  LinuxDoc-Tools User's Guide
  written by Matt Welsh as the LinuxDoc-SGML User's Guide.
  Updated by Greg Hankins, and rewritten by Eric S. Raymond
  for SGML-Tools.
  Updated and renamed by Taketoshi Sano, for LinuxDoc-Tools
  $Date: 2002/03/18 13:39:10 $ ($Revision: 1.2 $)

  This document is a user's guide to the LinuxDoc-Tools formatting
  system, a SGML-based system which allows you to produce a variety of
  output formats. You can create plain text output (ASCII, ISO-8859-1,
  and EUC-JP), DVI, PostScript, PDF, HTML, GNU info, LyX, and RTF output
  from a single document source file. LinuxDoc-Tools is a new branch
  from SGML-Tools 1.0.9, and an descendant of the original LinuxDoc-
  SGML.
  ______________________________________________________________________

  Table of Contents

  1. Introduction
     1.1 What's the DTD ?
     1.2 History of the LinuxDoc

  2. Installation
     2.1 Where to get the linuxdoc-tools sources
     2.2 What LinuxDoc-Tools Needs
     2.3 Installing The Software

  3. Writing Documents With LinuxDoc-Tools
     3.1 Basic Concepts
     3.2 Special Characters
     3.3 Verbatim and Code Environments
     3.4 Overall Document Structure
        3.4.1 The Preamble
        3.4.2 Sectioning And Paragraphs
        3.4.3 Ending The Document
     3.5 Internal Cross-References
     3.6 Web References
     3.7 Fonts
     3.8 Lists
     3.9 Conditionalization
     3.10 Index generation
     3.11 Controlling justification

  4. Formatting SGML Documents
     4.1 Checking SGML Syntax
     4.2 Creating Plain Text Output
     4.3 Creating LaTeX, DVI, PostScript or PDF Output
     4.4 Creating HTML Output
     4.5 Creating GNU Info Output
     4.6 Creating LyX Output
     4.7 Creating RTF Output

  5. Internationalization Support
  6. How LinuxDoc-Tools Works
     6.1 Overview of SGML
     6.2 How SGML Works
     6.3 What Happens When LinuxDoc-Tools Processes A Document
     6.4 Further Information

  ______________________________________________________________________

  1.  Introduction

  This document is the user's guide to the LinuxDoc-Tools document
  processing system. LinuxDoc-Tools is a suite of programs to help you
  write source documents that can be rendered as plain text, hypertext,
  or LaTeX files. It contains what you need to know to set up LinuxDoc-
  Tools and write documents using it.  See example.sgml for an example
  of an LinuxDoc DTD SGML document that you can use as a model for your
  own documents.  The ``LinuxDoc'' means the name of a specific SGML DTD
  here.

  1.1.  What's the DTD ?

  The DTD specifies the names of ``elements'' within the document.  An
  element is just a bit of structure; like a section, a subsection, a
  paragraph, or even something smaller like emphasized text.  You may
  know the HTML has their own DTD.

  Don't be confusing. SGML is not a text-formatting system.  SGML itself
  is used only to specify the document structure. There are no text-
  formatting facilities or ``macros'' intrinsic to SGML itself.  All of
  those things are defined within the DTD.  You can't use SGML without a
  DTD; a DTD defines what SGML does.  For more Detail, please refer the
  later section of this document (``How LinuxDoc-Tools Works'').

  1.2.  History of the LinuxDoc

  The LinuxDoc DTD is created by Matt Welsh as the core part of his
  Linuxdoc-SGML document processing system. This DTD is based heavily on
  the QWERTZ DTD by Tom Gordon, thomas.gordon@gmd.de.  The target of the
  QWERTZ DTD is to provide the simple way to create LaTeX source for
  document publishing. Matt Welsh took and shaped it into Linuxdoc-SGML
  because he needed it to produce a lot of Linux Documentations. It can
  convert a single source of documentation into various output formats
  such as plain text, html, and PS. No work for synchronization between
  various output formatted documents are needed.

  The Linuxdoc-SGML system had been maintained for years by Matt Welsh
  and many others, but it has some limitations. Then Cees de Groot came
  and created the new system using perl. The new system is called as
  ``SGML-Tools''. The perl based version for LinuxDoc had been
  maintained for a year, then totally new system using the original
  python scripts and some stylesheets with the jade has been released.
  This system is called as ``SGML-Tools 2.0'' and it does not use the
  LinuxDoc DTD as the main DTD, but uses the new standard one, the
  DocBook DTD.  Now ``SGML-Tools 2.0'' becomes ``SGMLtools-Lite'' and is
  distributed from  <http://sgmltools-lite.sourceforge.net/>.

