%  \iffalse  
%    The first part is a comment to the reader(s) of `deleq.dtx'.
%  deleq.dtx    Version 4.1, Dec. 27, 1994
%  (c) 1994 by Mats Dahlgren  (matsd@physchem.kth.se)
%  Please see the information in file `deleq.ins' on how you 
%  may use and (re-)distribute this file.  Run LaTeX on the file 
%  `deleq.ins' to get a .sty-file, instructions, and an example.
%  This file may NOT be distributed if not accompanied by 'deleq.ins'.
%  \fi
%  \def\filename{deleq.dtx}
%  \def\fileversion{4.1}
%  \def\filedate{1994/12/27}\def\docdate{\filedate}
%  \MakeShortVerb{\|}
%  \title{\LaTeX{} Macro for Partial Numbering of Equations}
%  \author{Mats Dahlgren\\ (\texttt{matsd@physchem.kth.se})} 
%  \maketitle
%  \begin{abstract}
%  To enable a more flexible equation numbering, especially 
%  ``partial'' equation numbers (`3a', `3b' \textit{etc.}), 
%  the |deleq| package has been developed.  It can produce 
%  partial equation numbers intermixed with ordinary 
%  equation numbers also in an |eqnarray|-like environment, 
%  the intermixing can occur within one environment.  The 
%  package also provides commands for putting commentatory 
%  text in an |eqnarray| environment without requiering the 
%  |amstex| package.\\ \small This file and the package:\
%  Copyright \copyright\ 1994 by Mats Dahlgren.  All rights
%  reserved. 
%  \end{abstract}
%  \section{Introduction}
%  |deleq| is a \LaTeX{} package which makes partial
%  numbering of equations possible.  It is meant to be used
%  when numbering such as 3a, 3b, \textit{etc.} is desired.
%  The default is to give an equation number like `3a'
%  without period and with the `a' typeset in roman font. 
%  It can be used in |article| as well as |book| and
%  |report| document classes.  The examples given below are
%  valid for the |article| class.  |deleq| is fully
%  compatible with the |leqno| documentclass option.
%  Equations can be numbered either 3, 3a, 3b, $\ldots$, or
%  2, 3a, 3b, $\ldots$. Also, equation numbers within
%  |eqnarray|-like environments can be numbered 3, 3a, 3b,
%  $\ldots$ or starting at 3a, which can follow both after
%  equation 2b or 3.  An |eqnarray|-like environment can have 
%  its first equation numbered 3b if the nearest previous
%  equation is 3a.  Within one |eqnarray|-like  environment, 
%  equations can be numbered 3a, 3b, 4, 4a, 4b, 5a, 5b,
%  \textit{etc.}, and also not numbered lines are possible
%  (`|\nonumber|' works). Furthermore, equation numbers can
%  be ``recycled''.  If equation 3 is repeated after
%  equation 8, it can still have the numbering set to 3,
%  and be followed by (a new) equation 9.  ``Recycled''
%  equation numbers can receive partial numbers (3a, 3b,
%  $\ldots$); partially numbered equations can also be
%  ``recycled'' (but at this stage, the latter can not
%  receive new partial numbers neatly, it will come out
%  like `3ba' if eqution 3b is the ``recycled'' equation). 
%  Two commands are supplied which enable the user to write
%  commnetary tests in |eqnarray|-like environments without
%  interfering with the alignment. 
%  \section{History}
%  |deleq.sty| was originally written for \LaTeX{} 2.09,
%  and contained several commands which became obsolete
%  with the introduction of \LaTeXe.  These commands have
%  been removed.  The previous release (v.4.0, Oct.~14,
%  1994) was the first for \LaTeXe. The present release 
%  (v.4.1) is the first which is compatible with the 
%  |docstrip| utility of Johannes Braams, Denys Duchier and 
%  Frank Mittelbach.
