\def\fileversion{v1.0d}
\def\filedate{92/07/02}
\def\docdate{92/07/02}

%% \CheckSum{139}
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%%
%
%\iffalse   % this is a METACOMMENT !
%% Style-option `exercise' to use with LaTeX v2.09
%% Copyright (C) 1991 by Anselm Lingnau, all rights reserved.
%
% Copying of this file is authorized only if either
% (1) you make absolutely no changes to your copy, including name, or
% (2) if you do make changes, you name it something other than
%     exercise.doc.
% This restriction helps ensure that all styles developed here
% remain identical.
%
% Error reports please to: Anselm Lingnau
%                          Buchenweg 1
%                          D-6239 Eppstein
%                          Federal Republic of Germany
%                  Email:  <lingnau@math.uni-frankfurt.de>
%
%
%\fi
%
% \changes{v1.0d}{92/07/02}{New Internet address.}
% \changes{v1.0c}{91/10/08}{Write version number to transcript file only.}
% \changes{v1.0b}{91/10/08}{Polished the documentation.}
%
% \DoNotIndex{\@addtoreset,\@ifundefined,\@makeother,\@spaces,\@tempa}
% \DoNotIndex{\@warning,\^,\active,\arabic,\begin,\bf,\catcode}
% \DoNotIndex{\closeout,\csname,\def,\do,\docdate,\dospecials}
% \DoNotIndex{\edef,\else,\end,\endcsname,\endinput,\endtrivlist}
% \DoNotIndex{\errhelp,\errmessage,\expandafter,\fi,\filedate}
% \DoNotIndex{\fileversion,\ifx,\immediate,\input,\item,\jobname,\let}
% \DoNotIndex{\medbreak,\newcounter,\newif,\newenvironment,\newwrite}
% \DoNotIndex{\openout,\par,\refstepcounter,\relax,\renewcommand,\section}
% \DoNotIndex{\space,\string,\subsection,\the,\trivlist,\typeout,\wlog}
% \DoNotIndex{\write}
%
% \title{A \LaTeX\ Environment for Typesetting Exercises\thanks{%
%        This file has version number \fileversion{} dated \filedate.
%        The documentation was last revised on \docdate.
%       }}
% \def\rtitlex{A \LaTeX\ Environment for Typesetting Exercises}
% \thispagestyle{plain}
%
% \author{Anselm Lingnau}
% \address{Buchenweg 1\\
%         D-6239 Eppstein\\
%         Federal Republic of Germany}
% \netaddress[\network{Internet}]{lingnau@informatik.uni-frankfurt.de}
%
% \date{October 8, 1991}
%
% \maketitle
%
% \begin{abstract}
%   This style option implements a \LaTeX{} environment for
%   typesetting exercises.
%   The {\tt exercise} environment is modelled along the lines of
%   the exercise macros in {\sl The \TeX book},
%   i.\ e.\ the answer can be given right after the exercise.
%   Answers are collected in a file that can be included afterwards.
%   The macros presented here are based on Rainer Sch\"opf's
%   reimplementation of the {\tt verbatim} environment.
% \end{abstract}
%
% \section{Introduction}
%
% \TeX\ users everywhere are looking up to Donald Knuth's {\sl \TeX book\/}
% as a source of inspiration and good ideas.
% When I began writing a major software manual,
% I thought that it would be nice to have little exercises
% that would let the readers practise using the input language and so on.
% On pages 422--423,
% {\sl The \TeX book\/} lists some macros for exercises.
% Their most interesting feature is that the answer can be given
% immediately after the exercise itself.
% Thus exercises and answers always conveniently stay together,
% making it easy to insert, delete or move exercises without having to
% worry about numbering.
% The following example was lifted from {\sl The \TeX book\/}:
%\begin{verbatim}
%   \exercise After you have mastered the material in this book,
%   what will you be: A \TeX pert, or a \TeX nician?
%   \answer A \TeX nician (underpaid); sometimes also called
%   a \TeX acker.
%\end{verbatim}
% Obviously, these macros are geared towards \PlainTeX,
% and while they can be hacked to seemingly work with \LaTeX\ as well,
% I have experienced some nasty incompatibilities
% that didn't look straightforward to fix.
%
% Luckily, I came across Rainer Sch\"opf's reimplementation
% of the {\tt verbatim} environment \& Co.,
% and using this it was very easy to reimplement the exercise macros
% and make them fit within \LaTeX.