  Recently, the DocBook DTD is the standard DTD for the technical
  software documentation, and used by many project such as GNOME and
  KDE, as well as many professional authors and commercial publishers.
  But some people in the LDP, and users of the various LinuxDoc SGML
  documents, still needs the support of the tools for the LinuxDoc.
  This ``LinuxDoc-Tools'' is created for those people. If you need the
  tools for the LinuxDoc DTD, then you may wish to use this. But
  remember, the LinuxDoc DTD is not the standard way now even in the
  Linux world. If you can, try the DocBook DTD. It is the standard, and
  full-featured way of writing the documentations.

  2.  Installation

  2.1.  Where to get the linuxdoc-tools sources

  o  You can get the source archive of the Linuxdoc-Tools from the page:
     <https://packages.debian.org/source/unstable/linuxdoc-tools/>.

     The name of the archive will be something like linuxdoc-
     tools_x.y.z.orig.tar.gz.

  o  Linuxdoc-Tools git repository can currently be reached at
     <https://gitlab.com/agmartin/linuxdoc-tools>.

     It contains an issue tracker to report problems.

  2.2.  What LinuxDoc-Tools Needs

  LinuxDoc-Tools depends on the usage of sgml parser from Jade or
  OpenJade (nsgmls or onsgmls). You have to install either of them to
  use this.

  The source archive of the linuxdoc-tools contains the tools and data
  that you need to write SGML documents and convert them to groff,
  LaTeX, PostScript, HTML, GNU info, LyX, and RTF. In addition to this
  package, you will need some additional tools for generating formatted
  output.

  1. groff.  You need version 1.08 or greater.  You can get this from
     <https://www.gnu.org/software/groff>.  You will need groff to
     produce plain text from your SGML documents.  nroff will not work!
     You can find the version of your groff from groff -v < /dev/null.

  2. TeX and LaTeX.  This is available more or less everywhere; you
     should have no problem getting it and installing it (there is a
     Linux binary distribution on sunsite.unc.edu).  Of course, you only
     need TeX/LaTeX if you want to format your SGML documents with
     LaTeX.  So, installing TeX/LaTeX is optional.  If you need PDF
     output, then you need pdfLaTeX also.

  3. flex.  (lex will probably not work).  You can get flex from
     <http://flex.sourceforge.net/>.

  4. The GNU info tools, for formatting and viewing info files.  These
     are available on <https://www.gnu.org/software/texinfo/>.

  5. LyX (a quasi-WYSIWYG interface to LaTeX, with SGML layouts), is
     available on <https://www.lyx.org/>.

  2.3.  Installing The Software

  The steps needed to install and configure the LinuxDoc-Tools are:

  1. First, unpack the tar file of the source archive somewhere.  This
     will create the directory linuxdoc-tools-x.y.z.  It doesn't matter
     where you unpack this file; just don't move things around within
     the extracted source tree.

  2. Read the INSTALL file - it has detailed installation instructions.
     Follow them. If all went well, you should be ready to use the
     system immediately once you have done so.

  3.  Writing Documents With LinuxDoc-Tools

  For the most part, writing documents using LinuxDoc-Tools is very
  simple, and rather like writing HTML. However, there are some caveats
  to watch out for. In this section we'll give an introduction on
  writing SGML documents. See the file example.sgml for a SGML example
  document (and tutorial) which you can use as a model when writing your
  own documents. Here we're just going to discuss the various features
  of LinuxDoc-Tools, but the source is not very readable as an example.
  Instead, print out the source (as well as the formatted output) for
  example.sgml so you have a real live case to refer to.

  3.1.  Basic Concepts

  Looking at the source of the example document, you'll notice right off
  that there are a number of ``tags'' marked within angle brackets (<
  and >). A tag simply specifies the beginning or end of an element,
  where an element is something like a section, a paragraph, a phrase of
  italicized text, an item in a list, and so on. Using a tag is like
  using an HTML tag, or a LaTeX command such as \item or \section{...}.

  As a simple example, to produce this boldfaced text, you would type

       As a simple example, to produce <bf>this boldfaced text</bf>, ...

  in the source. <bf> begins the region of bold text, and </bf> ends it.
  Alternately, you can use the abbreviated form

       As a simple example, to produce <bf/this boldfaced text/, ...

  which encloses the bold text within slashes. (Of course, you'll need
  to use the long form if the enclosed text contains slashes, such as
  the case with Unix filenames).

  There are other things to watch out with respect to special characters
  (that's why you'll notice all of these bizarre-looking ampersand
  expressions if you look at the source; I'll talk about those shortly).

  In some cases, the end-tag for a particular element is optional. For
  example, to begin a section, you use the <sect> tag, however, the end-
  tag for the section (which could appear at the end of the section body
  itself, not just after the name of the section!)  is optional and
  implied when you start another section of the same depth.  In general
  you needn't worry about these details; just follow the model used in
  the tutorial (example.sgml).