%  |deleq.sty| version 4.0 has been tested with \LaTeXe%  
%  /\TeX{} 3.141 under UNIX and \LaTeXe /\TeX{} 3.1415 under
%  OpenVMS.  Version 4.1 has only been tested on the 
%  OpenVMS platform.  Please send bug reports (see below),
%  corrections, additions, suggestions, \textit{etc.}\ to
%  me at \texttt{matsd@physchem.kth.se}.  (Command names
%  are a mixture of \LaTeX , Swedish and a bit of the
%  author's fantasy$\ldots$) With |deleq| v.4.1, the 
%  commentatory commands |\where| and |\rem| were included.
%  \section{Known Problems}
%  \begin{itemize}
%  \item  Earlier versions (3.0) of |deleq.sty| had
%    problems with \LaTeXe   compatibility mode (reported by
%    Tom Scavo   (\texttt{trscavo@mailbox.syr.edu})).  These
%    problems are likely to remain.   There are also problems
%    with the font selections when using older   versions of
%    |deleq| than 4.0 in \LaTeXe{} native mode.  These error 
%    messages can be ignored, \LaTeXe{} will produce the
%    correct output   anyway.  There will not be any attempts
%    to correct this by the author   of |deleq| in the near
%    future. 
%  \item In \LaTeX{} 2.09, the use of |deleq.sty| version
%    3.0 sometimes caused the   |\em| command not to work
%    properly in a theorem-like environment   (reported by
%    Patrick Dan\`es (\texttt{danes@laas.fr})).  This appears
%    not to be   a problem in \LaTeXe{} with |deleq| version
%    4.0 and later. 
%  \end{itemize}
%  \section{``Userguide''}
%  \subsection{Requirements}     
%  The file |deleq.sty| must be available in the user's
%  |TEXINPUTS| directories.
%  It requires \LaTeXe{} of 1994/06/01 (or newer).
%  \subsection{Usage}
%  The package is included by stating\\
%  |  \usepackage{|$\ldots$|,deleq,|$\ldots$|}| \\
%  In the document preamble.
%  The documentclass option |leqno| is fully supported,
%  no other class or package options have any effect on 
%  |deleq|.
%  \subsection{Environments}     
%  The package defines the following four environments:
%  \begin{description}
%  \item{\texttt{deqn}} environment for automatic numbering of 
%    the  first equation of type 3a; 
%  \item{\texttt{ddeqn}} used for automatic numbering of the
%    equations following the one in |deqn|, see also details
%    below; 
%  \item{\texttt{deqarr}} environment for automatic partial equation
%    numbering used in |eqnarray| environment with 
%    |deqn| functionality for the first equation;
%  \item{\texttt{ddeqar}} similar to |deqarr| environment, but 
%    with |ddeqn| behaviour for the first equation, see details below.
%  \end{description}
%  \subsection{Commands}         
%  The package defines the following nine user commands:\\
%  \hspace*{2mm}|\nydeqno|\\
%  \hspace*{2mm}|\heqno|\\
%  \hspace*{2mm}|\nydeleqno|\\
%  \hspace*{2mm}|\deleqno|\\
%  \hspace*{2mm}|\reqno{FOO}|\\
%  \hspace*{2mm}|\rndeqno{FOO}|\\
%  \hspace*{2mm}|\rdeqno{FOO}|\\
%  \hspace*{2mm}|\where|\\
%  \hspace*{2mm}|\rem{|\textit{text}|}|\\
%  \subsection{Notice}
%  \begin{enumerate}
%  \item If you use |\nydeleqno|, |\deleqno|, |\nydeqno|, 
%  |\heqno|, |\reqno|, |\rndeqno|, or |\rdeqno| in 
%  combination with |\label{FOO}|, |\label{FOO}| should
%  appear after the |deleq|-command;
%  \item If you change the appearance of equation numbers 
%  (\textit{e.g.}\ use
%  `[2]' instead of `(2)'),  |deleq|'s commands may not typeset
%  the partial equation numbers like your ordinary ones.