% The remainder of this document discusses the use and implementation
% of these macros.
%
% {\bf Warning:} This style option assumes
% that you have Rainer Sch\"opf's {\tt verbatim} macros.
% It won't work otherwise.
% You can get these macros from major software servers everywhere
% or from your local \TeX\ Users Group, e.\ g., DANTE e.\ V. in Germany. 
%
% \section{The User Interface}
%
% To use {\tt exercise.sty}, the style option {\tt exercise}
% must be included among the document style options.
% Be sure that you say {\tt verbatim} somewhere in the
% \verb|\documentstyle| command
% in order to load Rainer Sch\"opf's {\tt verbatim} stuff.
% Thus,
%\begin{verbatim}
%   \documentstyle[foo,verbatim,bar,exercise,baz]{article}
%\end{verbatim}
%
% \DescribeEnv{exercise}
% The {\tt exercise} environment is the most important part
% of our new style option.
% It will format its contents as an exercise,
% namely preceding it with an exercise number and, optionally,
% a ``title''. (Refer to the Implementation to find out how to change
% the formatting of exercise ``heads''.)
% The answer to the exercise is given right after the exercise itself,
% following the |\answer| command.
% Generally, an exercise looks like this:\\[2pt]
%   \hspace*{\MacroIndent}|\begin{exercise}|\\
%   \hspace*{\MacroIndent}\meta{The exercise itself}\\
%   \hspace*{\MacroIndent}|\answer|\\
%   \hspace*{\MacroIndent}\meta{The answer to this exercise}\\
%   \hspace*{\MacroIndent}|\end{exercise}|\\[2pt]
% The \meta{answer} is copied out to the file {\tt answers.tex}.
% This file can be read back in later with the |\listanswers| command.
%
% In {\sl The \TeX book}, the answer is finished by a blank line.
% With the {\tt exercise} environment, however, the answer includes
% everything up to the |\end{exercise}| command.
% Note that the |\answer| command may not occur inside another
% environment opened within the {\tt exercise}, that is, not at the same
% level of nesting as the |\begin{exercise}| \dots |\end{exercise}|
% commands. The implementation tries to detect this error and issues
% an appropriate diagnostic.
%
% Sometimes you may want to state an exercise without any answer
% (for example, a research problem).
% You can use the {\tt exercise} environment to typeset an
% answer-less exercise as well,
% thereby keeping the style and numbering of exercises uniform.
%
% \DescribeMacro{\listanswers}
% At some stage you will want to list the collected answers, presumably
% in an appendix of some sort. The |\listanswers| command will take
% care of that.
% You should say something along the lines of
%\begin{verbatim}
%   \appendix
%   \chapter{Answers to the Exercises}
%   Don't peek.
%   \listanswers
%\end{verbatim}
%
% \StopEventually{\makesignature}
%
% \section{The Implementation}
%
% As usual, the file identifies itself. Following Nico Poppelier's
% suggestion to Frank Mittelbach, the rather verbose headers are only written
% to the transcript file.
% \changes{v1.0c}{91/10/08}{Write version number to transcript file only.}
% We read this file only once.
%    \begin{macrocode}
\@ifundefined{exercise}{}{\endinput}
\wlog{Style-Option: `exercise'
   \fileversion \space <\filedate>  (AL)}
\wlog{English Documentation
   \@spaces \@spaces \space <\docdate>  (AL)}
%    \end{macrocode}
% We need a new counter for counting the exercises.
% Tricky manipulations are necessary because exercises are normally
% numbered within some sectional unit, e.\ g., the chapter.
% This, of course, is for the user to decide---we can't simply
% stick in a |\newcounter{exercise}[chapter]| because there's
% no way we'd be able to undo this, should our guess prove
% wrong\footnote{Oh well, I suppose we could always keep track
% of which sectional unit the exercises are numbered within.
% Then we could remove the entry from the corresponding list.
% What a mess.}.
% So, by default, the exercises are numbered consecutively throughout
% the whole document.
% (This is what you want when you're typing up homework assignments.)
%    \begin{macrocode}
\newcounter{exercise}
%    \end{macrocode}
%
% \begin{macro}{\numberexercises}
% The |\numberexercises| command lets the user specify
% where the exercises are actually supposed to be numbered.
% I. e., |\numberexercises{chapter}| will reset the {\tt exercise}
% counter to zero whenever a new chapter begins.
