# How to superscript text on \rightarrowtail?

How to get superscripted text on \rightarrowtail? I have included mathtools package, but there isn't any command like \xrightarrowtail[]{}.

It is not extensible, but still may suit.

\documentclass[12pt]{article}
\usepackage{mathtools,amssymb,stackengine}
\newcommand\Rightarrowtail[2][]{\ensurestackMath{\mathrel{%
\stackengine{1pt}{%
\stackengine{0pt}{\rightarrowtail}{\scriptstyle#2}{O}{c}{F}{F}{S}%
}{\scriptstyle#1}{U}{c}{F}{F}{S}%
}}}
\begin{document}
$x \xrightarrow[a]{b} y$
$x \Rightarrowtail[a]{b} y$
\end{document}