%  \end{enumerate}       
%  \section{Syntax}
%  Here follows a more detailed description of the
%  different environments and commands. \\[1em]
%  \noindent|\begin{deqn} ... \end{deqn}|\\    
%  Typesets an equation just like 
%  |\begin{equation} ... \end{equation}| does, but gives it
%  a number such as `3a' instead of `3' (always `a'). \\[1em]
%  \noindent|\begin{ddeqn} ... \end{ddeqn}|\\    
%  Typesets an equation just like 
%  |\begin{equation} ... \end{equation}| does, but gives it
%  a number such as `3b', `3c', $\ldots$ instead of `3',
%  `4', $\ldots$ when following after another partially
%  numbered equation.  When following after an ordinarily
%  numbered equation (`3') it gives a partial equation
%  number with the last used ordinary equation number,
%  \textit{e.g.}\  `3a'.  Can be made to produce the result
%  of |deqn| environment by the use of |\nydeqno| (see 
%  below). \\[1em]
%  \noindent|\begin{deqarr} ... \end{deqarr}|\\    
%  Typesets an equation array just like 
%  |\begin{eqnarray} ... \end{eqnarray}| does, but gives
%  the first numbered equation a number like `3a' instead
%  of `3' (always `a'), and the following `3b', `3c'
%  \textit{etc.}  The |\nonumber| command works just like
%  in the |eqnarray| environment. \\[1em]
%  \noindent|\begin{ddeqar} ... \end{ddeqar}|\\    
%  Typesets an equation array just like 
%  |\begin{eqnarray} ... \end{eqnarray}| does, but gives
%  the equation numbers such as `3a' if following after an
%  equation numbered `3' (ordinary equation number) and
%  numbers such as `3c' if following after a partially
%  numbered equation `3b'. Can be made to produce the
%  result of |deqarr| environment by the use of |\nydeqno|.
%  The |\nonumber| command works just like in the
%  |eqnarray| environment. \\[1em]
%  \noindent|\nydeqno|\\
%  Used within |deqarr| and |ddeqar| environments to
%  step the main equation number by one and reset the 
%  partial equation number to `a'; thus, |\nydeqno| gives 
%  equation number `4a' when following after equation `3c'.\\[1em]
%  \noindent|\heqno|\\
%  Used within |deqarr| and |ddeqar| 
%  environments to step the main equation number by one and  
%  to produce an ordinary equation number; thus, gives 
%  equation number `4' when following equation `3c'. 
%  Equations following the |\heqno|-ed will be partially  
%  numbered with the |\heqno|-ed equation's number as the
%  main number, \textit{e.g.}\  `4a' (unless it has a |\nydeqno| 
%  command, which in this case would produce the equation 
%  number `5a').\\[1em]
%  \noindent|\nydeleqno|\\
%  Gives a new partial equation number when used within |$$ ... $$|.
%  Thus, writing |$$ ...| |\nydeleqno $$| is equivalent to 
%  writing |\begin{deqn} ... \end{deqn}|.\\[1em]
%  \noindent|\deleqno|\\
%  Gives a partial equation number when used within |$$ ... $$|.
%  Thus, writing |$$ ... \deleqno $$| is equivalent to 
%  writing |\begin{ddeqn} ... \end{ddeqn}|. \\[1em]
%  \noindent|\reqno{FOO}|\\
%  Is used when repeating equations with its original number.
%  |\reqno| takes the argument |FOO|, which has to be defined
%  by a |\label{FOO}| in the original equation.  It can
%  only be used within |$$ ... $$|.  It does not affect the 
%  equation number counter, nor the partial equation number 
%  counter.  However, it resets the partial equation number
%  counter for the |\rndeqno| and |\rdeqno| 
%  commands.  If used with a |\label{FOOO}| 
%  command, the |.aux|-file will only contain the page number 
%  of label |FOOO|. \\[1em]
%  \noindent|\rndeqno{FOO}|\\
%  Adds a partial equation number to an old equation number,
%  specified by the |FOO| label.  If |FOO| refers to equation 
%  `4', |\rdeqno{FOO}| will result in equation number `4a'
%  (always `a').  It can only be used within 
%  |$$ ... $$|.  It does not affect the equation number counter,
%  nor the partial equation number counter.  However, it
%  resets the partial equation number counter for the 
%  |\rndeqno| and  |\rdeqno| commands.
