense’,
     canonicalUrl: [

  ’UPL’: {
     licenseName: ’Universal Permissive License’,
     identifier: ’UPL-1.0’,
     canonicalUrl: [

  ’WTFPL’: {
Chapter 7: Setting Your JavaScript Free                                                     14

         licenseName: ’Do What The F*ck You Want To Public License (WTFPL)’,
         identifier: ’WTFPL’,
         canonicalUrl: [

    ’X11’: {
       licenseName: ’X11 License’,
       identifier: ’X11’,
       canonicalUrl: [

    ’XFree86-1.1’: {
       licenseName: "XFree86 1.1 License",
       identifier: ’XFree86-1.1’,
       canonicalUrl: [

7.2 Undetected Free Licenses
If you are using a free license that isn’t detected by LibreJS and isn’t listed in the previous
section, please send a message to regarding this license, where code
released under this license can be found, and where to find the license text and information.
    Many free licenses are listed in this page:   

7.3 Known limitations
Service workers may cause false positives and false negatives, and there are multiple ways
to disable them. The cleanest way is by setting dom.serviceWorkers.enabled to false in
the about:config page of your browser.

8 LibreJS Development Notes

8.1 Dependencies
LibreJS 7.21 depends on a number of Node.js-based libraries that can be installed using the
npm utility:
  $ npm install acorn-loose jassha browserify
  $ export PATH=$PATH:./node_modules/.bin

8.2 Building
To build the extension run:
  $ browserify main_background.js -o bundle.js
   To build the extension plus create a .xpi package run:
  $ ./
   To build the extension including the automated test suite (see TEST below) run:
  $ ./ -t
  $ ./ --test
   Note: this script relies on no new source files being created.

8.3 Debugging
To debug LibreJS, visit the special URL about:debugging. Click on ‘Enable add-on de-
bugging‘ then ‘Load Temporary Add-on‘. Navigate to LibreJS’s unpacked source directory
and select manifest.json.
   Lines 39 and 40 in main_background.js assign two variables controlling the verbosity
of dbg_print() statements. Make sure these are set to false before building a release.

8.4 Testing
An automated test suite runs automatically in its own tab whenever the extension is loaded
as a "Temporary add-on" from about:debugging. Otherwise (if included in the xpi) it can
be launched from the UI by clicking the "Automated self test..." button.

8.5 Headless testing
To launch the test suite from the command line, ensure the xpi package has been built
with automated test suite. Then install selenium-webdriver and geckodriver, and ensure
the latter is in $PATH:
  $ npm install selenium-webdriver geckodriver
  $ export PATH=$PATH:./node_modules/.bin
   Now you can invoke the test with
  $ node ./test.js
   which will print out a summary of test results.
Chapter 8: LibreJS Development Notes                                                    16

   Optionally you can also test with a chosen seed
  $ node ./test.js 12345

8.6 Headless compliance check
To check whether a webpage is LibreJS-compliant from the command line, ensure that the
xpi package has been built WITHOUT the automated test suite. Then install selenium-
webdriver and geckodriver, and ensure the latter is in $PATH:
  $ npm install selenium-webdriver
  $ npm install geckodriver
  $ export PATH=$PATH:./node_modules/.bin
   Now you can check a webpage for compliance with
  $ node ./compliance.js <url>
   It will open the url in a headless browser, save a screenshot, and output the compliance
check result.
   For example, to check the compliance of the FSF homepage, do
  $ node ./compliance.js

8.7 Adding new whitelisted libraries
The script index.js in ./hash_script generates the default whitelist. Run it with the
following command:
    node index.js > output
    Then, just copy the contents of the file "output" to the appropriate place in
main background.js.

8.8 Releasing a new version
Update the version number in manifest.json.
  Make sure debug statements are set to false on lines 39/40 in main_background.js.
  Update the version number in docs/version.texi
  Then run the build script

Appendix A Installation Requirements

A.1 Mozilla Browser
You will need one of the many flavors of the Mozilla browser to use LibreJS. It can be
installed on the following:
   GNU IceCat, Mozilla Firefox, Trisquel Abrowser, Debian Iceweasel.
   LibreJS works on these browsers starting from version 60. We recommend that you
use the latest version of your Mozilla browser. LibreJS has been tested on a GNU/Linux
distribution, but it is compatible any operating system as long as you’re using a compatible
Mozilla browser.

