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% Filename: modguide.tex
 
\NeedsTeXFormat{LaTeX2e}[1994/12/01]
 
\documentclass{ltxguide}[1994/11/20]

\setcounter{secnumdepth}{-1}

\title{Modifying \LaTeXe}
 
\author{\copyright~Copyright 1995, \LaTeX3 Project Team.\\
   All rights reserved.}
 
\date{14 June 1995}
 
\begin{document}
 
\maketitle
 
\tableofcontents
 
\section{Introduction}
\label{sec:intro}

This document describes the principles underlying our policy on
distribution and modification of the files comprising the \LaTeX{}
system.  It has been produced as a result of wide-ranging discussions
of the issues involved in the support and maintenance of a widely
distributed document processing system used by diverse people for many
applications.  These discussions have involved users, maintainers of
installations that support \LaTeX{} and various types of organisations
that distribute it.  The discussions are continuing and we are hoping
that this document will be widely distributed and make a useful
contribution to the debate.

Our aim is to produce a system that which users of all types can trust
to fulfill their needs.  Such a system must be stable and
well-maintained.  This implies that it must be reasonably easy to
maintain it (otherwise it will simply not get maintained at all).
So here is a summary of our basic philosophy:
\begin{quotation}
  We believe that the freedom to rely on a widely-used standard for
  document interchange and formatting is as important as the freedom to
  experiment with the contents of files.

  We are therefore adopting a policy similar to that which Donald
  Knuth applies to modifications of the underlying \TeX{} system: that
  certain files, together with their names, are part of the system and
  therefore cannot be changed unless the following conditions are met:
  \begin{itemize}
  \item they are clearly marked as being no longer part of the
    standard system;
  \item the name of the file is changed.
  \end{itemize}
\end{quotation}


\section{The system}
\label{sec:sys}

In developing this philosophy, and the consequent limitations on how
modifications of the system should be carried out, we were heavily
influenced by the following facts concerning the current widespread
and wide-ranging uses of the \LaTeX{} system.
\begin{enumerate}
\item  \LaTeX{} is not just a document processing system;
  it also defines a language for document exchange.

\item The standard document class files, and some other files, also
  define a particular formatting of a document.

\item  The packages that we maintain define a particular document
  interface and, in some cases, particular formatting of parts of a
  document. 
  
\item  The interfaces between different parts of the \LaTeX{} system
  are very complex and it is therefore very difficult to check that a
  change to one file does not affect the functionality of both that
  file and also other parts of the system that are not obviously
  connected to the file that has been changed.
\end{enumerate}

This leads us to the general principle that:
\begin{quotation}
  with certain special exceptions, if you change the contents of a
  file then the changed version should have a different file name.
\end{quotation}

We certainly do not wish to prevent people from experimenting with the
code in different ways and adapting it to their purposes.  However, we
are concerned that any distribution of modifications to the code
should be very clearly identified as not being a part of the standard
distribution.  The exact wording and form of the distribution
conditions is thus something that is flexible, but only within the
constraint of keeping \LaTeX{} as a standardised, reliable product for
the purposes described above: the exchange and formatting of
documents.


\section{Some examples}
\label{sec:expl}

Here we elaborate the arguments that have led us to the above
conclusion.


\subsection{Separate development considered harmful!}
\label{sec:ja}

In many fields, the use of \LaTeX{} as a language for communication
is just as important as its capacity for fine typesetting; this is a
very important consideration for a large population of authors,
journal editors, archivists, etc.

Related to this issue of portability is the fact that the file names
are part of the end-user syntax.

To take a real example, the \LaTeX{} `tools' collection contains the
package `array.sty'.  A new user-level feature was added to this file
at the end of 1994 and a document using this feature can contain the
line:
\begin{verbatim}
   \usepackage{array}[1994/10/16]
\end{verbatim}
  
By supplying the optional argument, the document author is indicating
that a version of the file \texttt{array.sty} dated no earlier than
that date is required to run this document without error.

This feature would be totally worthless if we were to allow an
alternative version of the array package to be distributed under the
same name since it would mean that there would be in circulation files
of a later date, but without the new feature.  If the document
were processed using this `alternative array' then it would certainly
produce `undefined command' errors and would probably not be
processable at all.
 

