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&gt;       Elements of SGML
&gt;       by 
&gt;       Jonathan Fine                           11 October 1994
&gt;
&gt;       203 Coldhams Lane
&gt;       Cambridge
&gt;       CB1 3HY
&gt;       Tel:0223 215389
&gt;       Email: J.Fine@pmms.cam.ac.uk

This is the LaTeX file for the first of a series of articles which
will appear in "Baskerville", which is the journal of the UK TeX
User's Group.  Comments are welcome.
\fi

\title{Elements of SGML}
\author[Jonathan Fine]{Jonathan Fine\\
203 Coldhams Lane\\ Cambridge CB1 3HY\\\texttt{J.Fine@pmms.cam.ac.uk}}
\begin{Article}

\noindent This is the first of a series of articles on various aspects
of SGML.  It is intended to be a general introduction.  Subsequent
articles will discuss the SGML concepts of document elements and their
content, attributes, entities, markup minimization and data notation.
The author welcomes queries and requests. He is also available for
professional consultation.

\subsection{Introduction}
Here is the opening paragraph of the SGML standard:
\begin{quotation}
\noindent
This International Standard specifies a language for document
representation referred to as the ``Standard Generalized Markup
Language'' (SGML).  SGML can be used for publishing in its broadest
definition, ranging from single medium conventional publishing to
multi-media data base publishing.  SGML can also be used in office
document processing when the benefits of human readability and
interchange with publishing systems are required.

\raggedleft
from \em Clause 0, Introduction, ISO 8879, October 1986
\end{quotation}

Necessarily, this article will give an incomplete picture of SGML.
Here are five professional activities involved in modern publishing. 
It is the \emph{author} whose words are published.  Most of the time
we assume for simplicity that it is the author who keys the
manuscript into a computer, creating what I shall call a {\em
compuscript}, or \emph{script} for short.  This assumption is of
course not true for Shakespeare and many other authors.  The {\em
designer} will establish the structure of the author's work---perhaps
retrospectively---and establish standards for its printed
representation.  The \emph{typesetter} or application programmer will
cause software tools to produce from the author's script printed
pages or whatever meeting the designer's requirements.  These software
tools will have been created by an \emph{implementor} or systems
programmer.  The \emph{publisher} will have an overall responsibility
for, and financial interest in, the whole process.

This quintet---author, designer, typesetter, implementor, and
publisher---are all involved in the production of a book before it
goes to printer and binder. This production process is generally of little
concern (except when it goes wrong) to the final sixth party, the
\emph{reader}.  However, SGML can be used to offer the reader new
electronically published products.

In this article I shall show you, in its entirety, an extremely
simple SGML document.  By and large I will take the author's point of
view, in part in hope of alleviating any distrust there may be
amongst the humanities towards technology.  I hope that the more
technically minded will bear with me during this apparently pedestrian
exposition.  They may wish to reflect on how SGML allows cooperation
and division of responsibility within the production process.

\section{Field of Application}
It is useful to know what SGML can be used for, and what lies outside
its province.
\begin{quotation}
\noindent
The Standard Generalized Markup Language can be used for documents
that are processed by any text processing or word processing system. 
It is particularly applicable to:
\begin{list}{}{}
\item a) Documents that are interchanged among systems with differing
text processing languages.
\item b) Documents that are processed in more than one way, even when
the procedures use the same text processing language.
\end{list}

Documents that exist solely in final imaged form are not within the
field of application of this International Standard.

\raggedleft
\em Clause 2, Field of Application, ISO 8879
\end{quotation}

This is the whole of Clause~2 of the Standard, which altogether has
15~clauses and 9~annexes.  Unfortunately, they are all much harder
and longer than this clause.  This clause may be all that the
publisher needs to know.

It is well worth noting that SGML is first of all a standard for {\em
documents}, of particular use when documents are interchanged among
systems (the author sends the compuscript to the publisher who sends
it on to the typesetter) or processed in several ways (for paper or
electronic publication, in a journal or a book, or extracts in a
secondary journal, or even just a second edition).  Just as the ASCII
character codes provide a standard for the expression and thus
interchange of sequences of characters, so SGML is to provide a
standard for the expression of structured or marked-up documents. 
However (note to Clause~1) the SGML International Standard does not
specify standard document types, or standard SGML applications, nor
the implementation or architecture of either the application or the
electronic storage representation of the documents.  It is an
abstract standard for documents, deliberately indifferent to the
specifics of application and implementation.  

