[bpt/emacs.git] / doc / emacs / nxml-mode.texi

\input texinfo @c -*- texinfo -*-
@c %**start of header
@setfilename nxml-mode.info
@settitle nXML Mode
@c %**end of header

@dircategory Emacs
@direntry
* nXML Mode: (nxml-mode.info).
@end direntry

@node Top
@top nXML Mode

This manual documents nxml-mode, an Emacs major mode for editing
XML with RELAX NG support.  This manual is not yet complete.

@menu
* Completion::                  
* Inserting end-tags::          
* Paragraphs::                  
* Outlining::                   
* Locating a schema::           
* DTDs::                        
* Limitations::                 
@end menu

@node Completion
@chapter 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.

The traditional GNU Emacs key combination for completion in a
buffer is @kbd{M-@key{TAB}}. However, many window systems
and window managers use this key combination themselves (typically for
switching between windows) and do not pass it to applications. It's
hard to find key combinations in GNU Emacs that are both easy to type
and not taken by something else.  @kbd{C-@key{RET}} (i.e.
pressing the Enter or Return key, while the Ctrl key is held down) is
available.  It won't be available on a traditional terminal (because
it is indistinguishable from Return), but it will work with a window
system.  Therefore we adopt the following solution by default: use
@kbd{C-@key{RET}} when there's a window system and
@kbd{M-@key{TAB}} when there's not.  In the following, I
will assume that a window system is being used and will therefore
refer to @kbd{C-@key{RET}}.

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 @point{}
indicates point):

@example
<html xmlns="http://www.w3.org/1999/xhtml">
<h@point{}
@end example

@noindent
and the schema is XHTML.  In this context, the symbol to be completed
is @samp{h}.  The possible completions consist of just
@samp{head}.  Another example, is

@example
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<@point{}
@end example

@noindent
In this case, the symbol to be completed is empty, and the possible
completions are @samp{base}, @samp{isindex},
@samp{link}, @samp{meta}, @samp{script},
@samp{style}, @samp{title}.  Another example is:

@example
<html xmlns="@point{}
@end example

@noindent
In this case, the symbol to be completed is empty, and the possible
completions are just @samp{http://www.w3.org/1999/xhtml}.

When you type @kbd{C-@key{RET}}, what happens depends
on what the set of possible completions are.

@itemize @bullet
@item
If the set of completions is empty, nothing
happens.
@item
If there is one possible completion, then that completion is
inserted, together with any following characters that are
required. For example, in this case:

@example
<html xmlns="http://www.w3.org/1999/xhtml">
<@point{}
@end example

@noindent
@kbd{C-@key{RET}} will yield

@example
<html xmlns="http://www.w3.org/1999/xhtml">
<head@point{}
@end example
@item
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:

@example
<html x@point{}
@end example

@noindent
The symbol to be completed is @samp{x}. The possible completions
are @samp{xmlns} and @samp{xml:lang}.  These share a
common prefix of @samp{xml}.  Thus, @kbd{C-@key{RET}}
will yield:

@example
<html xml@point{}
@end example

@noindent
Typically, you would do @kbd{C-@key{RET}} again, which would
have the result described in the next item.
@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:

@example
<html xml@point{}
@end example

@noindent
Emacs will prompt you in the minibuffer with

@example
Attribute: xml@point{}
@end example

@noindent
and the buffer showing possible completions will contain

@example
Possible completions are:
xml:lang			   xmlns
@end example

@noindent
If you input @kbd{xmlns}, the result will be:

@example
<html xmlns="@point{}
@end example

@noindent
(If you do @kbd{C-@key{RET}} again, the namespace URI will
be inserted. Should that happen automatically?)
@end itemize

@node Inserting end-tags
@chapter 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 @kbd{C-@key{RET}} after @samp{</}
to complete the rest of the end-tag.

@kbd{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 @samp{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 @kbd{C-c C-i}. This inserts a @samp{>}, then inserts
the end-tag and leaves point before the end-tag.  @kbd{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 @samp{i} is mnemonic for inline and the
@samp{b} is mnemonic for block.

