for version 7.21, 21 July 2022
Loic J. Duros (firstname.lastname@example.org)
Yuchen Pei (email@example.com)
This manual is for GNU LibreJS (version 7.21, 21 July 2022), a GNU IceCat extension to
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
6 Free Licenses Detection . . . . . . . . . . . . . . . . . . . . . . . . . 7
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
GNU LibreJS —an add-on for GNU IceCat and Mozilla Firefox— detects and blocks nonfree
trivial and/or free.
tasks; many use it gratuitously for minor jobs that could be done easily with plain HTML.
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.
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
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 https://www.gnu.org/software/librejs/ or https://
addons.mozilla.org/addon/librejs/, 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
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.
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
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 identi.ca links, and phone numbers.
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
your web browser.
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
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.
scripts. This is meant to maximimize compatibility with websites that haven’t tried to be
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:
• 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.
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
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.
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.
licenseName: ’GNU AFFERO GENERAL PUBLIC LICENSE version 3’,
licenseName: ’Apache License, Version 2.0’,
licenseName: "Artistic License 2.0",
licenseName: "BSD 2-Clause License",
licenseName: "BSD 3-Clause License",
licenseName: ’Boost Software License 1.0’,
licenseName: ’Creative Commons Attribution 1.0 Generic’,
licenseName: ’Creative Commons Attribution 2.0 Generic’,
licenseName: ’Creative Commons Attribution 2.5 Generic’,
licenseName: ’Creative Commons Attribution 3.0 Unported’,
licenseName: ’Creative Commons Attribution 4.0 International’,
licenseName: ’Creative Commons Attribution-ShareAlike 1.0 Generic’,
licenseName: ’Creative Commons Attribution-ShareAlike 2.0 Generic’,
licenseName: ’Creative Commons Attribution-ShareAlike 2.5 Generic’,
licenseName: ’Creative Commons Attribution-ShareAlike 3.0 Unported’,
licenseName: ’Creative Commons Attribution-ShareAlike 4.0 International’,
licenseName: ’Creative Commons CC0 1.0 Universal’,
licenseName: ’Common Public Attribution License Version 1.0 (CPAL)’,
licenseName: "Eclipse Public License Version 1.0",
licenseName: ’Expat License (sometimes called MIT Licensed)’,
licenseName: "FreeBSD License",
licenseName: ’GNU All-Permissive License’,
licenseName: ’GNU General Public License (GPL) version 2’,
licenseName: ’GNU General Public License (GPL) version 3’,
licenseName: "The ISC License",
licenseName: "jQuery Tools",
licenseName: ’GNU Lesser General Public License, version 2.1’,
licenseName: ’GNU Lesser General Public License, version 3’,
licenseName: ’Mozilla Public License Version 2.0’,
licenseName: ’Public Domain’,
licenseName: ’Universal Permissive License’,
licenseName: ’Do What The F*ck You Want To Public License (WTFPL)’,
licenseName: ’X11 License’,
licenseName: "XFree86 1.1 License",
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 firstname.lastname@example.org 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: http://www.gnu.org/licenses/
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
LibreJS 7.21 depends on a number of Node.js-based libraries that can be installed using the
$ npm install acorn-loose jassha browserify
$ export PATH=$PATH:./node_modules/.bin
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:
$ ./build.sh -t
$ ./build.sh --test
Note: this build.sh script relies on no new source files being created.
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.
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
For example, to check the compliance of the FSF homepage, do
$ node ./compliance.js https://fsf.org
8.7 Adding new whitelisted libraries
The script index.js in ./hash_script generates the default whitelist. Run it with the
node index.js > output
Then, just copy the contents of the file "output" to the appropriate place in
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 build.sh.
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
Appendix B LibreJS Internals
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
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
whether it is published as a printed book. We recommend this License principally for
works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a
notice placed by the copyright holder saying it can be distributed under the terms
of this License. Such a notice grants a world-wide, royalty-free license, unlimited in
duration, to use that work under the conditions stated herein. The “Document”,
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is addressed as “you”. You accept the license if you copy, modify or distribute the work
in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or
a portion of it, either copied verbatim, or with modifications and/or translated into
A “Secondary Section” is a named appendix or a front-matter section of the Document
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relationship could be a matter of historical connection with the subject or with related
matters, or of legal, commercial, philosophical, ethical or political position regarding
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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The “Cover Texts” are certain short passages of text that are listed, as Front-Cover
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A “Transparent” copy of the Document means a machine-readable copy, represented
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The Document may include Warranty Disclaimers next to the notice which states that
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2. VERBATIM COPYING
Appendix C: GNU Free Documentation License 21
You may copy and distribute the Document in any medium, either commercially or
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notice saying this License applies to the Document are reproduced in all copies, and
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If you distribute a large enough number of copies you must also follow the conditions
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You may also lend copies, under the same conditions stated above, and you may publicly
3. COPYING IN QUANTITY
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Appendix C: GNU Free Documentation License 22
be listed in the History section of the Document). You may use the same title as
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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
F. Include, immediately after the copyright notices, a license notice giving the public
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Texts given in the Document’s license notice.
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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
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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
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License,
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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.”
6. COLLECTIONS OF DOCUMENTS
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
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
Appendix C: GNU Free Documentation License 24
7. AGGREGATION WITH INDEPENDENT WORKS
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
10. FUTURE REVISIONS OF THIS LICENSE
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
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.
Loic J. Duros