[XeTeX] misaligned math if \smash used in subscripts

[jfbu]

Hi,

just to point out that there is interesting effect if the
very first \smash. is simply replaced by an x

\hrule
$$
\hbox{$\left)\vbox to15pt{}\right.$}_{x} % or also _{\hbox{x}}
\hbox{$\left)\vbox to15pt{}\right.$}_{\smash.}
\hbox{$\left)\vbox to15pt{}\right.$}_{\smash.}
$$
\hrule

Jean-François