  3.2.  Special Characters

  Obviously, the angle brackets are themselves special characters in the
  SGML source. There are others to watch out for. For example, let's say
  that you wanted to type an expression with angle brackets around it,
  as so: <foo>. In order to get the left angle bracket, you must use the
  &lt; element, which is a ``macro'' that expands to the actual left-
  bracket character. Therefore, in the source, I typed

       angle brackets around it, as so: <tt>&lt;foo&gt;</tt>.

  Generally, anything beginning with an ampersand is a special
  character. For example, there's &percnt; to produce %, &verbar; to
  produce |, and so on. For every special character that might otherwise
  confuse LinuxDoc-Tools if typed by itself, there is an ampersand
  "entity" to represent it. The most commonly used are:

  o  Use &amp; for the ampersand (&),

  o  Use &lt; for a left bracket (<),

  o  Use &gt; for a right bracket (>),

  o  Use &etago; for a left bracket with a slash (</)

  o  Use &dollar; for a dollar sign ($),

  o  Use &num; for a hash (#),

  o  Use &percnt; for a percent (%),

  o  Use &tilde; for a tilde (~),

  o  Use `` and '' for quotes, or use &dquot; for ".

  o  Use &shy; for a soft hyphen (that is, an indication that this is a
     good place to break a word for horizontal justification).

  Here is a complete list of the entities recognized by 0.1. Note that
  not all back-ends will be able to make anything useful from every
  entity -- if you see parantheses with nothing between them in the
  list, it means that the back-end that generated what you're looking at
  has no replacement for the entity. The ``common'' ones listed above
  are pretty reliable.

     &half; (1/2)
        vertical 1/2 fraction

     &frac12; (1/2)
        typeset 1/2 fraction

     &frac14; (1/4)
        typeset 1/4 fraction

     &frac34; (3/4)
        typeset 3/4 fraction

     &frac18; (1/8)
        typeset 1/8 fraction

     &frac38; (3/8)
        typeset 3/8 fraction

     &frac58; (5/8)
        typeset 5/8 fraction

     &frac78; (7/8)
        typeset 7/8 fraction

     &sup1; (^1)
        superscript 1

     &sup2; (^2)
        superscript 2

     &sup3; (^3)
        superscript 3

     &plus; (+)
        plus sign

     &plusmn; (+-)
        plus-or-minus sign

     &lt; (<)
        less-than sign

     &equals; (=)
        equals sign

     &gt; (>)
        greater-than sign

     &divide; (/)
        division sign

     &times; (x)
        multiplication sign

     &curren; ({curren})
        currency symbol

     &pound; (L)
        symbol for ``pounds''

     &dollar; ($)
        dollar sign

     &cent; (c)
        cent sign

     &yen; (Y)
        yen sign

     &num; (#)
        number or hash sign

     &percnt; (%)
        percent sign

     &amp; (&)
        ampersand

     &ast; (*)
        asterisk

     &commat; (@)
        commercial-at sign

     &lsqb; ([)
        left square bracket

     &bsol; (\)
        backslash

     &rsqb; (])
        right square bracket

     &lcub; ({)
        left curly brace

     &horbar; (-)
        horizontal bar

     &verbar; (|)
        vertical bar

     &rcub; (})
        right curly brace

     &micro; (u)
        greek mu (micro prefix)

     &ohm; ({ohm})
        greek capital omega (Ohm sign)

     &deg; ({deg})
        small superscript circle sign (degree sign)

     &ordm; ({ordm})
        masculine ordinal

     &ordf; ({ordf})
        feminine ordinal

     &sect; (S)
        section sign

     &para; (P)
        paragraph sign

     &middot; (.)
        centered dot

     &larr; (<-)
        left arrow

     &rarr; (->)
        right arrow

     &uarr; ({uarr})
        up arrow

     &darr; ({darr})
        down arrow

     &copy; ((C))
        copyright

     &reg; ((R))
        r-in-circle marl

     &trade; ((TM))
        trademark sign

     &brvbar; (|)
        broken vertical bar

     &not; (~)
        logical-negation sign

     &sung; ({sung})
        sung-note sign

     &excl; (!)
        exclamation point

     &iexcl; (!)
        inverted exclamation point

     &quot; (")
        double quote

     &apos; (')
        apostrophe (single quote)

     &lpar; (()
        left parenthesis

     &rpar; ())
        right parenthesis

     &comma; (,)
        comma

     &lowbar; (_)
        under-bar

     &hyphen; (-)
        hyphen

     &period; (.)
        period

     &sol; (/)
        solidus

     &colon; (:)
        colon

     &semi; (;)
        semicolon

     &quest; (?)
        question mark

     &iquest; (?)
        interrobang

     &laquo; (<<)
        left guillemot

     &raquo; (>>)
        right guillemot

     &lsquo; (`)
        left single quote

     &rsquo; (')
        right single quote

     &ldquo; (``)
        left double quote

     &rdquo; ('')
        right double quote

     &nbsp; ( )
        non-breaking space

     &shy; ()
        soft hyphen

  3.3.  Verbatim and Code Environments

  While we're on the subject of special characters, we might as well
  mention the verbatim ``environment'' used for including literal text
  in the output (with spaces and indentation preserved, and so on). The
  verb element is used for this; it looks like the following:

       <verb>
        Some literal text to include as example output.
       </verb>

  The verb environment doesn't allow you to use everything within it
  literally. Specifically, you must do the following within verb
  environments.

  o  Use &ero; to get an ampersand,

  o  Use &etago; to get </,

  o  Don't use \end{verbatim} within a verb environment, as this is what
     LaTeX uses to end the verbatim environment. (In the future, it
     should be possible to hide the underlying text formatter entirely,
     but the parser doesn't support this feature yet.)

