label double arrows in both directions

Question

How can I do something like this

$L_1 \xrightarrow[\alpha]{\xleftarrow{\gamma}}L_2$

where both arrows are of the same length?

Answer 1

The excellent Mathmode document explains how to make extensible arrows of all kinds (§38; pp.78-79). Here is an xleftrightarrow:

\documentclass{article}
\usepackage{mathtools, amssymb}

\makeatletter
\newcommand\xlongleftrightarrow[2][]{%
\ext@arrow 0059{\longleftrightarrowfill@}{#1}{#2}%
}
\def\longleftrightarrowfill@{%
\arrowfill@ ← \relbar → }
\makeatother

\begin{document}

\begin{equation*}
  A \xlongleftrightarrow[\quadα\quad]{γ\quadχ}B%
  \xlongleftrightarrow[\alpha]{γ} C \xlongleftrightarrow[ζ\,η\,θ\,ι\,\varkappa]{\,α\,β\,γ\,δ\,ε\,}D
\end{equation*}

\end{document}