% The actual exercise number is appended to the number of
% whatever we are numbering the exercises within.
% |\numberexercises| can occur only once in a document,
% and it must appear before the first exercise.
% Otherwise we issue a warning message.
%    \begin{macrocode}
\newif\if@exnofixed
\def\numberexercises#1{%
  \if@exnofixed\@warning{Exercise numbering already fixed}
  \else\@addtoreset{exercise}{#1}%
     \renewcommand{\theexercise}{\csname
        the#1\endcsname--\arabic{exercise}}\fi
  \@exnofixedtrue}
%    \end{macrocode}
% \end{macro}
% \begin{macro}{\exercisehead}
% One can imagine quite a lot of different ways to start an exercise:
% compare ``{$\bullet$\,\small\bf EXERCISE \normalsize\bf 2--3}'' to
% ``{\sl Exercise 2--3.}'' or even ``\fbox{2--3}''.
% The |\exercisehead| command lets the user specify
% how the exercise headings are to be formatted---its
% first argument is inserted right in front of the exercise number,
% while the second argument is inserted right afterwards.
% The default is to print just the exercise number in boldface;
% the examples above would be realized by the commands
%\begin{verbatim}
%   \exercisehead{$\bullet$\,\small\bf EXERCISE \normalsize\bf}{}
%   \exercisehead{\sl Exercise }{.}
%   \exercisehead{\fbox}{}
%\end{verbatim}
% respectively.
% Note the (seemingly extraneous) braces around |\theexercise|
% in the definition of |\exercisehead|.
% They make the last example possible.
%    \begin{macrocode}
\def\exercisehead#1#2{\def\exercise@head{#1{\theexercise}#2}}
\exercisehead{\bf}{}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\exenvname}
% We'll need the name of the {\tt exercise} environment
% later on, to check whether the |\answer| occurs at `top level'
% inside the environment.
%    \begin{macrocode}
\def\exenvname{exercise}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\exercise}
% Now we define the {\tt exercise} environment.
% The |\@exnofixedtrue| inhibits use of the
% |\numberexercises| command after the first exercise.
% The |\refstepcounter| command lets you |\label|
% exercises for cross-reference.
%    \begin{macrocode}
\def\exercise{\medbreak\@exnofixedtrue
     \refstepcounter{exercise}\trivlist\item[]{\exercise@head}}
%    \end{macrocode}
% \end{macro}
% \begin{macro}{\endexercise}
% The |\endexercise| command finishes up an exercise.
%    \begin{macrocode}
\def\endexercise{\endtrivlist}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\answer}
% Now for the tougher bits.
% We check whether the name of the current environment equals
% ``{\tt exercise}''.
% If not, then some other environment has been opened but not closed
% before the |\answer|.
% That's bad, because the verbatim scanning mechanism will look for
% the wrong |\end|.
% It certainly rates an error message,
% which is output by |\@answererr|.
% Otherwise, all is well and we delegate the work to |\@answer|.
% Note that we call |\@answer| in a non-straightforward fashion:
% if we had
%\begin{verbatim}
%   \ifx\@currenvir\exenvname\@answer\else\@answererr\fi
%\end{verbatim}
% then |\@answer| would copy the |\else| and following
% tokens to the answer file.
% We have to make sure that the answer text is immediately next
% when |\@answer| is called.
%    \begin{macrocode}
\def\answer{%
  \ifx\@currenvir\exenvname\let\next\@answer
    \else\let\next\@answererr\fi\next}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@answererr}
% This macro issues an error message when the current environment name
% is found not to match {\tt exercise}.
% It tries to correct the error by finishing the current environment
% and repeating the test.
%    \begin{macrocode}
\def\@answererr{\errhelp{There may be no unmatched `\begin' between
     the^^J`\begin{exercise}' and `\answer'.}%
    \errmessage{LaTeX error: Wrong \string\answer\space (current
      environment is `\@currenvir')}%
    \edef\@tempa{{\@currenvir}} \expandafter\end\@tempa \answer}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@answer}
% The |\@answer| macro diverts subsequent lines to the answer file,
% up to the line that finishes the {\tt exercise} environment,
% namely |\end{exercise}|.
% Rainer Sch\"opf's {\tt verbatim} macros do the hard work;
% this macro is essentially adapted from his {\tt verbatimwrite}
% environment.
%
% Before we start copying the answer,
% we write a line containing the current exercise number.
