%%
%% The LaTeX Companion, 3ed
%%
%% Example A-1-14 on page II-641 in "Using Embellishments".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{63.0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
\NewDocumentCommand\xgets{ E{_^}{{}{}} }{\xleftarrow[#1]{#2}}
%StopShownPreambleCommands

\begin{document}
\[ \xgets        \xgets^x       \xgets_y         \xgets^{x+y}_{x-y}    \]
\[ \xleftarrow{} \xleftarrow{x} \xleftarrow[y]{} \xleftarrow[x-y]{x+y} \]
\end{document}