     The code environment is much just like the verb environment, except
     that horizontal rules are added to the surrounding text, as so:

     ___________________________________________________________________
     Here is an example code environment.
     ___________________________________________________________________

  You should use the tscreen environment around any verb environments,
  as so:

       <tscreen><verb>
       Here is some example text.
       </verb></tscreen>

  tscreen is an environment that simply indents the text and sets the
  sets the default font to tt. This makes examples look much nicer, both
  in the LaTeX and plain text versions. You can use tscreen without
  verb, however, if you use any special characters in your example
  you'll need to use both of them. tscreen does nothing to special
  characters. See example.sgml for examples.

  The quote environment is like tscreen, except that it does not set the
  default font to tt. So, you can use quote for non-computer-interaction
  quotes, as in:

       <quote>
       Here is some text to be indented, as in a quote.
       </quote>

  which will generate:

       Here is some text to be indented, as in a quote.

  3.4.  Overall Document Structure

  Before we get too in-depth with details, we're going to describe the
  overall structure of an LinuxDoc-Tools document. Look at example.sgml
  for a good example of how a document is set up.

  3.4.1.  The Preamble

  In the document ``preamble'' you set up things such as the title
  information and document style:

       <!doctype linuxdoc system>

       <article>

       <title>Linux Foo HOWTO
       <author>Norbert Ebersol, <tt/norb@baz.com/
       <date>v1.0, 9 March 1994
       <abstract>
       This document describes how to use the <tt/foo/ tools to frobnicate
       bar libraries, using the <tt/xyzzy/ relinker.
       </abstract>

       <toc>

  The elements should go more or less in this order. The first line
  tells the SGML parser to use the linuxdoc DTD. We'll explain that in
  the later section on ``How LinuxDoc-Tools Works''; for now just treat
  it as a bit of necessary magic. The <article> tag forces the document
  to use the ``article'' document style.

  The title, author, and date tags should be obvious; in the date tag
  include the version number and last modification time of the document.

  The abstract tag sets up the text to be printed at the top of the
  document, before the table of contents. If you're not going to include
  a table of contents (the toc tag), you probably don't need an
  abstract.

  3.4.2.  Sectioning And Paragraphs

  After the preamble, you're ready to dive into the document. The
  following sectioning commands are available:

  o  sect: For top-level sections (i.e. 1, 2, and so on.)

  o  sect1: For second-level subsections (i.e. 1.1, 1.2, and so on.)

  o  sect2: For third-level subsubsections.

  o  sect3: For fourth-level subsubsubsections.

  o  sect4: For fifth-level subsubsubsubsections.

     These are roughly equivalent to their LaTeX counterparts section,
     subsection, and so on.

  After the sect (or sect1, sect2, etc.) tag comes the name of the
  section. For example, at the top of this document, after the preamble,
  comes the tag:

       <sect>Introduction

  And at the beginning of this section (Sectioning and paragraphs),
  there is the tag:

       <sect2>Sectioning And Paragraphs

  After the section tag, you begin the body of the section. However, you
  must start the body with a <p> tag, as so:

       <sect>Introduction
       <p>
       This is a user's guide to the LinuxDoc-Tools document processing...

  This is to tell the parser that you're done with the section title and
  are ready to begin the body. Thereafter, new paragraphs are started
  with a blank line (just as you would do in TeX). For example,

       Here is the end of the first paragraph.

       And we start a new paragraph here.

  There is no reason to use <p> tags at the beginning of every
  paragraph; only at the beginning of the first paragraph after a
  sectioning command.

  3.4.3.  Ending The Document

  At the end of the document, you must use the tag:

       </article>

  to tell the parser that you're done with the article element (which
  embodies the entire document).

  3.5.  Internal Cross-References

  Now we're going to move onto other features of the system.  Cross-
  references are easy. For example, if you want to make a cross-
  reference to a certain section, you need to label that section as so:

       <sect1>Introduction<label id="sec-intro">

  You can then refer to that section somewhere in the text using the
  expression:

       See section <ref id="sec-intro" name="Introduction"> for an introduction.

  This will replace the ref tag with the section number labeled as sec-
  intro. The name argument to ref is necessary for groff and HTML
  translations. The groff macro set used by LinuxDoc-Tools does not
  currently support cross-references, and it's often nice to refer to a
  section by name instead of number.

