nXML Mode

This manual documents nXML mode, an Emacs major mode for editing XML with RELAX NG support.

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 the Front-Cover Texts being “A GNU Manual,” and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled “GNU Free Documentation License”.

(a) The FSF’s Back-Cover Text is: “You have the freedom to copy and modify this GNU manual.”

This manual is not yet complete.

1 Introduction
2 Completion
3 Inserting end-tags
4 Paragraphs
5 Outlining
6 Locating a schema
- 6.1 Commands for locating a schema
- 6.2 Schema locating files
7 DTDs
8 Limitations
Appendix A GNU Free Documentation License

1 Introduction

nXML mode is an Emacs major-mode for editing XML documents. It supports editing well-formed XML documents, and provides schema-sensitive editing using RELAX NG Compact Syntax. To get started, visit a file containing an XML document, and, if necessary, use M-x nxml-mode to switch to nXML mode. By default, auto-mode-alist and magic-fallback-alist put buffers in nXML mode if they have recognizable XML content or file extensions. You may wish to customize the settings, for example to recognize different file extensions.

Once in nXML mode, you can type C-h m for basic information on the mode.

The etc/nxml directory in the Emacs distribution contains some data files used by nXML mode, and includes two files (test-valid.xml and test-invalid.xml) that provide examples of valid and invalid XML documents.

To get validation and schema-sensitive editing, you need a RELAX NG Compact Syntax (RNC) schema for your document (see Locating a schema). The etc/schema directory includes some schemas for popular document types. See https://relaxng.org/ for more information on RELAX NG. You can use the ‘Trang’ program from http://www.thaiopensource.com/relaxng/trang.html to automatically create RNC schemas. This program can:

infer an RNC schema from an instance document;
convert a DTD to an RNC schema;
convert a RELAX NG XML syntax schema to an RNC schema.

To convert a RELAX NG XML syntax (‘.rng’) schema to a RNC one, you can also use the XSLT stylesheet from https://github.com/oleg-pavliv/emacs/tree/master/xsl.

To convert a W3C XML Schema to an RNC schema, you need first to convert it to RELAX NG XML syntax using the RELAX NG converter tool rngconv (built on top of MSV). See https://github.com/kohsuke/msv and https://msv.dev.java.net/.

For historical discussions only, see the mailing list archives at http://groups.yahoo.com/group/emacs-nxml-mode/. Please make all new discussions on the ‘help-gnu-emacs’ and ‘emacs-devel’ mailing lists. Report any bugs with M-x report-emacs-bug.

2 Completion

Apart from real-time validation, the most important feature that nXML mode provides for assisting in document creation is "completion". Completion assists the user in inserting characters at point, based on knowledge of the schema and on the contents of the buffer before point.

nXML mode adapts the standard GNU Emacs command for completion in a buffer: completion-at-point, which is bound to C-M-i and M-TAB. Note that many window systems and window managers use M-TAB themselves (typically for switching between windows) and do not pass it to applications. In that case, you should type C-M-i or ESC TAB for completion, or bind completion-at-point to a key that is convenient for you. In the following, I will assume that you type C-M-i.

nXML mode completion works by examining the symbol preceding point. This is the symbol to be completed. The symbol to be completed may be the empty. Completion considers what symbols starting with the symbol to be completed would be valid replacements for the symbol to be completed, given the schema and the contents of the buffer before point. These symbols are the possible completions. An example may make this clearer. Suppose the buffer looks like this (where ∗ indicates point):

<html xmlns="http://www.w3.org/1999/xhtml">
<h∗

and the schema is XHTML. In this context, the symbol to be completed is ‘h’. The possible completions consist of just ‘head’. Another example, is

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<∗

In this case, the symbol to be completed is empty, and the possible completions are ‘base’, ‘isindex’, ‘link’, ‘meta’, ‘script’, ‘style’, ‘title’. Another example is:

<html xmlns="∗

In this case, the symbol to be completed is empty, and the possible completions are just ‘http://www.w3.org/1999/xhtml’.

When you type C-M-i, what happens depends on what the set of possible completions are.