%  The argument of |\rndeqno| follows the same rules as that of
%  |\reqno|.  When used with the |\label{FOOO}| command, a
%  reference to |FOOO| will only return the partial equation
%  number (and the page number).  To make a complete reference 
%  to an equation which has a |\rndeqno| command, say 
%  |\ref{FOO}\ref{FOOO}|. \\[1em]
%  \noindent|\rdeqno{FOO}|\\
%  Same as |\rndeqno{FOO}| but without resetting any 
%  equation number counter and giving consecutive partial
%  equation numbers (`4b', `4c', \textit{etc.}).
%  \textbf{Note: } There is nothing preventing the
%  repeated use of |\rndeqno{FOO}| and |\rdeqno{FOO}| for
%  the same label |FOO|.  This will result in numbering such
%  as `3a' (if |FOO| refers to equation `3') occuring several
%  times. \\[1em]
%  \noindent|\where|\\
%  The command |\where| typsets the text ``where'' (in 
%  the default LR-font) flush left on a seperate row in 
%  |eqnarray|, |deqarr|, and |ddeqar| environments, and 
%  preserves the environment's alignment for rows to come. \\[1em]
%  \noindent|\rem{|\textit{text}|}|\\
%  The command |\rem{|\textit{text}|}| typsets the text
%  ``\textit{text}'' (in the default LR-font) flush left on
%  a seperate row in |eqnarray|, |deqarr|, and |ddeqar|
%  environments, and preserves the environment's alignment
%  for rows to come.  |\where| is equivalent to 
%  |\rem{where}|. \\[1em]
%  \noindent|\mthref{FOO}|\\
%  This is not intended as a user command, but is
%  avaliable.  It is the same as ordinary |\ref{FOO}| but
%  does not type-set the `??' in |\bfseries| font if
%  occurring within mathmode. |deleq| makes use of this to
%  avoid interruptions when \LaTeX -ing documents
%  containing |\reqno|, |\rndeqno|, or |\rdeqno| with
%  undefined references.  It can be used also outside
%  mathmode, and behaves then just like |\ref{FOO}|. 
%  Warnings are the same as when using |\ref{FOO}|. 
%  \section{Example}
%  After running \LaTeX{} on |deleq.ins|, there is an 
%  example avaliable in the file |delex.tex|, 
%  which makes use of all the environments and commands 
%  defined in the |deleq| package.  Typset it and see with 
%  your own eyes what the results are!
%  \section{Sending a Bug Report}
%  |deleq| is most likely to contain bugs.
%  Reports of bugs in the package are most welcome.  
%  Before filing a bug report,
%  please take the following actions:
%  \begin{enumerate}
%  \item Ensure your problem is not due to your inputfile;
%  \item Ensure your problem is not due to 
%     your own package(s) or class(es);
%  \item Ensure your problem is not covered in the section 
%     ''Known Problems'' above;
%  \item  Try to locate the problem by writing a minimal 
%     \LaTeX{} input file which reproduces the problem.  
%     Include the command\\ 
%     |  \setcounter{errorcontextlines}{999}|\\ 
%     in your input;
%  \item Run your file through \LaTeX ;
%  \item Send a description of your problem, the input file 
%     and the log file via e-mail to:\\
%     \hspace*{5mm} \texttt{matsd@physchem.kth.se}.
%  \end{enumerate}
%  {\itshape Enjoy your \LaTeX!\raisebox{-\baselineskip}{mats d.}}
%  \section{The Code} 
%  For the interested reader(s), here is a short description 
%  of the code. \par
% \iffalse
%<*paketkod>
% Part 1: Identification etc.
% \fi
%  First, the package is to identify itself.
