%% 
%%  Example 06-02-1 
%%  Copyright (C) 2016 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
%StartShownPreambleCommands
\usepackage{amsmath}
\makeatletter
\def\rightharpoondownfill@{\arrowfill@\relbar\relbar\rightharpoondown}
\def\rightharpoonupfill@{\arrowfill@\relbar\relbar\rightharpoonup}
\def\leftharpoondownfill@{\arrowfill@\leftharpoondown\relbar\relbar}
\def\leftharpoonupfill@{\arrowfill@\leftharpoonup\relbar\relbar}
\newcommand\xrightharpoondown[2][]{\ext@arrow 0359\rightharpoondownfill@{#1}{#2}}
\newcommand\xrightharpoonup[2][]{\ext@arrow 0359\rightharpoonupfill@{#1}{#2}}
\newcommand\xleftharpoondown[2][]{\ext@arrow 3095\leftharpoondownfill@{#1}{#2}}
\newcommand\xleftharpoonup[2][]{\ext@arrow 3095\leftharpoonupfill@{#1}{#2}}
\newcommand\xleftrightharpoons[2][]{\mathrel{%
  \raise.22ex\hbox{$\ext@arrow 3095\leftharpoonupfill@{\phantom{#1}}{#2}$}%
    \setbox0=\hbox{$\ext@arrow 0359\rightharpoondownfill@{#1}{\phantom{#2}}$}%
     \kern-\wd0 \lower.22ex\box0}}
\newcommand\xrightleftharpoons[2][]{\mathrel{%
  \raise.22ex\hbox{$\ext@arrow 3095\rightharpoonupfill@{\phantom{#1}}{#2}$}%
    \setbox0=\hbox{$\ext@arrow 0359\leftharpoondownfill@{#1}{\phantom{#2}}$}%
      \kern-\wd0 \lower.22ex\box0}}
\makeatother
%StopShownPreambleCommands
\begin{document}
$\xrightharpoondown[under]{over} $\quad$\xrightharpoonup[under]{over}$\quad
$\xleftharpoondown[under]{over}  $\quad$\xleftharpoonup[under]{over} $\quad
$\xleftrightharpoons[under]{over}$\quad$\xrightleftharpoons[under]{over}$
\end{document}