If the set of completions is empty, nothing happens.
If there is one possible completion, then that completion is inserted, together with any following characters that are required. For example, in this case:
```
<html xmlns="http://www.w3.org/1999/xhtml">
<∗
```
C-M-i will yield
```
<html xmlns="http://www.w3.org/1999/xhtml">
<head∗
```
If there is more than one possible completion, but all possible completions share a common non-empty prefix, then that prefix is inserted. For example, suppose the buffer is:
```
<html x∗
```
The symbol to be completed is ‘x’. The possible completions are ‘xmlns’ and ‘xml:lang’. These share a common prefix of ‘xml’. Thus, C-M-i will yield:
```
<html xml∗
```
Typically, you would do C-M-i again, which would have the result described in the next item.
If there is more than one possible completion, but the possible completions do not share a non-empty prefix, then Emacs will prompt you to input the symbol in the minibuffer, initializing the minibuffer with the symbol to be completed, and popping up a buffer showing the possible completions. You can now input the symbol to be inserted. The symbol you input will be inserted in the buffer instead of the symbol to be completed. Emacs will then insert any required characters after the symbol. For example, if it contains:
```
<html xml∗
```
Emacs will prompt you in the minibuffer with
```
Attribute: xml∗
```
and the buffer showing possible completions will contain
```
Possible completions are:
xml:lang                           xmlns
```
If you input xmlns, the result will be:
```
<html xmlns="∗"
```
(If you do C-M-i again, the namespace URI will be inserted. Should that happen automatically?)

3 Inserting end-tags

The main redundancy in XML syntax is end-tags. nXML mode provides several ways to make it easier to enter end-tags. You can use all of these without a schema.

You can use C-M-i after ‘</’ to complete the rest of the end-tag.

C-c C-f inserts an end-tag for the element containing point. This command is useful when you want to input the start-tag, then input the content and finally input the end-tag. The ‘f’ is mnemonic for finish.

If you want to keep tags balanced and input the end-tag at the same time as the start-tag, before inputting the content, then you can use C-c C-i. This inserts a ‘>’, then inserts the end-tag and leaves point before the end-tag. C-c C-b is similar but more convenient for block-level elements: it puts the start-tag, point and the end-tag on successive lines, appropriately indented. The ‘i’ is mnemonic for inline and the ‘b’ is mnemonic for block.

Finally, you can customize nXML mode so that / automatically inserts the rest of the end-tag when it occurs after ‘<’, by doing

M-x customize-variable RET nxml-slash-auto-complete-flag RET

and then following the instructions in the displayed buffer.

4 Paragraphs

Emacs has several commands that operate on paragraphs, most notably M-q. nXML mode redefines these to work in a way that is useful for XML. The exact rules that are used to find the beginning and end of a paragraph are complicated; they are designed mainly to ensure that M-q does the right thing.

A paragraph consists of one or more complete, consecutive lines. A group of lines is not considered a paragraph unless it contains some non-whitespace characters between tags or inside comments. A blank line separates paragraphs. A single tag on a line by itself also separates paragraphs. More precisely, if one tag together with any leading and trailing whitespace completely occupy one or more lines, then those lines will not be included in any paragraph.

A start-tag at the beginning of the line (possibly indented) may be treated as starting a paragraph. Similarly, an end-tag at the end of the line may be treated as ending a paragraph. The following rules are used to determine whether such a tag is in fact treated as a paragraph boundary:

If the schema does not allow text at that point, then it is a paragraph boundary.
If the end-tag corresponding to the start-tag is not at the end of its line, or the start-tag corresponding to the end-tag is not at the beginning of its line, then it is not a paragraph boundary. For example, in
```
<p>This is a paragraph with an
<emph>emphasized</emph> phrase.
```
the ‘<emph>’ start-tag would not be considered as starting a paragraph, because its corresponding end-tag is not at the end of the line.
If there is text that is a sibling in element tree, then it is not a paragraph boundary. For example, in
```
<p>This is a paragraph with an
<emph>emphasized phrase that takes one source line</emph>
```
the ‘<emph>’ start-tag would not be considered as starting a paragraph, even though its end-tag is at the end of its line, because there the text ‘This is a paragraph with an’ is a sibling of the ‘emph’ element.
Otherwise, it is a paragraph boundary.