%    \begin{macrocode} 
\NeedsTeXFormat{LaTeX2e}[1994/06/01]
\ProvidesPackage{deleq}[1994/12/27 v.4.1 Partial equation numbering]
\typeout{deleq.sty included, version 4.1 (Dec. 27, 1994).  (c) 1994 by Mats Dahlgren}
\def\deleqver{\texttt{deleq} version 4.1 (Dec.~27, 1994)}
%    \end{macrocode}
% \iffalse
% Part 2: Counters and internal commands, etc.
% \fi
%  \par  Then, we need to define the counters used for the partial 
%  part of the equation number.  The counter |Deleq| is used for 
%  ``recycled'' equation numbers, hence no dependence on any other 
%  counter.  Also the lettering of the counters are to be defined.
%    \begin{macrocode}
\newcounter{deleq}[equation]
\newcounter{Deleq}
\newcount\@deqpen
\newcount\@deqcnt
\renewcommand{\thedeleq}{\ensuremath{\theequation\mathrm{\aLph{deleq}}}}
\renewcommand{\theDeleq}{\ensuremath{\mathrm{\aLph{Deleq}}}}
\def\@deleqnnum{(\thedeleq)}
\def\aLph#1{\@aLph{\@nameuse{c@#1}}}
\def\@aLph#1{\ifcase#1\or a\or b\or c\or d\or e\or f\or g\or h\or i\or j\or
   k\or l\or m\or n\or o\or p\or q\or r\or s\or t\or u\or v\or w\or x\or y\or
   z\else\@ctrerr\fi}
%    \end{macrocode}
%  The parts making up the |\\| command in the multiline environments 
%  are extremely similar to those used by the standard environments:
%    \begin{macrocode}
\def\@deqncr{{\ifnum0=`}\fi\@ifstar{\global\@deqpen\@M
    \@ydeqncr}{\global\@deqpen\interdisplaylinepenalty \@ydeqncr}} 
\def\@ydeqncr{\@ifnextchar [{\@xdeqncr}{\@xdeqncr[\z@]}} 
\def\@xdeqncr[#1]{\ifnum0=`{\fi}\@@deqncr
   \noalign{\penalty\@deqpen\vskip\jot\vskip #1\relax}}
\def\@@deqncr{\let\@tempa\relax
    \ifcase\@deqcnt \def\@tempa{& & &}\or \def\@tempa{& &}%
      \else \def\@tempa{&}\fi
     \@tempa \if@eqnsw\@deleqnnum\stepcounter{deleq}\fi
     \global\@eqnswtrue\global\@deqcnt\z@\cr}  
%    \end{macrocode}
% \iffalse
% Part 3:  Commands:  \nydeleqno, \deleqno, \reqno, \rndeqno, \rdeqno,  
%                     \nydeqno, and \heqno
% \fi
%  \par Here the definitions of the user commands are.
%    \begin{macrocode}
\newcommand{\nydeleqno}{\refstepcounter{equation}\refstepcounter{deleq}
  \let\@currentlabel\thedeleq \eqno (\thedeleq)}
\newcommand{\deleqno}{\refstepcounter{deleq} \let\@currentlabel\thedeleq 
  \eqno (\thedeleq)}
\newcommand{\reqno}[1]{\setcounter{Deleq}{-1}\refstepcounter{Deleq}
   \eqno (\ref{#1}) }
\newcommand{\rdeqno}[1]{\refstepcounter{Deleq}
   \eqno (\ref{#1}\theDeleq)}
\newcommand{\rndeqno}[1]{\setcounter{Deleq}{0}\refstepcounter{Deleq}
   \eqno (\ref{#1}\theDeleq)}
\newcommand{\nydeqno}{\stepcounter{equation}\stepcounter{deleq}}
\newcommand{\heqno}{\stepcounter{equation}}
%    \end{macrocode}
%  And the two commands |\where| and |\rem| follow suit:
%    \begin{macrocode}
\def\where{\let\@tempa\relax \def\@tempa{& & &}
     \@tempa {\hbox to .01\p@{}\rlap{\hskip -\displaywidth where}}\cr}
\def\rem#1{\let\@tempa\relax \def\@tempa{& & &}
     \@tempa {\hbox to .01\p@{}\rlap{\hskip -\displaywidth #1}}\cr}
%    \end{macrocode}
% \iffalse
% Part 4:  Environments: 'deqarr', 'ddeqar', 'deqn', and 'ddeqn'  
% \fi
%  \par The four environments are slight modifications of the 
%  corresponding \LaTeX{} standard environments.  The main difference 
%  lies in which counter(s) is stepped and which internals are used to 
%  finish off the lines.