\subsection{What's in a file-name?}
\label{sec:jb}

In a pure markup language, such as SGML, it is reasonably clear that
control over the final presentation lies with the receiver of a
document and not with the author.

However, the way that \LaTeX{} is often used in practice means that
most people (at least when using the standard classes and packages)
expect the formatting to be preserved when they send the document to
another site.

For example, suppose, as is still the most common use of \LaTeX{} in
publishing, you produce a document for `camera-ready-copy' using the
class `article' and that you carefully tune the formatting by, for
example, adding some explicit line breaks etc, to ensure that it fits
the 8 page limit set by the editor a journal or proceedings.

It then gets sent to the editor or a referee who, without anyone
knowing, has a non-standard version of the class file `article' and so
it then runs to 9 pages.  The consequence of this will, at the least,
be a lot of wasted time whilst everyone involved works out what has
gone wrong; it will probably also lead to everyone blaming each other
for something which was in fact caused by a misguided distribution
policy.

It should also be noted that, for most people, the version of the
class file `article' that gets used is decided by a site maintainer
or the compilers of a CD-ROM distribution.  To most users,
the symbols \texttt{a\,r\,t\,i\,c\,l\,e} in:
\begin{verbatim}
   \documentclass{article}
\end{verbatim}
are just as much part of \LaTeX{}'s syntax as are the symbols
\texttt{1\,2\,p\,t} in:
\begin{verbatim}
   \hspace{12pt}
\end{verbatim}
Thus they should both define a standard formatting rather than
sometimes producing 1 more page or a 5pt larger space.

Users rely on the fact that the command (or menu item) `LaTeX'
produces a completely standard LaTeX, including the fact that
`article' is the `standard article'.  They would not be at all happy
if the person who installed and maintains \LaTeX{} for them were
allowed to customise `article' every second day so as (in her or his
opinion) to improve the layout; or because another user wanted to write
a document in a different language or typeset one with different fonts.

\subsection{\TeX{} itself}
\label{sec:tex}

We have modelled our policies on those of the \TeX{} system since this
has for some time now been widely acknowledged as a very stable and
high quality typesetting system.

The distribution policy set up by Donald Knuth for \TeX{} has the
following features:
\begin{itemize}
\item There is a clearly specified method for changing parts of the
  software by the use of `change files'.
\item Although arbitrary changes are allowed, the resulting program
  can be called \TeX{} only if its functionality is precisely the same
  as that of \TeX{} (i.e.~neither less nor more) in all important
  areas.
\item There are many files in the system that cannot be changed at all
  (without changing the name): examples are the file plain.tex and the
  files associated with fonts, including the Metafont source files.
\end{itemize}


\subsection{Maintaining complexity}
\label{sec:compl}

Our experience of maintaining \LaTeX{} has shown us just how complex
are the interactions between different parts of the system.

We have therefore, with lots of help from the bug reports you send in,
developed a large suite of test files which we run to check the
effects of every change we make.  A non-negligible percentage of these
test runs give unexpected results and hence show up some unexpected
dependency in the system.


\section{Some assurances}
\label{sec:conc}

We are certainly not attempting to stop people reformatting \LaTeX{}
documents in any way they wish.  There are many ways of customising
incoming documents to your personal style that do not involve changing
the contents of \LaTeX{}'s standard files; indeed, this freedom is one
of the system's many advantages.
The simplest way to achieve this is to replace
\begin{quotation}
\verb|\documentclass{article}|\quad by\quad
\verb|\documentclass{myart}| 
\end{quotation}

Nor do we wish to discourage the production of new packages improving
on the functionality or implementation of those we distribute.  All we
ask is that, in the best interests of all \LaTeX{} users, you give
your superbly improved class or package file some other name. 


\section{What do you think?}
\label{sec:you}

We are interested in your views on the issues raised in this document.
The best way to let us know what you think, and to discuss your ideas
with others, is to join the \texttt{LaTeX-L} mailing list and send your
comments there.
To subscribe to this list, mail to:
\begin{verbatim}
   listserv@vm.urz.uni-heidelberg.de
\end{verbatim}
the following one line message:
\begin{verbatim}
   subscribe LATEX-L <your-first-name> <your-second-name>
\end{verbatim}

\end{document}