\section{Hello world!}
It is traditional to begin the explanation of a computer language
with the code that will print the words:
\begin{verbatim}
Hello world!
\end{verbatim}
say on the user's screen.  An author might object to using a computer
language to write the great English novel.  But SGML is quite unlike
other computer languages, and anyway, all word processors use their
own computer language to represent your documents.  Do you know what
they are doing to your words?  With SGML you do, or at least can if
you wish to.  You don't need to be a programmer to write an SGML
document.  SGML is not even a programming language---it is a document
structure and markup language.  Moreover, SGML does not understand
the English language, and so will not correct your spelling or
criticize your plot.

Instead, with SGML you make statements, which are called {\em
declarations}.  Most other computer languages are concerned with
giving instructions to the computer.  However, SGML does little more
than record your declarations.  It also checks that you are doing
only what you allowed yourself to do.  Though this may sound onerous,
according to Hegel ``one who will do something great must learn to
limit oneself."  It can be a healthy discipline.

Back to our example.  We wish to express
\begin{verbatim}
Hello world!
\end{verbatim}
in SGML.  If this is too trivial for you, replace this text by
Shakespeare's sonnet
\begin{verbatim}
Shall I compare thee to a summer's day?
\end{verbatim}
or even the collection of his sonnets, complete with editorial and
critical apparatus, and publishing information.  It is to such that
we wish to add markup, which is defined in the Standard as \emph{text
that is added to the data of a document in order to convey
information about it.} We shall mark up our message so that it is an
SGML document. 

What sort of document is it?  It is a message.  So we mark it up as
\begin{verbatim}
&lt;message&gt;Hello world!&lt;/message&gt;
\end{verbatim}
where the added text is markup, sharing with the computer our
knowledge, that the original text is a message.  The \emph{content} of
the message \emph{element} is our original text, which lies between
the \emph{start-tag} and the \emph{end-tag}.

Although the author may be satisfied by this, the programmer or
typesetter will not.  This person would like to know, without reading
the whole document (which might be quite long and perhaps still in
progress), what elements (tags) are to be found in it.  And the
author might like to be told when a tag name (called for historic
reasons the \emph{generic identifier}) has been misspelt.  Before we
use a tag---or any other textual markup---we must first declare that
it exists to be used.

Although it may appear to be overly fussy, it is arguably to
everyone's benefit that all declarations required for a document
should appear before any of its text.  In any case, SGML insists that
a document consists of a \emph{prolog} (which contains all the markup
declarations) followed by the \emph{document instance}, which is the
author's text, marked up in conformity to the declarations in the
prolog.  This is important.  To repeat: an SGML document consists of
the prolog followed by the conforming document instance.  It is this
requirement which allows documents to be interchanged among systems.

Before our marked up document instance
\begin{verbatim}
&lt;message&gt;Hello world!&lt;/message&gt;
\end{verbatim}
can be allowed, we must create a prolog which declares the markup
construction(s) that can be used.  The text to do this is here 
needlessly verbose, except that it will later enable the powerful
CONCUR and LINK capabilities of SGML.

Here is our message, marked up as an SGML document.
\begin{verbatim}
&lt;!DOCTYPE message [
  &lt;!ELEMENT message ANY &gt;
]&gt;

&lt;message&gt;Hello world!&lt;/message&gt;
\end{verbatim}

There are four occurrences of the character string \verb"message".  The
first tells us that the document instance to follow is to consist of
a \verb"message" element.  The second tells us that there are no
restrictions on what may appear in a \verb"message" element. 
\verb"ANY" words or characters, \verb"ANY" elements, and \verb"ANY"
other markup constructions, repeated as often as one likes, and in
\verb"ANY" order.  The third and fourth delimit the content of the
\verb"message" element, which the first occurrence of \verb"message"
had promised.

There are 55 \emph{reserved names} such as \verb"DOCTYPE", \verb"ELEMENT"
and \verb"ANY" in SGML, which have special r\^oles in the prolog,
and also some special character strings.  Neither the author nor the
publisher needs to know what they are or what they do.

For more complicated documents, particularly those that are to conform
to a house style, the prolog---which declares the elements and
structure of the document---cannot be left to chance.  In particular,
the author should be relieved of responsibilty for the prolog, and
not given the impression that it is something that he or she could
change, if they so wished.  (This last remark is directed to the
publisher.)  The designer (or someone else) will create a set of
declarations which the author is able to invoke simply by placing
a line such as:
\begin{verbatim}
&lt;!DOCTYPE message SYSTEM "message.dtd" &gt;
\end{verbatim}
at the top of the compuscript.  The publisher (or someone else)
should supply the author with guidance and examples as to how the
document structure declared by the designer is to be used.  The
author should not need to consult the invoked prolog.  Ideally, the
publisher's tag set and its description ideally should, together with
general SGML guidance, be enough to allow the author to mark up the
document instance.  (A proviso.  Specialised \emph{data content
notations}, such as for mathematics, may require additional non-SGML

\section{But wait, there's more}
The designers of SGML wanted a scheme able to encode the most
complicated document structures, which was at the same time easy to
learn and implement for the simpler documents.  They did this by
giving SGML a number of parameters and optional features, to be set
even before the prolog was read.  For example, the powerful CONCUR
feature allows a single document to support two independent tagging
schemes!  For a historic printed (or manuscript) Bible or other text
one might wish to record not only the division of the text into books
and verses, but also into pages and lines!