  For example, this section is ``Cross-References''.

  Some back-ends may get upset about special characters in reference
  labels.  In particular, latex2e chokes on underscores (though the
  latex back end used in older versions of this package didn't). Hyphens
  are safe.

  3.6.  Web References

  There is also a url element for Universal Resource Locators, or URLs,
  used on the World Wide Web. This element should be used to refer to
  other documents, files available for FTP, and so forth. For example,

       You can get the Linux HOWTO documents from
       <url url="http://sunsite.unc.edu/mdw/HOWTO/"
          name="The Linux HOWTO INDEX">.

  The url argument specifies the actual URL itself. A link to the URL in
  question will be automatically added to the HTML document.  The
  optional name argument specifies the text that should be anchored to
  the URL (for HTML conversion) or named as the description of the URL
  (for LaTeX and groff). If no name argument is given, the URL itself
  will be used.

  A useful variant of this is htmlurl, which suppresses rendering of the
  URL part in every context except HTML. What this is useful for is
  things like a person's email addresses; you can write

       <htmlurl url="mailto:esr@snark.thyrsus.com"
             name="esr@snark.thyrsus.com">

  and get ``esr@snark.thyrsus.com'' in text output rather than the
  duplicative ``esr@snark.thyrsus.com <mailto:esr@snark.thyrsus.com>''
  but still have a proper URL in HTML documents.

  3.7.  Fonts

  Essentially, the same fonts supported by LaTeX are supported by
  LinuxDoc-Tools. Note, however, that the conversion to plain text
  (through groff) does away with the font information. So, you should
  use fonts as for the benefit of the conversion to LaTeX, but don't
  depend on the fonts to get a point across in the plain text version.

  In particular, the tt tag described above can be used to get constant-
  width ``typewriter'' font which should be used for all e-mail
  addresses, machine names, filenames, and so on.  Example:

       Here is some <tt>typewriter text</tt> to be included in the document.

  Equivalently:

       Here is some <tt/typewriter text/ to be included in the document.

  Remember that you can only use this abbreviated form if the enclosed
  text doesn't contain slashes.

  Other fonts can be achieved with bf for boldface and em for italics.
  Several other fonts are supported as well, but we don't suggest you
  use them, because we'll be converting these documents to other formats
  such as HTML which may not support them.  Boldface, typewriter, and
  italics should be all that you need.

  3.8.  Lists

  There are various kinds of supported lists. They are:

  o  itemize for bulleted lists such as this one.

  o  enum for numbered lists.

  o  descrip for ``descriptive'' lists.

     Each item in an itemize or enum list must be marked with an item
     tag. Items in a descrip are marked with tag.  For example,

       <itemize>
       <item>Here is an item.
       <item>Here is a second item.
       </itemize>

  Looks like this:

  o  Here is an item.

  o  Here is a second item.

     Or, for an enum,

       <enum>
       <item>Here is the first item.
       <item>Here is the second item.
       </enum>

  You get the idea. Lists can be nested as well; see the example
  document for details.

  A descrip list is slightly different, and slightly ugly, but you might
  want to use it for some situations:

       <descrip>
       <tag/Gnats./ Annoying little bugs that fly into your cooling fan.
       <tag/Gnus./ Annoying little bugs that run on your CPU.
       </descrip>

  ends up looking like:

     Gnats.
        Annoying little bugs that fly into your cooling fan.

     Gnus.
        Annoying little bugs that run on your CPU.

  3.9.  Conditionalization

  The overall goal of LinuxDoc-tools is to be able to produce from one
  set of masters output that is semantically equivalent on all back
  ends.  Nevertheless, it is sometimes useful to be able to produce a
  document in slightly different variants depending on back end and
  version.  LinuxDoc-Tools supports this through the <#if> and <#unless>
  bracketing tags.

  These tags allow you to selectively include and uninclude portions of
  an SGML master in your output, depending on filter options set by your
  driver. Each tag may include a set of attribute/value pairs. The most
  common are ``output'' and ``version'' (though you are not restricted
  to these) so a typical example might look like this:

       Some <#if output=latex2e version=drlinux>conditional</#if> text.

  Everything from this <#if> tag to the following </#if> would be
  considered conditional, and would not be included in the document if
  either the filter option ``output'' were set to something that doesn't
  match ``latex2e'' or the filter option ``version'' were set to
  something that doesn't match ``drlinux''. The double negative is
  deliberate; if no ``output'' or ``version'' filter options are set,
  the conditional text will be included.

