%% 
%%  Example 02-14-1 
%%  Copyright (C) 2016 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
\makeatletter
\newcommand\xLongLeftRightArrow[2][]{\ext@arrow 0055{\LongLeftRightArrowfill@}{#1}{#2}}
\def\LongLeftRightArrowfill@{\arrowfill@\Leftarrow\Relbar\Rightarrow}
\newcommand\xlongleftrightarrow[2][]{\ext@arrow 0055{\longleftrightarrowfill@}{#1}{#2}}
\def\longleftrightarrowfill@{\arrowfill@\leftarrow\relbar\rightarrow}
\makeatother
%StopShownPreambleCommands
\begin{document}
$\xlongleftrightarrow[\text{below}]{\text{above the arrow}}$
$\xLongLeftRightArrow[\text{below}]{\text{above the arrow}}$
\end{document}
