% There's no easy way to generate equation numbers like (1a), (1b), (1') etc.
% However these are both easily fixed by reading in eqalign.sty (via the
% optional arg to \documentstyle) where eqalign.sty contains:

% eqalign (style option for all style) to reenable PLAIN TeX's
% \eqalign command and generalize the LaTeX's equation numbering.
% Written by Charles Karney (Karney%PPC.MFENET@NMFECC.ARPA) 1986/01/03.

% This style option can be used with any style. E.g.,
%     \documentstyle[11pt,eqalign]{article}
% It provides:

% (1) \eqalign, \displaylines, \eqalignno, and \leqalignno from Plain
% TeX.

% (2) \eqn(A5c) gives \eqno\hbox{(A5c)}.  This ensures that the "A" and
% "c" are set in Roman.

% (3) \eqnum increments the equation counter and prints it.  E.g.,
%     $$x^2+y^2=z^2, \eqn(\eqnum)$$

% (4) \eqlab{<label>} is the same as \eqnum except that \ref{<label>} can
% be used to refer to the equation.  E.g.,
%     $$x^2+y^2=z^2, \eqn(\eqlab{Pythag})$$
%     ...  as is shown in Eq.~(\ref{Pythag}). ...

% (5) \eqlab and \ref in combination allow the inclusion of alphabetic
% suffices to equation numbers.  E.g.,
%     $$\eqalignno{
%                 a&=b+c, &(\eqlab{triv}a)\cr             % gives (10a)
%                 d&=e+f, &(\ref{triv}b)\cr               % gives (10b)
%                 g&=h+i. &(\ref{triv}c)\cr}$$            % gives (10c)
%     ... as is shown in Eq.~(\ref{triv}a) ...
%     ... as is shown in Eqs.~(\ref{triv}) ...
% and so on.

\def\eqalign#1{\null\,\vcenter{\openup\jot\m@th
  \ialign{\strut\hfil$\displaystyle{##}$&$\displaystyle{{}##}$\hfil
      \crcr#1\crcr}}\,}
\def\@lign{\tabskip\z@skip\everycr{}} % restore inside \displ@y
\def\displaylines#1{\displ@y
  \halign{\hbox to\displaywidth{$\@lign\hfil\displaystyle##\hfil$}\crcr
    #1\crcr}}
\def\eqalignno#1{\displ@y \tabskip\@centering
  \halign to\displaywidth{\hfil$\@lign\displaystyle{##}$\tabskip\z@skip
    &$\@lign\displaystyle{{}##}$\hfil\tabskip\@centering
    &\llap{$\@lign\hbox{##}$}\tabskip\z@skip\crcr
    #1\crcr}}
\def\leqalignno#1{\displ@y \tabskip\@centering
  \halign to\displaywidth{\hfil$\@lign\displaystyle{##}$\tabskip\z@skip
    &$\@lign\displaystyle{{}##}$\hfil\tabskip\@centering
    &\kern-\displaywidth\rlap{$\@lign\hbox{##}$}\tabskip\displaywidth\crcr
    #1\crcr}}

\def\eqn(#1){\eqno\hbox{(#1)}}
\def\eqnum{{\refstepcounter{equation}\theequation}}
\def\eqlab#1{{\refstepcounter{equation}\label{#1}\theequation}}