  Filter options are set in one of two ways. Your format driver sets the
  ``output'' option to the name of the back end it uses; thus, in
  particular, ``linuxdoc -B latex'' sets ``output=latex2e'', Or you may
  set an attribute-value pair with the ``-D'' option of your format
  driver. Thus, if the above tag were part of a file a file named
  ``foo.sgml'', then formatting with either

       % linuxdoc -B latex -D version=drlinux foo.sgml

  or

       % linuxdoc -B latex foo.sgml

  would include the ``conditional'' part, but neither

       % linuxdoc -B html -D version=drlinux foo.sgml

  nor

       % linuxdoc -B latex -D private=book foo.sgml

  would do so.

  So that you can have conditionals depending on one or more of several
  values matching, values support a simple alternation syntax using
  ``|''. Thus you could write:

       Some <#if output="latex2e|html" version=drlinux>conditional</#if> text.

  and formatting with either ``-B latex'' or ``-B html'' will include
  the ``conditional'' text (but formatting with, say, ``-B txt'' will
  not).

  The <#unless> tag is the exact inverse of <#if>; it includes when
  <#if>; would exclude, and vice-versa.

  Note that these tags are implemented by a preprocessor which runs
  before the SGML parser ever sees the document. Thus they are
  completely independent of the document structure, are not in the DTD,
  and usage errors won't be caught by the parser. You can seriously
  confuse yourself by conditionalizing sections that contain unbalanced
  bracketing tags.

  The preprocessor implementation also means that standalone SGML
  parsers will choke on LinuxDoc-Tools documents that contain
  conditionals.  However, you can validity-check them with ``linuxdoc -B
  check''.

  Also note that in order not to mess up the source line numbers in
  parser error messages, the preprocessor doesn't actually throw away
  everything when it omits a conditionalized section. It still passes
  through any newlines. This leads to behavior that may suprise you if
  you use <if> or <unless> within a <verb> environment, or any other
  kind of bracket that changes SGML's normal processing of whitespace.

  These tags are called ``#if'' and ``#unless'' (rather than ``if'' and
  ``unless'') to remind you that they are implemented by a preprocessor
  and you need to be a bit careful about how you use them.

  3.10.  Index generation

  To support automated generation of indexes for book publication of
  SGML masters, LinuxDoc-Tools supports the <idx> and <cdx> tags. These
  are bracketing tags which cause the text between them to be saved as
  an index entry, pointing to the page number on which it occurs in the
  formatted document. They are ignored by all backends except LaTeX,
  which uses them to build a .ind file suitable for processing by the
  TeX utility makeindex.

  The two tags behave identically, except that <idx> sets the entry in a
  normal font and <cdx> in a constant-width one.

  If you want to add an index entry that shouldn't appear in the text
  itself, use the <nidx> and <ncdx> tags.

  3.11.  Controlling justification

  In order to get proper justification and filling of paragraphs in
  typeset output, LinuxDoc-Tools includes the &shy; entity. This becomes
  an optional or `soft' hyphen in back ends like latex2e for which this
  is neaningful.

  The bracketing tag <file> can be used to surround filenames in running
  text. It effectively inserts soft hyphens after each slash in the
  filename.

  One of the advantages of using the <url> and <htmlurl> tags is that
  they do likewise for long URLs.

  4.  Formatting SGML Documents

  Let's say you have the SGML document foo.sgml, which you want to
  format.  Here is a general overview of formatting the document for
  different output.  For a complete list of options, consult the man
  pages.

  4.1.  Checking SGML Syntax

  If you just want to capture your errors from the SGML conversion, use
  the ``linuxdoc -B check''. For example.

       % linuxdoc -B check foo.sgml

  If you see no output from this check run other than the
  ``Processing...'' message, that's good. It means there were no errors.

  4.2.  Creating Plain Text Output

  If you want to produce plain text, use the command:

       % linuxdoc -B txt foo.sgml

  You can also create groff source for man pages, which can be formatted
  with groff -man. To do this, do the following:

       % linuxdoc -B txt --man foo.sgml

  4.3.  Creating LaTeX, DVI, PostScript or PDF Output

  To create a LaTeX documents from the SGML source file, simply run:

       % linuxdoc -B latex foo.sgml

  If you want to produce PostScript output (via dvips), use the ``-o''
  option:

       % linuxdoc -B latex --output=ps foo.sgml

  Or you can produce a DVI file:

       % linuxdoc -B latex --output=dvi foo.sgml

  Also, you can produce a PDF file:

       % linuxdoc -B latex --output=pdf foo.sgml

  4.4.  Creating HTML Output

  If you want to produce HTML output, do this:

       % linuxdoc -B html --imagebuttons foo.sgml

  This will produce foo.html, as well as foo-1.html, foo-2.html, and so
  on -- one file for each section of the document. Run your WWW browser
  on foo.html, which is the top level file. You must make sure that all
  of the HTML files generated from your document are all installed in
  the directory, as they reference each other with local URLs.

  The ``--imagebuttons'' option tells html backend driver to use graphic
  arrows as navigation buttons. The names of these icons are
  ``next.png'', ``prev.png'', and ``toc.png'', and the LinuxDoc-Tools
  system supplies appropriate PNGs in its library directory.