% This line is of the form\\[2pt]
%   \hspace*{\MacroIndent}|\ansno{|\meta{exercise number}|}|\\[2pt]
% and |\ansno| will be suitably defined later on.
% |\ans| is the file descriptor for the answer file.
%    \begin{macrocode}
\def\@answer{%
   \immediate\write\ans{\string\ansno{\theexercise}}
%    \end{macrocode}
% Now we get ready for copying the answer.
% All special characters are disabled,
% while the end of line is made active.
% According to Rainer Sch\"opf's paper the |\verbatim@processline|
% macro is called from within the {\tt verbatim} environment when
% a whole line of stuff has been accumulated.
% We will define this macro such that this line is then output
% to the answer file.
% |\verbatim@start| launches the copying process.
%    \begin{macrocode}
  \let\do\@makeother\dospecials \catcode`\^^M\active
  \let\verbatim@processline=\answer@processline
  \verbatim@start}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\answer@processline}
% The {\tt verbatim} macros gather a complete line of input into the
% token variable |\verbatim@line|.
% Here we output the contents of this variable to the answer file.
% This macro is called whenever a line has been completely read.
%    \begin{macrocode}
\def\answer@processline{\immediate\write\ans{\the\verbatim@line}}
%    \end{macrocode}
% \end{macro}
%
% \DescribeEnv{exercises}
% We define another environment that will introduce a special section
% for the exercises. The name of the environment is unfortunate,
% but I couldn't think of anything else. Maybe {\tt homework} or
% {\tt assignment} might have been more sensible. (Any ideas, anyone?)
% Anyhow, you can say\\[2pt]
%   \hspace*{\MacroIndent}|\begin{exercises}|\\
%   \hspace*{\MacroIndent}|Make sure you attempt all of them.|\\
%   \hspace*{\MacroIndent}\quad|\begin{exercise}|\\
%   \hspace*{\MacroIndent}\quad\meta{exercise$_1$}\\
%   \hspace*{\MacroIndent}\quad|\end{exercise}|\\
%   \hspace*{\MacroIndent}\dots\\
%   \hspace*{\MacroIndent}\quad|\begin{exercise}|\\
%   \hspace*{\MacroIndent}\quad\meta{exercise$_n$}\\
%   \hspace*{\MacroIndent}\quad|\end{exercise}|\\
%   \hspace*{\MacroIndent}|\end{exercises}|\\[2pt]
% With {\tt article}, the exercises will be placed in a subsection,
% whereas, with {\tt report} and {\tt book}, the exercises will
% be put into a section.
% \DescribeMacro{\exercisetitle}
% The |\exercisetitle| can be redefined according to
% the desired language.
%    \begin{macrocode}
\def\exercisetitle{Exercises}
\@ifundefined{chapter}{\let\exsection\subsection}%
    {\let\exsection\section}
\newenvironment{exercises}{\exsection*{\exercisetitle}}{}
%    \end{macrocode}
%
% Now we open the file for the answers.
% Its name is the current job name plus {\tt.ans},
% i.\ e., if the document is {\tt paper.tex},
% the answer file is {\tt paper.ans}. In the original, the
% file name is always {\tt answers.tex}; using the job name makes
% it possible to put the answers into a separate document without
% causing undue confusion, since every document gets its own
% answer file.
%    \begin{macrocode}
\newwrite\ans
\immediate\openout\ans=\jobname.ans
\typeout{Writing answers to \jobname.ans.}
%    \end{macrocode}
%
% \begin{macro}{\listanswers}
% The |\listanswers| command will close the answer file
% and replace itself by its contents.
%    \begin{macrocode}
\def\listanswers{\immediate\closeout\ans \input\jobname.ans\relax}
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\ansno}
% Finally we have to define how the exercise numbers will appear
% with the answers.
% By default, we reuse whatever was defined for
% |\exercise@head|.
% Typographically speaking, this should be the sound thing to do.
% But conceivably one might want to let the exercise numbers stick
% out into the margin, by saying something like
%\begin{verbatim}
%   \def\ansno#1{\par{\noindent\strut\llap{\Ansno#1 }}}
%\end{verbatim}
% where |\Ansno| contains any commands necessary for formatting
% the exercise number.
%    \begin{macrocode}
\def\ansno#1{\par{\def\theexercise{#1}\exercise@head}}
%    \end{macrocode}
% \end{macro}
% \Finale
\endinput