Appendix B LibreJS Internals
LibreJS intercepts HTTP responses and rewrites their contents after analyzing JavaScript
within them. It does not remove script nodes and attributes from the page, but instead
“deactivates” them by replacing their content with a commented notice.
   LibreJS detects the most common cases using the HTTP response method described
above, but in less common edge cases, or when running code locally, LibreJS cannot detect
JavaScript during the response stage.

Appendix C GNU Free Documentation License
                          Version 1.3, 3 November 2008
    Copyright c 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.
   The purpose of this License is to make a manual, textbook, or other functional and
   useful document free in the sense of freedom: to assure everyone the effective freedom
   to copy and redistribute it, with or without modifying it, either commercially or non-
   commercially. Secondarily, this License preserves for the author and publisher a way
   to get credit for their work, while not being considered responsible for modifications
   made by others.
   This License is a kind of “copyleft”, which means that derivative works of the document
   must themselves be free in the same sense. It complements the GNU General Public
   License, which is a copyleft license designed for free software.
   We have designed this License in order to use it for manuals for free software, because
   free software needs free documentation: a free program should come with manuals
   providing the same freedoms that the software does. But this License is not limited to
   software manuals; it can be used for any textual work, regardless of subject matter or
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   A “Modified Version” of the Document means any work containing the Document or
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   The “Invariant Sections” are certain Secondary Sections whose titles are designated, as
   being those of Invariant Sections, in the notice that says that the Document is released
Appendix C: GNU Free Documentation License                                                20

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Appendix C: GNU Free Documentation License                                                21

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    the Document, numbering more than 100, and the Document’s license notice requires
    Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all
    these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
    the back cover. Both covers must also clearly and legibly identify you as the publisher
    of these copies. The front cover must present the full title with all words of the title
    equally prominent and visible. You may add other material on the covers in addition.
    Copying with changes limited to the covers, as long as they preserve the title of the
    Document and satisfy these conditions, can be treated as verbatim copying in other
    If the required texts for either cover are too voluminous to fit legibly, you should put
    the first ones listed (as many as fit reasonably) on the actual cover, and continue the
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    If you publish or distribute Opaque copies of the Document numbering more than 100,
    you must either include a machine-readable Transparent copy along with each Opaque
    copy, or state in or with each Opaque copy a computer-network location from which
    the general network-using public has access to download using public-standard network
    protocols a complete Transparent copy of the Document, free of added material. If
    you use the latter option, you must take reasonably prudent steps, when you begin
    distribution of Opaque copies in quantity, to ensure that this Transparent copy will
    remain thus accessible at the stated location until at least one year after the last time
    you distribute an Opaque copy (directly or through your agents or retailers) of that
    edition to the public.
    It is requested, but not required, that you contact the authors of the Document well
    before redistributing any large number of copies, to give them a chance to provide you
    with an updated version of the Document.
    You may copy and distribute a Modified Version of the Document under the conditions
    of sections 2 and 3 above, provided that you release the Modified Version under precisely
    this License, with the Modified Version filling the role of the Document, thus licensing
    distribution and modification of the Modified Version to whoever possesses a copy of
    it. In addition, you must do these things in the Modified Version:
     A. Use in the Title Page (and on the covers, if any) a title distinct from that of the
         Document, and from those of previous versions (which should, if there were any,
Appendix C: GNU Free Documentation License                                                 22