Finally, you can customize nxml-mode so that @kbd{/}
automatically inserts the rest of the end-tag when it occurs after
@samp{<}, by doing

@display
@kbd{M-x customize-variable @key{RET} nxml-slash-auto-complete-flag @key{RET}}
@end display

@noindent
and then following the instructions in the displayed buffer.

@node Paragraphs
@chapter Paragraphs

Emacs has several commands that operate on paragraphs, most
notably @kbd{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 @kbd{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:

@itemize @bullet
@item
If the schema does not allow text at that point, then it
is a paragraph boundary.
@item
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

@example
<p>This is a paragraph with an
<emph>emphasized</emph> phrase.
@end example

@noindent
the @samp{<emph>} start-tag would not be considered as
starting a paragraph, because its corresponding end-tag is not at the
end of the line.
@item
If there is text that is a sibling in element tree, then
it is not a paragraph boundary.  For example, in

@example
<p>This is a paragraph with an
<emph>emphasized phrase that takes one source line</emph>
@end example

@noindent
the @samp{<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 @samp{This is a paragraph with an}
is a sibling of the @samp{emph} element.
@item
Otherwise, it is a paragraph boundary.
@end itemize

@node Outlining
@chapter Outlining

nXML mode allows you to display all or part of a buffer as an
outline, in a similar way to Emacs' 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 @code{div} to enclose each
section, rather than having sections implicitly delimited by different
@code{h@var{n}} elements.  This limitation may be removed
in a future version.

The variable @code{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
@code{nxml-heading-element-name-regexp} gives a regexp for the
local names of heading elements. For an element to be recognized
as a section

@itemize @bullet
@item
its start-tag must occur at the beginning of a line
(possibly indented);
@item
its local name must match
@code{nxml-section-element-name-regexp};
@item
either its first child element or a descendant of that
first child element must have a local name that matches
@code{nxml-heading-element-name-regexp}; the first such element
is treated as the section's heading.
@end itemize

@noindent
You can customize these variables using @kbd{M-x
customize-variable}.

There are three possible outline states for a section:

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

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

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

@noindent
would be displayed on a single line like this:

@example
<-section>Food...</>
@end example

@noindent
If there are hidden subsections, then a @code{+} will be used
instead of a @code{-} like this:

@example
<+section>Food...</>
@end example

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

@example
<-section>Food...
  <-section>Delicious Food...</>
  <-section>Distasteful Food...</>
</-section>
@end example

@noindent
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 @code{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 @kbd{C-c C-o} (@kbd{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.

@itemize @bullet
@item
@kbd{C-c C-o C-a} shows all sections in the buffer
normally.
@item
@kbd{C-c C-o C-t} hides the text content
of all sections in the buffer.
@item
@kbd{C-c C-o C-c} hides the text content
of the current section.
@item
@kbd{C-c C-o C-e} shows the text content
of the current section.
@item
@kbd{C-c C-o C-d} hides the text content
and subsections of the current section.
@item
@kbd{C-c C-o C-s} shows the current section 
and all its direct and indirect subsections normally.
@item
@kbd{C-c C-o C-k} shows the headings of the
direct and indirect subsections of the current section.
@item
@kbd{C-c C-o C-l} hides the text content of the
current section and of its direct and indirect
subsections.
@item
@kbd{C-c C-o C-i} shows the headings of the
direct subsections of the current section.
@item
@kbd{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.
@end itemize

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

You can also use the mouse to change the outline state:
@kbd{S-mouse-2} hides the text content of a section in the same
way as@kbd{C-c C-o C-c}; @kbd{mouse-2} on a specially
displayed heading shows the text content of the section in the same
way as @kbd{C-c C-o C-e}; @kbd{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. @kbd{C-c C-o C-r} can be used to refresh
the display so it is correct again.

@node Locating a schema
@chapter 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 @samp{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
@var{x} occurs in the list before file @var{y} then rules
from file @var{x} have precedence over rules from file
@var{y}.  A filename specified in
@samp{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
@samp{rng-schema-locating-files} do not not exist. You can use
@kbd{M-x customize-variable @key{RET} rng-schema-locating-files
@key{RET}} to customize the list of schema locating
files.