  If you use ``linuxdoc -B html'' without the ``-img'' flag, HTML
  documents will by default have the English labels ``Previous'',
  ``Next'', and ``Table of Contents'' for navigation.  If you specify
  one of the accepted language codes in a ``--language'' option,
  however, the labels will be given in that language.

  4.5.  Creating GNU Info Output

  If you want to format your file for the GNU info browser, just run the
  following command:

       % linuxdoc -B info foo.sgml

  4.6.  Creating LyX Output

  For LyX output, use the the command:

       % linuxdoc -B lyx foo.sgml

  4.7.  Creating RTF Output

  If you want to produce RTF output, run the command:

       % linuxdoc -B rtf foo.sgml

  This will produce foo.rtf, as well as foo-1.rtf, foo-2.rtf, and so on;
  one file for each section of the document.

  5.  Internationalization Support

  The ISO 8859-1 (latin1) character set may be used for international
  characters in plain text, LaTeX, HTML, LyX, and RTF output (GNU info
  support for ISO 8859-1 may be possible in the future). To use this
  feature, give the formatting scripts the ``--charset=latin'' flag, for
  example:

       % linuxdoc -B txt --charset=latin foo.sgml

  You also can use ISO 8859-1 characters in the SGML source, they will
  automatically be translated to the proper escape codes for the
  corresponding output format.

  Currently, EUC-JP (ujis) character set is partially supported.  Source
  SGML file using this character set can be converted in plain text,
  HTML, and LaTeX. Other output formats are not tested fully.

  6.  How LinuxDoc-Tools Works

  Technically, the tags and conventions we've explored in previous
  sections of this use's guide are what is called a markup language -- a
  way to embed formatting information in a document so that programs can
  do useful things with it. HTML, Tex, and Unix manual-page macros are
  well-known examples of markup languages.

  6.1.  Overview of SGML

  LinuxDoc-Tools uses a way of describing markup languages called SGML
  (Standard Generalized Markup Language). SGML itself doesn't describe a
  markup language; rather, it's a language for writing specifications
  for markup languages. The reason SGML is useful is that an SGML markup
  specification for a language can be used to generate programs that
  ``know'' that language with much less effort (and a much lower
  bugginess rate!) than if they had to be coded by hand.

  In SGML jargon, a markup language specification is called a ``DTD''
  (Document Type Definition). A DTD allows you to specify the structure
  of a kind of document; that is, what parts, in what order, make up a
  document of that kind. Given a DTD, an SGML parser can check a
  document for correctness. An SGML-parser/DTD combination can also make
  it easy to write programs that translate that structure into another
  markup language -- and this is exactly how LinuxDoc-Tools actually
  works.

  LinuxDoc-Tools provides a SGML DTD called ``linuxdoc'' and a set of
  ``replacement files'' which convert the linuxdoc documents to groff,
  LaTeX, HTML, GNU info, LyX, and RTF source. This is why the example
  document has a magic cookie at the top of it that says ``linuxdoc
  system''; that is how one tells an SGML parser what DTD to use.

  Actually, LinuxDoc-Tools provides a couple of closely related DTDs.
  But the ones other than linuxdoc are still experimental, and you
  probably do not want to try working with them unless you are an
  LinuxDoc-Tools guru.

  If you are an SGML guru, you may find it interesting to know that the
  LinuxDoc-Tools DTDs are based heavily on the QWERTZ DTD by Tom Gordon,
  thomas.gordon@gmd.de.

  If you are not an SGML guru, you may not know that HTML (the markup
  language used on the World Wide Web) is itself defined by a DTD.

  6.2.  How SGML Works

  An SGML DTD like linuxdoc specifies the names of ``elements'' within a
  document type. An element is just a bit of structure; like a section,
  a subsection, a paragraph, or even something smaller like emphasized
  text.

  Unlike in LaTeX, however, these elements are not in any way intrinsic
  to SGML itself. The linuxdoc DTD happens to define elements that look
  a lot like their LaTeX counterparts---you have sections, subsections,
  verbatim ``environments'', and so forth. However, using SGML you can
  define any kind of structure for the document that you like. In a way,
  SGML is like low-level TeX, while the linuxdoc DTD is like LaTeX.

  Don't be confused by this analogy. SGML is not a text-formatting
  system.  There is no ``SGML formatter'' per se. SGML source is only
  converted to other formats for processing. Furthermore, SGML itself is
  used only to specify the document structure. There are no text-
  formatting facilities or ``macros'' intrinsic to SGML itself. All of
  those things are defined within the DTD. You can't use SGML without a
  DTD, a DTD defines what SGML does.