Each SGML document should, to be really official, begin with an
SGML declaration, whose purpose is to state which of the optional
features are in use, that \verb"&lt;" and \verb"&lt;/" (or some other
character strings) are used to open start- and end- tags, and so
forth.

The SGML declaration will occupy about a page of text.  It begins
\begin{verbatim}
&lt;!SGML "ISO 8879:1986"
\end{verbatim}
which specifies the version of SGML being used.  Most likely, as it
wasn't born perfect, there will at some future time be a revision of
SGML, but for now there is only one version to choose from.  It is
expected that many many documents will share the same SGML
declaration, and that all documents on a single system will share the
same declaration. For these reasons, the standard allows the system
to \emph{imply} the SGML declaration, which means that should no SGML
declaration be supplied by the document, the system can provide one
of its own choosing.  The prolog and document instance which follow
had better conform to the chosen SGML declaration.  There is also an
SGML declaration for each SGML application, which records its SGML
capabilities.

Once the various features of SGML have been understood, the SGML
declaration becomes a control panel that turns them on and off, sets
delimiter strings such as \verb"&lt;" and \verb"&lt;/" and \verb"&lt;!", and
describes the character set used by the document.  Such is a {\em
concrete syntax}.  Until then, it is perhaps best left to one side. 
The standard contains two SGML declarations, which define the {\em
reference} and \emph{core} concrete syntaxes respectively, at least
one of which (Clause~15.3.3) must be supported by any conforming SGML
system.

\section{Summary}

In this article we have seen a complete SGML document, from prolog to
the final end-tag, and we even had a glimpse of the implied SGML
declaration.  Different people have different interests in the
document.  The implementor is particularly interested in the SGML
declaration.  The designer will control the prolog.  The author cares
about the words in the document instance.  The typesetter has broad
concerns.  He or she must understand the prolog and thus the tags in
the document instance, the typographic qualities of the author's
words (but not their meaning, if any), the tools supplied by the
implementor, and also the designer's output requirements. The
publisher is interested in everyone getting on with everyone else.

This article has said some about SGML, and some about the wider
context in which it is used.  The next will describe how to create
some more complicated documents.  We shall also look at SGML from the
typesetter or application programmer point of view.

\section{Exercises}
Finally, here are some exercises to test and develop your
comprehension and understanding.  First you might like to read
through the article again, pretending that you are some other person
involved in the publishing process.  Particularly someone with whom
you are having difficulties you do not understand or sympathise with
or, failing that, a colleague.  You might also like to write down all
the \emph{emphasised} definitions, and express them in your own words.

\smallskip
\noindent \textbf{Exercises}

\noindent \textbf{1. }
When was the SGML standard published?

\noindent \textbf{2. }
Who are the five persons involved in text processing?

\noindent \textbf{3. }
Who is the sixth?

\noindent \textbf{4. }
How does SGML differ from other computer languages?

\noindent \textbf{5. }
What is the purpose of the prolog?  Who writes it?

\noindent \textbf{6. }
What is the purpose of the SGML declaration?

\noindent \textbf{7. }
What is the purpose of the document instance?  Who writes it?

\noindent \textbf{8. }
How many lines in a sonnet?  Is that every sonnet?

\noindent \textbf{9. }
What is the difference between \emph{mark up} and \emph{markup}?

\noindent \textbf{10. }
What is a declaration?

\noindent \textbf{11. }
%% How is SGML is a compromise.  Between who?  Is this good, or bad?
How is SGML a compromise?  Between whom?  Is this good or bad?

\noindent \textbf{12. }
(Experts only)  In explaining SGML for beginners, have I made any
false statements?  
\smallskip

Answers in the next article.

\end{Article}
\endinput
\copyright{} 1994\qquad\qquad (this version 12 October 1994)\par
Jonathan Fine\par
203 Coldhams Lane Cambridge, CB1 3HY, England.\par
Tel: 0223 215389\par
Email: \verb"J.Fine@pmms.cam.ac.uk"\par

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