5 Outlining

nXML mode allows you to display all or part of a buffer as an outline, in a similar way to Emacs’s outline mode. An outline in nXML mode is based on recognizing two kinds of element: sections and headings. There is one heading for every section and one section for every heading. A section contains its heading as or within its first child element. A section also contains its subordinate sections (its subsections). The text content of a section consists of anything in a section that is neither a subsection nor a heading.

Note that this is a different model from that used by XHTML. nXML mode’s outline support will not be useful for XHTML unless you adopt a convention of adding a div to enclose each section, rather than having sections implicitly delimited by different hn elements. This limitation may be removed in a future version.

The variable nxml-section-element-name-regexp gives a regexp for the local names (i.e., the part of the name following any prefix) of section elements. The variable nxml-heading-element-name-regexp gives a regexp for the local names of heading elements. For an element to be recognized as a section

its start-tag must occur at the beginning of a line (possibly indented);
its local name must match nxml-section-element-name-regexp;
either its first child element or a descendant of that first child element must have a local name that matches nxml-heading-element-name-regexp; the first such element is treated as the section’s heading.

You can customize these variables using M-x customize-variable.

There are three possible outline states for a section:

normal, showing everything, including its heading, text content and subsections; each subsection is displayed according to the state of that subsection;
showing just its heading, with both its text content and its subsections hidden; all subsections are hidden regardless of their state;
showing its heading and its subsections, with its text content hidden; each subsection is displayed according to the state of that subsection.

In the last two states, where the text content is hidden, the heading is displayed specially, in an abbreviated form. An element like this:

<section>
<title>Food</title>
<para>There are many kinds of food.</para>
</section>

would be displayed on a single line like this:

<-section>Food...</>

If there are hidden subsections, then a + will be used instead of a - like this:

<+section>Food...</>

If there are non-hidden subsections, then the section will instead be displayed like this:

<-section>Food...
  <-section>Delicious Food...</>
  <-section>Distasteful Food...</>
</-section>

The heading is always displayed with an indent that corresponds to its depth in the outline, even it is not actually indented in the buffer. The variable nxml-outline-child-indent controls how much a subheading is indented with respect to its parent heading when the heading is being displayed specially.

Commands to change the outline state of sections are bound to key sequences that start with C-c C-o (o is mnemonic for outline). The third and final key has been chosen to be consistent with outline mode. In the following descriptions current section means the section containing point, or, more precisely, the innermost section containing the character immediately following point.

C-c C-o C-a shows all sections in the buffer normally.
C-c C-o C-t hides the text content of all sections in the buffer.
C-c C-o C-c hides the text content of the current section.
C-c C-o C-e shows the text content of the current section.
C-c C-o C-d hides the text content and subsections of the current section.
C-c C-o C-s shows the current section and all its direct and indirect subsections normally.
C-c C-o C-k shows the headings of the direct and indirect subsections of the current section.
C-c C-o C-l hides the text content of the current section and of its direct and indirect subsections.
C-c C-o C-i shows the headings of the direct subsections of the current section.
C-c C-o C-o hides as much as possible without hiding the current section’s text content; the headings of ancestor sections of the current section and their child section sections will not be hidden.

When a heading is displayed specially, you can use RET in that heading to show the text content of the section in the same way as C-c C-o C-e.

You can also use the mouse to change the outline state: S-mouse-2 hides the text content of a section in the same way asC-c C-o C-c; mouse-2 on a specially displayed heading shows the text content of the section in the same way as C-c C-o C-e; mouse-1 on a specially displayed start-tag toggles the display of subheadings on and off.

The outline state for each section is stored with the first character of the section (as a text property). Every command that changes the outline state of any section updates the display of the buffer so that each section is displayed correctly according to its outline state. If the section structure is subsequently changed, then it is possible for the display to no longer correctly reflect the stored outline state. C-c C-o C-r can be used to refresh the display so it is correct again.

6 Locating a schema

nXML mode has a configurable set of rules to locate a schema for the file being edited. The rules are contained in one or more schema locating files, which are XML documents.

