update fdl with orginal FDL eng

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diff --git a/docs/fdl.md b/docs/fdl.md
index 1faa364..e1b0b30 100644
--- a/docs/fdl.md
+++ b/docs/fdl.md
@@ -179,3 +179,461 @@ with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts
 如果你有不具封面文字的恒常章节，或一些其它这三者的组合，将可选择的二项合并以符合实际情形。
 
 如果你的文件中包含有并非微不足道的程序码范例，我们建议这些范例平行地在你的自由软件授权选择下，比如以 GNU General Public License 的自由软件授权来发布，从而允许它们作为自由软件而使用。
+
+**因为本书是基于 GNU FPL 许可证发布的，所以下面附加 FDL 许可证的英文原版，同时亦可与前面中文翻译版共同帮助理解。特此注明。**
+
+### GNU Free Documentation License {#gnu-free-documentation-license .chapter}
+
+Version 1.3, 3 November 2008
+
+> Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/
+> Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
+
+1.  **PREAMBLE**
+
+    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 noncommercially. 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.
+
+2.  **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”, below,
+    refers to any such manual or work. Any member of the public is a
+    licensee, and 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 another language.
+
+    A “Secondary Section” is a named appendix or a front-matter section
+    of the Document that deals exclusively with the relationship of the
+    publishers or authors of the Document to the Document’s overall
+    subject (or to related matters) and contains nothing that could fall
+    directly within that overall subject. (Thus, if the Document is in
+    part a textbook of mathematics, a Secondary Section may not explain
+    any mathematics.) The 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 them.
+
+    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 under this License. If a
+    section does not fit the above definition of Secondary then it is
+    not allowed to be designated as Invariant. The Document may contain
+    zero Invariant Sections. If the Document does not identify any
+    Invariant Sections then there are none.
+
+    The “Cover Texts” are certain short passages of text that are
+    listed, as Front-Cover Texts or Back-Cover Texts, in the notice that
+    says that the Document is released under this License. A Front-Cover
+    Text may be at most 5 words, and a Back-Cover Text may be at most
+    25 words.
+
+    A “Transparent” copy of the Document means a machine-readable copy,
+    represented in a format whose specification is available to the
+    general public, that is suitable for revising the document
+    straightforwardly with generic text editors or (for images composed
+    of pixels) generic paint programs or (for drawings) some widely
+    available drawing editor, and that is suitable for input to text
+    formatters or for automatic translation to a variety of formats
+    suitable for input to text formatters. A copy made in an otherwise
+    Transparent file format whose markup, or absence of markup, has been
+    arranged to thwart or discourage subsequent modification by readers
+    is not Transparent. An image format is not Transparent if used for
+    any substantial amount of text. A copy that is not “Transparent” is
+    called “Opaque”.
+
+    Examples of suitable formats for Transparent copies include plain
+    ASCII without markup, Texinfo input format, LaTeX input format, SGML
+    or XML using a publicly available DTD, and standard-conforming
+    simple HTML, PostScript or PDF designed for human modification.
+    Examples of transparent image formats include PNG, XCF and JPG.
+    Opaque formats include proprietary formats that can be read and
+    edited only by proprietary word processors, SGML or XML for which
+    the DTD and/or processing tools are not generally available, and the
+    machine-generated HTML, PostScript or PDF produced by some word
+    processors for output purposes only.
+
+    The “Title Page” means, for a printed book, the title page itself,
+    plus such following pages as are needed to hold, legibly, the
+    material this License requires to appear in the title page. For
+    works in formats which do not have any title page as such, “Title
+    Page” means the text near the most prominent appearance of the
+    work’s title, preceding the beginning of the body of the text.
+
+    The “publisher” means any person or entity that distributes copies
+    of the Document to the public.
+
+    A section “Entitled XYZ” means a named subunit of the Document whose
+    title either is precisely XYZ or contains XYZ in parentheses
+    following text that translates XYZ in another language. (Here XYZ
+    stands for a specific section name mentioned below, such as
+    “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To
+    “Preserve the Title” of such a section when you modify the Document
+    means that it remains a section “Entitled XYZ” according to
+    this definition.
+
+    The Document may include Warranty Disclaimers next to the notice
+    which states that this License applies to the Document. These
+    Warranty Disclaimers are considered to be included by reference in
+    this License, but only as regards disclaiming warranties: any other
+    implication that these Warranty Disclaimers may have is void and has
+    no effect on the meaning of this License.
+
+3.  **VERBATIM COPYING**
+
+    You may copy and distribute the Document in any medium, either
+    commercially or noncommercially, provided that this License, the
+    copyright notices, and the license notice saying this License
+    applies to the Document are reproduced in all copies, and that you
+    add no other conditions whatsoever to those of this License. You may
+    not use technical measures to obstruct or control the reading or
+    further copying of the copies you make or distribute. However, you
+    may accept compensation in exchange for copies. If you distribute a
+    large enough number of copies you must also follow the conditions in
+    section 3.
+
+    You may also lend copies, under the same conditions stated above,
+    and you may publicly display copies.
+
+4.  **COPYING IN QUANTITY**
+
+    If you publish printed copies (or copies in media that commonly have
+    printed covers) of 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 respects.
+
+    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 rest onto
+    adjacent pages.
+
+    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.
+
+5.  **MODIFICATIONS**
+
+    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:
+
+    1.  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, 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.
+    2.  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.
+    3.  State on the Title page the name of the publisher of the
+        Modified Version, as the publisher.
+    4.  Preserve all the copyright notices of the Document.
+    5.  Add an appropriate copyright notice for your modifications
+        adjacent to the other copyright notices.
+    6.  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.
+    7.  Preserve in that license notice the full lists of Invariant
+        Sections and required Cover Texts given in the Document’s
+        license notice.
+    8.  Include an unaltered copy of this License.
+    9.  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
+        Document, 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.
+    10. 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.
+    11. 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.
+    12. 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.
+    13. Delete any section Entitled “Endorsements”. Such a section may
+        not be included in the Modified Version.
+    14. Do not retitle any existing section to be Entitled
+        “Endorsements” or to conflict in title with any
+        Invariant Section.
+    15. 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
+    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 Version.
+
+6.  **COMBINING DOCUMENTS**
+
+    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 work.
+
+    In the combination, you must combine any sections Entitled “History”
+    in the various 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.”
+
+7.  **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 other respects.
+
+    You may extract a single document from such a collection, and
+    distribute it individually 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.
+
+8.  **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.
+
+9.  **TRANSLATION**
+
+    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 prevail.
+
+    If a section in the Document is Entitled “Acknowledgements”,
+    “Dedications”, or “History”, the requirement (section 4) to Preserve
+    its Title (section 1) will typically require changing the
+    actual title.
+
+10. **TERMINATION**
+
+    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 notice.
+
+    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.
+
+11. **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. See
+    <http://www.gnu.org/copyleft/>.
+
+    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.
+
+12. **RELICENSING**
+
+    “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 published 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.
+
+#### ADDENDUM: How to use this License for your documents {#addendum-how-to-use-this-license-for-your-documents .heading}
+
+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.
+