% subeqn.sty 
% ----------------------------------------------------------------------
%                     This LaTeX environment  is for
% printing   subequations.   To   use   this   environment,  include  in  the
% \documentstyle header  a command to  load in the  .sty file containing this
% macro. For example:
%     \documentstyle[subeqn]{article}
% if you  have the  macro in  a file subeqn.sty. The environment seems pretty
% well documented in the comments.
%
% Modified : June 8, 1989.  You can now reference either individual
%            equations in the subequations environment, or all of
%            them.  If you use a \label command immediately after the
%            \begin{subequations} command, then a reference to that
%            label will generate a reference to the equation number
%            without the alphabetic extension.
%
% Modified : 16 - january - 1989 by Johannes Braams ( BRAAMS@HLSDNL5)
%            Added \global\@ignoretrue in the definition of
%            \endsubequations in order to prevent a spurious space
%            at the beginning of the next text-line. Also added %'s
%            at the end of each command-line for the same reasons.
%
%%%----------------------------------------------------------------
%%% File: subeqn.sty
%%% The subequations environment %%%
%
% Within the subequations environment, the only change is that
% equations are labeled differently.  The number stays the same,
% and lower case letters are appended.  For example, if after doing
% three equations, numbered 1, 2, and 3, you start a subequations
% environmment and do three more equations, they will be numbered
% 4a, 4b, and 4c.  After you end the subequations environment, the
% next equation will be numbered 5.
%
% Both text and equations can be put inside the subequations environment.
%
% If you make any improvements, I'd like to hear about them.
%
%
\newtoks\@stequation

\def\subequations{\refstepcounter{equation}%
\edef\@savedequation{\the\c@equation}%
\@stequation=\expandafter{\theequation}%   %only want \theequation
\edef\@savedtheequation{\the\@stequation}% %expanded once
\edef\oldtheequation{\theequation}%
\setcounter{equation}{0}%
\def\theequation{\oldtheequation\alph{equation}}}%

\def\endsubequations{%
\setcounter{equation}{\@savedequation}%
\@stequation=\expandafter{\@savedtheequation}%
\edef\theequation{\the\@stequation}\global\@ignoretrue}