        be listed in the History section of the Document). You may use the same title as
        a previous version if the original publisher of that version gives permission.
    B. List on the Title Page, as authors, one or more persons or entities responsible for
       authorship of the modifications in the Modified Version, together with at least five
       of the principal authors of the Document (all of its principal authors, if it has fewer
       than five), unless they release you from this requirement.
    C. State on the Title page the name of the publisher of the Modified Version, as the
    D. Preserve all the copyright notices of the Document.
    E. Add an appropriate copyright notice for your modifications adjacent to the other
       copyright notices.
    F. Include, immediately after the copyright notices, a license notice giving the public
       permission to use the Modified Version under the terms of this License, in the form
       shown in the Addendum below.
    G. Preserve in that license notice the full lists of Invariant Sections and required Cover
       Texts given in the Document’s license notice.
    H. Include an unaltered copy of this License.
     I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item
        stating at least the title, year, new authors, and publisher of the Modified Version
        as given on the Title Page. If there is no section Entitled “History” in the Docu-
        ment, create one stating the title, year, authors, and publisher of the Document
        as given on its Title Page, then add an item describing the Modified Version as
        stated in the previous sentence.
     J. Preserve the network location, if any, given in the Document for public access to
        a Transparent copy of the Document, and likewise the network locations given in
        the Document for previous versions it was based on. These may be placed in the
        “History” section. You may omit a network location for a work that was published
        at least four years before the Document itself, or if the original publisher of the
        version it refers to gives permission.
    K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title
       of the section, and preserve in the section all the substance and tone of each of the
       contributor acknowledgements and/or dedications given therein.
    L. Preserve all the Invariant Sections of the Document, unaltered in their text and
       in their titles. Section numbers or the equivalent are not considered part of the
       section titles.
    M. Delete any section Entitled “Endorsements”. Such a section may not be included
       in the Modified Version.
    N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in
       title with any Invariant Section.
    O. Preserve any Warranty Disclaimers.
   If the Modified Version includes new front-matter sections or appendices that qualify
   as Secondary Sections and contain no material copied from the Document, you may at
   your option designate some or all of these sections as invariant. To do this, add their
Appendix C: GNU Free Documentation License                                                   23

   titles to the list of Invariant Sections in the Modified Version’s license notice. These
   titles must be distinct from any other section titles.
   You may add a section Entitled “Endorsements”, provided it contains nothing but
   endorsements of your Modified Version by various parties—for example, statements of
   peer review or that the text has been approved by an organization as the authoritative
   definition of a standard.
   You may add a passage of up to five words as a Front-Cover Text, and a passage of up
   to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified
   Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be
   added by (or through arrangements made by) any one entity. If the Document already
   includes a cover text for the same cover, previously added by you or by arrangement
   made by the same entity you are acting on behalf of, you may not add another; but
   you may replace the old one, on explicit permission from the previous publisher that
   added the old one.
   The author(s) and publisher(s) of the Document do not by this License give permission
   to use their names for publicity for or to assert or imply endorsement of any Modified
   You may combine the Document with other documents released under this License,
   under the terms defined in section 4 above for modified versions, provided that you
   include in the combination all of the Invariant Sections of all of the original documents,
   unmodified, and list them all as Invariant Sections of your combined work in its license
   notice, and that you preserve all their Warranty Disclaimers.
   The combined work need only contain one copy of this License, and multiple identical
   Invariant Sections may be replaced with a single copy. If there are multiple Invariant
   Sections with the same name but different contents, make the title of each such section
   unique by adding at the end of it, in parentheses, the name of the original author or
   publisher of that section if known, or else a unique number. Make the same adjustment
   to the section titles in the list of Invariant Sections in the license notice of the combined
   In the combination, you must combine any sections Entitled “History” in the vari-
   ous original documents, forming one section Entitled “History”; likewise combine any
   sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You
   must delete all sections Entitled “Endorsements.”
   You may make a collection consisting of the Document and other documents released
   under this License, and replace the individual copies of this License in the various
   documents with a single copy that is included in the collection, provided that you
   follow the rules of this License for verbatim copying of each of the documents in all
   other respects.
   You may extract a single document from such a collection, and distribute it individu-
   ally under this License, provided you insert a copy of this License into the extracted
   document, and follow this License in all other respects regarding verbatim copying of
   that document.
Appendix C: GNU Free Documentation License                                                 24