%    \begin{macrocode}
\newenvironment{deqarr}{\stepcounter{equation}\stepcounter{deleq}
  \let\@currentlabel\thedeleq
  \global\@eqnswtrue\m@th
  \global\@deqcnt\z@\tabskip\@centering\let\\\@deqncr
  $$\halign to\displaywidth\bgroup\@eqnsel\hskip\@centering
  $\displaystyle\tabskip\z@{##}$&\global\@deqcnt\@ne
  \hskip 2\arraycolsep \hfil${##}$\hfil
  &\global\@deqcnt\tw@ \hskip 2\arraycolsep $\displaystyle\tabskip\z@{##}$\hfil
   \tabskip\@centering&\llap{##}\tabskip\z@\cr}
{\@@deqncr\egroup
      \global\advance\c@deleq\m@ne$$\global\@ignoretrue}
\newenvironment{ddeqar}{\stepcounter{deleq}
  \let\@currentlabel\thedeleq
  \global\@eqnswtrue\m@th
  \global\@deqcnt\z@\tabskip\@centering\let\\\@deqncr
  $$\halign to\displaywidth\bgroup\@eqnsel\hskip\@centering
  $\displaystyle\tabskip\z@{##}$&\global\@deqcnt\@ne
  \hskip 2\arraycolsep \hfil${##}$\hfil
  &\global\@deqcnt\tw@ \hskip 2\arraycolsep $\displaystyle\tabskip\z@{##}$\hfil
   \tabskip\@centering&\llap{##}\tabskip\z@\cr} 
{\enddeqarr}
\newenvironment{deqn}{$$\refstepcounter{equation}\stepcounter{deleq} 
   \let\@currentlabel\thedeleq}
  {\eqno \hbox{\@deleqnnum} $$\global\@ignoretrue}
\newenvironment{ddeqn}{$$\refstepcounter{deleq} \let\@currentlabel\thedeleq}
  {\eqno \hbox{\@deleqnnum} $$\global\@ignoretrue}
%    \end{macrocode}
% \iffalse
% Part 5:  'leqno' compatibility
% \fi
%  \par If the user wants the equation numbers to the left, we have to 
%  modify some of the commands and internals defined above.  This is 
%  done in a |\DeclareOption|-call:
%    \begin{macrocode}
\DeclareOption{leqno}{\typeout{`leqno' implementation in progress.}
  \def\@deleqnnum{\hbox to .01\p@{}\rlap{\reset@font\rmfamily
        \hskip -\displaywidth(\thedeleq)}}
\renewcommand{\nydeleqno}{\refstepcounter{equation}\refstepcounter{deleq}
  \let\@currentlabel\thedeleq \leqno (\thedeleq)}
\renewcommand{\deleqno}{\refstepcounter{deleq} \let\@currentlabel\thedeleq 
  \leqno (\thedeleq)}
\renewcommand{\reqno}[1]{\setcounter{Deleq}{-1}\refstepcounter{Deleq}
   \leqno (\ref{##1}) }
\renewcommand{\rdeqno}[1]{\refstepcounter{Deleq}
   \leqno (\ref{##1}\theDeleq)}
\renewcommand{\rndeqno}[1]{\setcounter{Deleq}{0}\refstepcounter{Deleq}
   \leqno (\ref{##1}\theDeleq)}
%    \end{macrocode}
%  If other options were asked for, the package should inform the user 
%  that this has no effect on |deleq|.  Then, last, the option(s) 
%  is (are) processed.