  6.3.  What Happens When LinuxDoc-Tools Processes A Document

  Here's how processing a document with LinuxDoc-Tools works. First, you
  need a DTD, which sets up the structure of the document. A small
  portion of the normal (linuxdoc) DTD looks like this:

       <!element article - -
           (titlepag, header?,
            toc?, lof?, lot?, p*, sect*,
            (appendix, sect+)?, biblio?) +(footnote)>

  This part sets up the overall structure for an ``article'', which is
  like a ``documentstyle'' within LaTeX. The article consists of a
  titlepage (titlepag), an optional header (header), an optional table
  of contents (toc), optional lists of figures (lof) and tables (lot),
  any number of paragraphs (p), any number of top-level sections (sect),
  optional appendices (appendix), an optional bibliography (biblio) and
  footnotes (footnote).

  As you can see, the DTD doesn't say anything about how the document
  should be formatted or what it should look like. It just defines what
  parts make up the document. Elsewhere in the DTD the structure of the
  titlepag, header, sect, and other elements are defined.

  You don't need to know anything about the syntax of the DTD in order
  to write documents. We're just presenting it here so you know what it
  looks like and what it does. You do need to be familiar with the
  document structure that the DTD defines. If not, you might violate the
  structure when attempting to write a document, and be very confused
  about the resulting error messages.

  The next step is to write a document using the structure defined by
  the DTD. Again, the linuxdoc DTD makes documents look a lot like LaTeX
  or HTML -- it's very easy to follow. In SGML jargon a single document
  written using a particular DTD is known as an ``instance'' of that
  DTD.

  In order to translate the SGML source into another format (such as
  LaTeX or groff) for processing, the SGML source (the document that you
  wrote) is parsed along with the DTD by the SGML parser. LinuxDoc-Tools
  uses the onsgmls parser in OpenJade, or nsgmls parser in Jade.  The
  former is the successor of the latter. sgmls parser was written by
  James Clark, jjc@jclark.com, who also happens to be the author of
  groff. We're in good hands.  The parser (onsgmls or nsgmls) simply
  picks through your document and verifies that it follows the structure
  set forth by the DTD.  It also spits out a more explicit form of your
  document, with all ``macros'' and elements expanded, which is
  understood by sgmlsasp, the next part of the process.

  sgmlsasp is responsible for converting the output of sgmls to another
  format (such as LaTeX). It does this using replacement files, which
  describe how to convert elements in the original SGML document into
  corresponding source in the ``target'' format (such as LaTeX or
  groff).

  For example, part of the replacement file for LaTeX looks like:

       <itemize>    +    "\\begin{itemize}   +
       </itemize>   +    "\\end{itemize}    +

  Which says that whenever you begin an itemize element in the SGML
  source, it should be replaced with

       \begin{itemize}

  in the LaTeX source. (As I said, elements in the DTD are very similar
  to their LaTeX counterparts).

  So, to convert the SGML to another format, all you have to do is write
  a new replacement file for that format that gives the appropriate
  analogies to the SGML elements in that new format. In practice, it's
  not that simple---for example, if you're trying to convert to a format
  that isn't structured at all like your DTD, you're going to have
  trouble. In any case, it's much easier to do than writing individual
  parsers and translators for many kinds of output formats; SGML
  provides a generalized system for converting one source to many
  formats.

  Once sgmlsasp has completed its work, you have LaTeX source which
  corresponds to your original SGML document, which you can format using
  LaTeX as you normally would.

  6.4.  Further Information

  o  The QWERTZ User's Guide is available from
     ftp://ftp.cs.cornell.edu/pub/mdw/SGML.  QWERTZ (and hence,
     LinuxDoc-Tools) supports many features such as mathematical
     formulae, tables, figures, and so forth.  If you'd like to write
     general documentation in SGML, I suggest using the original QWERTZ
     DTD instead of the hacked-up linuxdoc DTD, which I've modified for
     use particularly by the Linux HOWTOs and other such documentation.

  o  Tom Gordon's original QWERTZ tools can be found at
     ftp://ftp.gmd.de/GMD/sgml.

  o  More information on SGML can be found at the following WWW pages:

     1. SGML and the Web <http://www.w3.org/hypertext/WWW/MarkUp/SGML/>

     2. SGML Web Page <http://www.sil.org/sgml/sgml.html>

     3. Yahoo's SGML Page
        <http://www.yahoo.com/Computers_and_Internet/Software/Data_Formats/SGML>

  o  James Clark's sgmls parser, and it's successor nsgmls and other
     tools can be found at ftp://ftp.jclark.com and at James Clark's WWW
     Page <http://www.jclark.com>.

  o  The emacs psgml package can be found at
     ftp://ftp.lysator.liu.se/pub/sgml. This package provides a lot of
     SGML functionality.

  o  More information on LyX can be found at the LyX WWW Page
     <http://wsiserv.informatik.uni-tuebingen.de/~ettrich/>. LyX is a
     high-level word processor frontend to LaTeX. Quasi-WYSIWYG
     interface, many LaTeX styles and layouts automatically generated.
     Speeds up learning LaTeX and makes complicated layouts easy and
     intuitive.

