%%% Bogus\l{}aw Jackowski & Piotr Strzelczyk: sigma tweak
A sum operator that adjusts its width automatically to the widest
subscript or superscript -- perhaps completely useless ;-)\par
\medskip
{\bf Note:} \pdfTeX{} is required for clippling; you may wish to use
         other tools for this purpose

defining the parts of the symbol:

\beginverbatim
\def\SIGMAleft{\copy0 }
\def\SIGMAright{\copy1 }
\def\SIGMAcenter{\copy2 }
\endverbatim

preparing the parts of the symbol:

\beginverbatim
\setbox0\hbox{\tenex \char88\kern-.6em}
\setbox1\hbox{\kern-.8em\tenex \char88}
\setbox2\hbox{\hbox{\kern-.8em\tenex \char88\kern-.6em}}
\endverbatim

clipping:

\beginverbatim
\pdfxform0 \setbox0\hbox{\pdfrefxform\pdflastxform}
\pdfxform1 \setbox1\hbox{\pdfrefxform\pdflastxform}
\pdfxform2 \setbox2\hbox{\pdfrefxform\pdflastxform}
\endverbatim

assembling:

\beginverbatim
\def\SIGMA{%
  \mathop{\hbox{\SIGMAleft \kern-.1em
  \xleaders\SIGMAcenter\hfill
  \kern-.1em\SIGMAright}}\limits}% with \nolimits behaves ``traditionally''
\endverbatim

usage:

\beginverbatim
$ \SIGMA^{\rm a}\quad
  \SIGMA^{\rm sum}\quad
  \SIGMA^{\rm operator}\quad
  \SIGMA^{\rm that\ adjusts}\quad
  \SIGMA^{\rm its\ width\ automatically}
        _{\rm to\ the\ widest\ subscript\ or\ superscript}
$
\endverbatim

%\endinput

As a result we get:\bigskip

% defining the parts of the symbol
\def\SIGMAleft{\copy0 }
\def\SIGMAright{\copy1 }
\def\SIGMAcenter{\copy2 }

% preparing the parts of the symbol
\setbox0\hbox{\tenex \char88\kern-.6em}
\setbox1\hbox{\kern-.8em\tenex \char88}
\setbox2\hbox{\hbox{\kern-.8em\tenex \char88\kern-.6em}}

% clipping:
\pdfxform0 \setbox0\hbox{\pdfrefxform\pdflastxform}
\pdfxform1 \setbox1\hbox{\pdfrefxform\pdflastxform}
\pdfxform2 \setbox2\hbox{\pdfrefxform\pdflastxform}

% assembling:
\def\SIGMA{%
  \mathop{\hbox{\SIGMAleft \kern-.1em
  \xleaders\SIGMAcenter\hfill
  \kern-.1em\SIGMAright}}\limits}% with \nolimits behaves ``traditionally''

$ \SIGMA^{\rm a}\quad
  \SIGMA^{\rm sum}\quad
  \SIGMA^{\rm operator}\quad
  \SIGMA^{\rm that\ adjusts}\quad
  \SIGMA^{\rm its\ width\ automatically}
        _{\rm to\ the\ widest\ subscript\ or\ superscript}
$