%    \begin{macrocode}
\DeclareOption*{\typeout{`\CurrentOption '\space has no effect on deleq.}%
  \OptionNotUsed}
\ProcessOptions
%    \end{macrocode}
% \iffalse
% END of it all!
%</paketkod>
%<*driver>
\documentclass[a4paper]{article}
\usepackage{doc}
\textwidth=150mm
\textheight=210mm
\topmargin=0mm
\oddsidemargin=5mm
\evensidemargin=5mm
\begin{document}
  \DocInput{deleq.dtx} 
  \PrintChanges
\end{document}
%</driver>
%<*exempelkod>
\documentclass[11pt]{article} %%% add `leqno' if you want left-aligned
\usepackage{deleq}            %%%                    equation numbers.
\oddsidemargin=0.5cm
\evensidemargin=0.5cm
\topmargin=-5mm           
\textheight=23.5cm
\textwidth=15.5cm
\begin{document}
  \begin{center}
  \Large Welcome to the \texttt{deleq} package!
  \end{center}
  This is a short document to demonstrate the use of the 
  \texttt{deleq} package and its commands.  It uses \deleqver .
  \texttt{deleq} was written by Mats Dahlgren
  (\texttt{matsd@physchem.kth.se}).  Suggestions for
  improvements and bug reports are most welcome, see the 
  documentation.  \texttt{deleq} is fully compatible with the
  \texttt{leqno} option. 
  We start this demonstration by a simple and well-known
  equation to get the equation number counter going: 
  \begin{equation}  
    \sin^2\alpha + \cos^2\alpha = 1  
  \end{equation}
  The first example will be to make use of the
  \texttt{deqn} environment to get a partially numbered
  equation: 
  \begin{deqn}  
    \sin (-\alpha) = - \sin \alpha  
  \end{deqn}
  This equation has its cosine companion, here written in
  the \texttt{ddeqn} environment: 
  \begin{ddeqn}  
    \cos (-\alpha) = \cos \alpha  
  \end{ddeqn}
  Not so exciting, so far. :--)
  In the next example we introduce the \texttt{deqarr}
  environment for writing equations: 
  \begin{deqarr}
    \sin ( \alpha + \beta ) & = 
      & \sin \alpha \cos \beta + \sin \beta \cos \alpha \\
    \sin ( \alpha - \beta ) & = 
      & \sin \alpha \cos \beta - \sin \beta \cos \alpha 
    \label{Demo1}
  \end{deqarr}
  where we also have put in the label \texttt{Demo1} in
  the second equation. Next, notice how the environment
  \texttt{ddeqar} uses the same main equation number as
  the previous equations: 
  \begin{ddeqar}
    \cos ( \alpha + \beta ) & = 
      & \cos \alpha \cos \beta - \sin \alpha \sin \beta \\
    \cos ( \alpha - \beta ) & = 
      & \cos \alpha \cos \beta + \sin \alpha \sin \beta 
  \end{ddeqar}
  Now, we will make another \texttt{eqnarray}-like
  structure, again in the \texttt{ddeqar} environment: 
  \begin{ddeqar}
    \sin 2\alpha & = 
      & 2 \sin \alpha \cos \alpha \heqno \label{Demo2} \\
    \cos 2\alpha & = 
      & \cos ^2 \alpha - \sin ^2 \alpha \nydeqno \\
  \rem{or}
      & = & 2\cos ^2 \alpha - 1 
  \end{ddeqar}
  This example shows the use of \verb$\heqno$ in the first
  equation, which produces an ordinary equation number. 
  The first equation is also labelled, with the label
  \texttt{Demo2}.  The next equation's number was produced
  by the command \verb$\nydeqno$.  Also, notice the use of
  \verb+\rem{or}+, which results in the text ``or'' without
  changing the alignment.
  To step the equation number counter, we want the
  following equations typeset in ordinary
  \texttt{eqnarray} environment: 
  \begin{eqnarray}
    \tan ^2 \alpha & = 
      & \frac{\sin ^2 \alpha}{\cos ^2 \alpha} \label{Demo3} \\
    \tan \alpha & = 
      & \frac{\sin \alpha}{\cos \alpha} 
  \end{eqnarray}
  The upper equation was given the label \texttt{Demo3}.
