答案1
我发现结果不太好看:
\documentclass{article}
\begin{document}
\[
\ooalign{%
\phantom{$a\,$}$\overbrace{\phantom{\!{}^vb^xc^w\!}}\phantom{\,d^y}$\cr
\noalign{\kern2pt}
\phantom{$a^vb\,$}$\overbrace{\phantom{\!{}^xc^wd^y\!}}\,$\cr
$a^vb^xc^wd^y$\cr
}
\]
\[
\ooalign{%
\phantom{$a\,$}$\overbrace{\phantom{\!{}^vb^xc^w\!}}\phantom{\,d^y}$\cr
$a^vb^xc^wd^y$\cr
\phantom{$a^vb\,$}$\underbrace{\phantom{\!{}^xc^wd^y\!}}\,$\cr
}
\]
\end{document}