::
Semplice:
`
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\overlay}[3][\mathord]{%
#1{\vphantom{#2#3}\mathpalette\overlay@{{#2}{#3}}}%
}
\newcommand{\overlay@}[2]{\overlay@@#1#2}
\newcommand{\overlay@@}[3]{%
\ooalign{\hfil$\m@th#1#2$\hfil\cr\hfil$\m@th#1#3$\hfil\cr}%
}
\makeatother
\begin{document}
$x\overlay[\mathrel]{\to}{\mapstochar}y$
$x\overlay[\mathrel]{\to}{\mapstochar\mspace{4mu}}y$
$\displaystyle\overlay[\mathop]{\bigcup}{\circ}$
$\textstyle\overlay[\mathop]{\bigcup}{\circ}$
$\scriptstyle\overlay[\mathop]{\bigcup}{\circ}$
$\scriptscriptstyle\overlay[\mathop]{\bigcup}{\circ}$
\end{document}
`
[attachment=2547]ScreenShot2020-11-18at13.38.44.png[/attachment]
Ciao
Enrico
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