The variable ‘rng-schema-locating-files’ specifies the list of the file-names of schema locating files that nXML mode should use. The order of the list is significant: when file x occurs in the list before file y then rules from file x have precedence over rules from file y. A filename specified in ‘rng-schema-locating-files’ may be relative. If so, it will be resolved relative to the document for which a schema is being located. It is not an error if relative file-names in ‘rng-schema-locating-files’ do not exist. You can use M-x customize-variable RET rng-schema-locating-files RET to customize the list of schema locating files.

By default, ‘rng-schema-locating-files’ list has two members: ‘schemas.xml’, and ‘dist-dir/schema/schemas.xml’ where ‘dist-dir’ is the directory containing the nXML distribution. The first member will cause nXML mode to use a file ‘schemas.xml’ in the same directory as the document being edited if such a file exist. The second member contains rules for the schemas that are included with the nXML distribution.

Commands for locating a schema
Schema locating files

6.1 Commands for locating a schema

The command C-c C-s C-w will tell you what schema is currently being used.

The rules for locating a schema are applied automatically when you visit a file in nXML mode. However, if you have just created a new file and the schema cannot be inferred from the file-name, then this will not locate the right schema. In this case, you should insert the start-tag of the root element and then use the command C-c C-s C-a, which reapplies the rules based on the current content of the document. It is usually not necessary to insert the complete start-tag; often just ‘<name’ is enough.

If you want to use a schema that has not yet been added to the schema locating files, you can use the command C-c C-s C-f to manually select the file containing the schema for the document in current buffer. Emacs will read the file-name of the schema from the minibuffer. After reading the file-name, Emacs will ask whether you wish to add a rule to a schema locating file that persistently associates the document with the selected schema. The rule will be added to the first file in the list specified ‘rng-schema-locating-files’; it will create the file if necessary, but will not create a directory. If the variable ‘rng-schema-locating-files’ has not been customized, this means that the rule will be added to the file ‘schemas.xml’ in the same directory as the document being edited.

The command C-c C-s C-t allows you to select a schema by specifying an identifier for the type of the document. The schema locating files determine the available type identifiers and what schema is used for each type identifier. This is useful when it is impossible to infer the right schema from either the file-name or the content of the document, even though the schema is already in the schema locating file. A situation in which this can occur is when there are multiple variants of a schema where all valid documents have the same document element. For example, XHTML has Strict and Transitional variants. In a situation like this, a schema locating file can define a type identifier for each variant. As with C-c C-s C-f, Emacs will ask whether you wish to add a rule to a schema locating file that persistently associates the document with the specified type identifier.

The command C-c C-s C-l adds a rule to a schema locating file that persistently associates the document with the schema that is currently being used.

6.2 Schema locating files

Each schema locating file specifies a list of rules. The rules from each file are appended in order. To locate a schema each rule is applied in turn until a rule matches. The first matching rule is then used to determine the schema.

Schema locating files are designed to be useful for other applications that need to locate a schema for a document. In fact, there is nothing specific to locating schemas in the design; it could equally well be used for locating a stylesheet.

Schema locating file syntax basics
Using the document’s URI to locate a schema
Using the document element to locate a schema
Using type identifiers in schema locating files
Using multiple schema locating files

6.2.1 Schema locating file syntax basics

There is a schema for schema locating files in the file ‘locate.rnc’ in the schema directory. Schema locating files must be valid with respect to this schema.

The document element of a schema locating file must be ‘locatingRules’ and the namespace URI must be ‘http://thaiopensource.com/ns/locating-rules/1.0’. The children of the document element specify rules. The order of the children is the same as the order of the rules. Here’s a complete example of a schema locating file:

<?xml version="1.0"?>
<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <namespace ns="http://www.w3.org/1999/xhtml" uri="xhtml.rnc"/>
  <documentElement localName="book" uri="docbook.rnc"/>
</locatingRules>

This says to use the schema ‘xhtml.rnc’ for a document with namespace ‘http://www.w3.org/1999/xhtml’, and to use the schema ‘docbook.rnc’ for a document whose local name is ‘book’. If the document element had both a namespace URI of ‘http://www.w3.org/1999/xhtml’ and a local name of ‘book’, then the matching rule that comes first will be used and so the schema ‘xhtml.rnc’ would be used. There is no precedence between different types of rule; the first matching rule of any type is used.