  Now one of the ``recycling'' commands is to be
  demonstrated.  First of all, let's make an ordinary
  reference to equation~\ref{Demo2}, and then ``recycle'' it: 
  $$ \sin 2\alpha = 2 \sin \alpha \cos \alpha \reqno{Demo2} $$
  This was obtained with \verb#$$ ... $$# with the command
  \verb#\reqno{Demo2}# at the end.  Also partially
  numbered equations can be recycled with the
  \verb#\reqno{FOO}# command, as with
  equation~\ref{Demo1}: 
  $$ \sin ( \alpha - \beta ) = \sin \alpha \cos \beta - 
    \sin \beta \cos \alpha \reqno{Demo1} $$
  (However, the use of \verb#\rndeqno{FOO}# and
  \verb#\rdeqno{FOO}# will produce strange results with
  two (different) partial equation numbers if \texttt{FOO}
  refers to a partially numbered equation.) 
  The next equation was written with \verb#$$ ... $$# and
  a \verb#\deleqno# command at the end: 
  $$ \tan 2\alpha = \frac{2\tan \alpha}{1 - \tan ^2 \alpha} \deleqno $$
  Notice how the main equation number counter is still the
  same, despite that we now are outside of the
  \texttt{ddeqar} environment. Also, notice how the use of
  \verb#\reqno# above did not affect the equation number
  counter. The following equation is also set within
  \verb#$$ ... $$#, but it uses the command
  \verb#\nydeleqno# to produce an equation number with a
  new main number: 
  $$ \sin ^2 \alpha = 1 - \cos ^2 \alpha \nydeleqno $$
  Now we will elaborate a little on \verb#\rndeqno{FOO}#
  and \verb#\rdeqno{FOO}#. Let us use
  equation~\ref{Demo3}, and see what happens if we use the
  \verb#\rndeqno{FOO}# command: 
  $$ \tan ^2 \alpha = \frac{\sin ^2 \alpha}
    {\cos ^2 \alpha} \rndeqno{Demo3} $$
  which we rewrite:
  $$ \tan ^2 \alpha = \frac{\sin ^2 \alpha}
    {1 - \sin ^2 \alpha} \rdeqno{Demo3}  \label{Demo4} $$
  with \verb#\rdeqno{Demo3}# at the end.  This is great
  fun, so why not one more: 
  $$ \tan ^2 \alpha = \frac{1 - \cos ^2 \alpha}
    {\cos ^2 \alpha} \rdeqno{Demo3} $$ 
  Also, the middle form of equation~\ref{Demo3} got a
  label, \texttt{Demo4}. 
  There is still one equation to write.  This time we
  again use the \texttt{deqarr} environment: 
  \begin{deqarr}
  \cot \alpha & = & \frac{\cos \alpha}{\sin \alpha} \\
  \where
    & = & \frac{1}{\tan \alpha}
  \end{deqarr}
  This example also shows the use of the \verb+\where+ 
  command, which is a special case of the \verb+\rem+ command.
  Now, the interesting thing of referring to the recycled
  equations is ahead. If one writes \verb#\ref{Demo4}#,
  this is what \LaTeX\ will return:~\ref{Demo4}.  That is
  not too instructive, since the partial equation numbers
  are quite common in this document.  To make the complete
  reference, use \verb#\ref{Demo3}\ref{Demo4}#.  The
  reference~\ref{Demo3}\ref{Demo4} is much more
  comprehensible, right? (If you get bad line-breaks at
  such references, put them in an \verb#\mbox{...}#.) 
  Finally, the $\mathcal{END}$!  If you want your
  equations left-aligned, just specify the \texttt{leqno}
  option for the \texttt{documentclass} you are using.  It
  should work!  If you have any suggestions, corrections
  or contributions, please contact me. Enjoy \LaTeX ! 
  {\itshape mats d.}
\end{document}
%</exempelkod>  
% \fi
% \Finale
\endinput