   A compilation of the Document or its derivatives with other separate and independent
   documents or works, in or on a volume of a storage or distribution medium, is called
   an “aggregate” if the copyright resulting from the compilation is not used to limit the
   legal rights of the compilation’s users beyond what the individual works permit. When
   the Document is included in an aggregate, this License does not apply to the other
   works in the aggregate which are not themselves derivative works of the Document.
   If the Cover Text requirement of section 3 is applicable to these copies of the Document,
   then if the Document is less than one half of the entire aggregate, the Document’s Cover
   Texts may be placed on covers that bracket the Document within the aggregate, or the
   electronic equivalent of covers if the Document is in electronic form. Otherwise they
   must appear on printed covers that bracket the whole aggregate.
   Translation is considered a kind of modification, so you may distribute translations
   of the Document under the terms of section 4. Replacing Invariant Sections with
   translations requires special permission from their copyright holders, but you may
   include translations of some or all Invariant Sections in addition to the original versions
   of these Invariant Sections. You may include a translation of this License, and all the
   license notices in the Document, and any Warranty Disclaimers, provided that you
   also include the original English version of this License and the original versions of
   those notices and disclaimers. In case of a disagreement between the translation and
   the original version of this License or a notice or disclaimer, the original version will
   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “His-
   tory”, the requirement (section 4) to Preserve its Title (section 1) will typically require
   changing the actual title.
   You may not copy, modify, sublicense, or distribute the Document except as expressly
   provided under this License. Any attempt otherwise to copy, modify, sublicense, or
   distribute it is void, and will automatically terminate your rights under this License.
   However, if you cease all violation of this License, then your license from a particular
   copyright holder is reinstated (a) provisionally, unless and until the copyright holder
   explicitly and finally terminates your license, and (b) permanently, if the copyright
   holder fails to notify you of the violation by some reasonable means prior to 60 days
   after the cessation.
   Moreover, your license from a particular copyright holder is reinstated permanently if
   the copyright holder notifies you of the violation by some reasonable means, this is the
   first time you have received notice of violation of this License (for any work) from that
   copyright holder, and you cure the violation prior to 30 days after your receipt of the
   Termination of your rights under this section does not terminate the licenses of parties
   who have received copies or rights from you under this License. If your rights have
   been terminated and not permanently reinstated, receipt of a copy of some or all of the
   same material does not give you any rights to use it.
Appendix C: GNU Free Documentation License                                                  25

    The Free Software Foundation may publish new, revised versions of the GNU Free
    Documentation License from time to time. Such new versions will be similar in spirit
    to the present version, but may differ in detail to address new problems or concerns.
    Each version of the License is given a distinguishing version number. If the Document
    specifies that a particular numbered version of this License “or any later version”
    applies to it, you have the option of following the terms and conditions either of that
    specified version or of any later version that has been published (not as a draft) by
    the Free Software Foundation. If the Document does not specify a version number of
    this License, you may choose any version ever published (not as a draft) by the Free
    Software Foundation. If the Document specifies that a proxy can decide which future
    versions of this License can be used, that proxy’s public statement of acceptance of a
    version permanently authorizes you to choose that version for the Document.
    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide
    Web server that publishes copyrightable works and also provides prominent facilities
    for anybody to edit those works. A public wiki that anybody can edit is an example of
    such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
    site means any set of copyrightable works thus published on the MMC site.
    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license pub-
    lished by Creative Commons Corporation, a not-for-profit corporation with a principal
    place of business in San Francisco, California, as well as future copyleft versions of that
    license published by that same organization.
    “Incorporate” means to publish or republish a Document, in whole or in part, as part
    of another Document.
    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works
    that were first published under this License somewhere other than this MMC, and
    subsequently incorporated in whole or in part into the MMC, (1) had no cover texts
    or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
    The operator of an MMC Site may republish an MMC contained in the site under
    CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is
    eligible for relicensing.
Appendix C: GNU Free Documentation License                                           26

ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the
document and put the following copyright and license notices just after the title page:
       Copyright (C) year your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts. A copy of the license is included in the section entitled ‘‘GNU
       Free Documentation License’’.
   If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the
“with. . . Texts.” line with this:
         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.
   If you have Invariant Sections without Cover Texts, or some other combination of the
three, merge those two alternatives to suit the situation.
   If your document contains nontrivial examples of program code, we recommend releasing
these examples in parallel under your choice of free software license, such as the GNU
General Public License, to permit their use in free software.