As usual with XML-related technologies, resources are identified by URIs. The ‘uri’ attribute identifies the schema by specifying the URI. The URI may be relative. If so, it is resolved relative to the URI of the schema locating file that contains attribute. This means that if the value of ‘uri’ attribute does not contain a ‘/’, then it will refer to a filename in the same directory as the schema locating file.

6.2.2 Using the document’s URI to locate a schema

A ‘uri’ rule locates a schema based on the URI of the document. The ‘uri’ attribute specifies the URI of the schema. The ‘resource’ attribute can be used to specify the schema for a particular document. For example,

<uri resource="spec.xml" uri="docbook.rnc"/>

specifies that the schema for ‘spec.xml’ is ‘docbook.rnc’.

The ‘pattern’ attribute can be used instead of the ‘resource’ attribute to specify the schema for any document whose URI matches a pattern. The pattern has the same syntax as an absolute or relative URI except that the path component of the URI can use a ‘*’ character to stand for zero or more characters within a path segment (i.e., any character other ‘/’). Typically, the URI pattern looks like a relative URI, but, whereas a relative URI in the ‘resource’ attribute is resolved into a particular absolute URI using the base URI of the schema locating file, a relative URI pattern matches if it matches some number of complete path segments of the document’s URI ending with the last path segment of the document’s URI. For example,

<uri pattern="*.xsl" uri="xslt.rnc"/>

specifies that the schema for documents with a URI whose path ends with ‘.xsl’ is ‘xslt.rnc’.

A ‘transformURI’ rule locates a schema by transforming the URI of the document. The ‘fromPattern’ attribute specifies a URI pattern with the same meaning as the ‘pattern’ attribute of the ‘uri’ element. The ‘toPattern’ attribute is a URI pattern that is used to generate the URI of the schema. Each ‘*’ in the ‘toPattern’ is replaced by the string that matched the corresponding ‘*’ in the ‘fromPattern’. The resulting string is appended to the initial part of the document’s URI that was not explicitly matched by the ‘fromPattern’. The rule matches only if the transformed URI identifies an existing resource. For example, the rule

<transformURI fromPattern="*.xml" toPattern="*.rnc"/>

would transform the URI ‘file:///home/jjc/docs/spec.xml’ into the URI ‘file:///home/jjc/docs/spec.rnc’. Thus, this rule specifies that to locate a schema for a document ‘foo.xml’, Emacs should test whether a file ‘foo.rnc’ exists in the same directory as ‘foo.xml’, and, if so, should use it as the schema.

6.2.3 Using the document element to locate a schema

A ‘documentElement’ rule locates a schema based on the local name and prefix of the document element. For example, a rule

<documentElement prefix="xsl" localName="stylesheet" uri="xslt.rnc"/>

specifies that when the name of the document element is ‘xsl:stylesheet’, then ‘xslt.rnc’ should be used as the schema. Either the ‘prefix’ or ‘localName’ attribute may be omitted to allow any prefix or local name.

A ‘namespace’ rule locates a schema based on the namespace URI of the document element. For example, a rule

<namespace ns="http://www.w3.org/1999/XSL/Transform" uri="xslt.rnc"/>

specifies that when the namespace URI of the document is ‘http://www.w3.org/1999/XSL/Transform’, then ‘xslt.rnc’ should be used as the schema.

6.2.4 Using type identifiers in schema locating files

Type identifiers allow a level of indirection in locating the schema for a document. Instead of associating the document directly with a schema URI, the document is associated with a type identifier, which is in turn associated with a schema URI. nXML mode does not constrain the format of type identifiers. They can be simply strings without any formal structure or they can be public identifiers or URIs. Note that these type identifiers have nothing to do with the DOCTYPE declaration. When comparing type identifiers, whitespace is normalized in the same way as with the ‘xsd:token’ datatype: leading and trailing whitespace is stripped; other sequences of whitespace are normalized to a single space character.