By default, @samp{rng-schema-locating-files} list has two
members: @samp{schemas.xml}, and
@samp{@var{dist-dir}/schema/schemas.xml} where
@samp{@var{dist-dir}} is the directory containing the nXML
distribution. The first member will cause nXML mode to use a file
@samp{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.

@menu
* Commands for locating a schema::  
* Schema locating files::       
@end menu

@node Commands for locating a schema
@section Commands for locating a schema

The command @kbd{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 @kbd{C-c
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 @samp{<@var{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 @kbd{C-c C-s C-f}
to manually select the file contaiing 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
@samp{rng-schema-locating-files}; it will create the file if
necessary, but will not create a directory. If the variable
@samp{rng-schema-locating-files} has not been customized, this
means that the rule will be added to the file @samp{schemas.xml}
in the same directory as the document being edited.

The command @kbd{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 @kbd{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 @kbd{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.

@node Schema locating files
@section 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.

@menu
* 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::  
@end menu

@node Schema locating file syntax basics
@subsection Schema locating file syntax basics

There is a schema for schema locating files in the file
@samp{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
@samp{locatingRules} and the namespace URI must be
@samp{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:

@example
<?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>
@end example

@noindent
This says to use the schema @samp{xhtml.rnc} for a document with
namespace @samp{http://www.w3.org/1999/xhtml}, and to use the
schema @samp{docbook.rnc} for a document whose local name is
@samp{book}.  If the document element had both a namespace URI
of @samp{http://www.w3.org/1999/xhtml} and a local name of
@samp{book}, then the matching rule that comes first will be
used and so the schema @samp{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 @samp{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 @samp{uri} attribute
does not contain a @samp{/}, then it will refer to a filename in
the same directory as the schema locating file.

@node Using the document's URI to locate a schema
@subsection Using the document's URI to locate a schema

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

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

@noindent
specifies that that the schema for @samp{spec.xml} is
@samp{docbook.rnc}.

The @samp{pattern} attribute can be used instead of the
@samp{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 @samp{*} character to stand for zero or more characters
within a path segment (i.e. any character other @samp{/}).
Typically, the URI pattern looks like a relative URI, but, whereas a
relative URI in the @samp{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,

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

@noindent
specifies that the schema for documents with a URI whose path ends
with @samp{.xsl} is @samp{xslt.rnc}.

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

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

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

@node Using the document element to locate a schema
@subsection Using the document element to locate a schema

A @samp{documentElement} rule locates a schema based on
the local name and prefix of the document element. For example, a rule

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

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

A @samp{namespace} rule locates a schema based on the
namespace URI of the document element. For example, a rule

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

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

@node Using type identifiers in schema locating files
@subsection 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 @samp{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
@samp{uri} attribute to specify a schema, can instead use a
@samp{typeId} attribute to specify a type identifier.  The type
identifier can be associated with a URI using a @samp{typeId}
element. For example,

@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>
@end example

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

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

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

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

@noindent
that makes the default variant of XHTML be XHTML Transitional.

@node Using multiple schema locating files
@subsection Using multiple schema locating files

The @samp{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 @samp{include} element.
Relative URIs are resolved into absolute URIs before the inclusion is
performed. For example,

@example
<include rules="../rules.xml"/>
@end example

@noindent
includes the rules from @samp{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

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

@noindent
followed by a public file

@example
<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>
@end example

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

@example
<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>
@end example

@node DTDs
@chapter 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
simulataneously 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.

@node Limitations
@chapter Limitations

nXML mode has some limitations:

@itemize @bullet
@item
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.
@item
The restrictions on RELAX NG schemas in section 7 of the RELAX NG
specification are not enforced.
@item
Unicode support has problems. This stems mostly from the fact that
the XML (and RELAX NG) character model is based squarely on Unicode,
whereas the Emacs character model is not.  Emacs 22 is slated to have
full Unicode support, which should improve the situation here.
@end itemize

@bye