%% 
%%  Example 01-15-2 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{110.31393pt}
\usepackage[T1]{fontenc}
\usepackage{kpfonts}
%StartShownPreambleCommands

%StopShownPreambleCommands
\begin{document}
$A_{abc_{xyz}123def}^{abc123def}aa$

\makeatletter
\catcode`\_\active
\def_#1{\sb{\operator@font#1}}
\makeatother

$A_{abc_{xyz}123def}^{abc123def}aa$
\end{document}