Each of the rules described in previous sections that uses a ‘uri’ attribute to specify a schema, can instead use a ‘typeId’ attribute to specify a type identifier. The type identifier can be associated with a URI using a ‘typeId’ element. For example,

<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <namespace ns="http://www.w3.org/1999/xhtml" typeId="XHTML"/>
  <typeId id="XHTML" typeId="XHTML Strict"/>
  <typeId id="XHTML Strict" uri="xhtml-strict.rnc"/>
  <typeId id="XHTML Transitional" uri="xhtml-transitional.rnc"/>
</locatingRules>

declares three type identifiers ‘XHTML’ (representing the default variant of XHTML to be used), ‘XHTML Strict’ and ‘XHTML Transitional’. Such a schema locating file would use ‘xhtml-strict.rnc’ for a document whose namespace is ‘http://www.w3.org/1999/xhtml’. But it is considerably more flexible than a schema locating file that simply specified

<namespace ns="http://www.w3.org/1999/xhtml" uri="xhtml-strict.rnc"/>

A user can easily use C-c C-s C-t to select between XHTML Strict and XHTML Transitional. Also, a user can easily add a catalog

<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <typeId id="XHTML" typeId="XHTML Transitional"/>
</locatingRules>

that makes the default variant of XHTML be XHTML Transitional.

6.2.5 Using multiple schema locating files

The ‘include’ element includes rules from another schema locating file. The behavior is exactly as if the rules from that file were included in place of the ‘include’ element. Relative URIs are resolved into absolute URIs before the inclusion is performed. For example,

<include rules="../rules.xml"/>

includes the rules from ‘rules.xml’.

The process of locating a schema takes as input a list of schema locating files. The rules in all these files and in the files they include are resolved into a single list of rules, which are applied strictly in order. Sometimes this order is not what is needed. For example, suppose you have two schema locating files, a private file

<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <namespace ns="http://www.w3.org/1999/xhtml" uri="xhtml.rnc"/>
</locatingRules>

followed by a public file

<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <transformURI pathSuffix=".xml" replacePathSuffix=".rnc"/>
  <namespace ns="http://www.w3.org/1999/XSL/Transform" typeId="XSLT"/>
</locatingRules>

The effect of these two files is that the XHTML ‘namespace’ rule takes precedence over the ‘transformURI’ rule, which is almost certainly not what is needed. This can be solved by adding an ‘applyFollowingRules’ to the private file.

<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <applyFollowingRules ruleType="transformURI"/>
  <namespace ns="http://www.w3.org/1999/xhtml" uri="xhtml.rnc"/>
</locatingRules>

7 DTDs

nXML mode is designed to support the creation of standalone XML documents that do not depend on a DTD. Although it is common practice to insert a DOCTYPE declaration referencing an external DTD, this has undesirable side-effects. It means that the document is no longer self-contained. It also means that different XML parsers may interpret the document in different ways, since the XML Recommendation does not require XML parsers to read the DTD. With DTDs, it was impractical to get validation without using an external DTD or reference to an parameter entity. With RELAX NG and other schema languages, you can simultaneously get the benefits of validation and standalone XML documents. Therefore, I recommend that you do not reference an external DOCTYPE in your XML documents.

One problem is entities for characters. Typically, as well as providing validation, DTDs also provide a set of character entities for documents to use. Schemas cannot provide this functionality, because schema validation happens after XML parsing. The recommended solution is to either use the Unicode characters directly, or, if this is impractical, use character references. nXML mode supports this by providing commands for entering characters and character references using the Unicode names, and can display the glyph corresponding to a character reference.

8 Limitations

nXML mode has some limitations:

DTD support is limited. Internal parsed general entities declared in the internal subset are supported provided they do not contain elements. Other usage of DTDs is ignored.
The restrictions on RELAX NG schemas in section 7 of the RELAX NG specification are not enforced.

Appendix A GNU Free Documentation License

Version 1.3, 3 November 2008

Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
https://fsf.org/

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

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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 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.
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.
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.
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.
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.”
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.
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
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.
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.
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 https://www.gnu.org/licenses/.

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.
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

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.