We had a definition like ‘\def\overbrace#1{{\m:CondPicture{% \mathop{\vbox{\m@th\ialign{##\crcr\noalign{\kern3\p@} \downbracefill\crcr\noalign{\kern3\p@\nointerlineskip} $\hfil\displaystyle{#1}\hfil$\crcr}}}\limits}{}}}’ which should have something like ‘’ but it seems like there is no need for it anymore. Similarly for ‘\underbrace’, \overrightarrow, \overleftarrow, and \angle.
<..over/under fontmath..>
\def\:tempc#1{\a:overrightarrow
\o:overrightarrow:{#1}\b:overrightarrow}
\HLet\overrightarrow=\:tempc
\NewConfigure{overrightarrow}{2}
\def\:tempc#1{\a:overleftarrow
\o:overleftarrow:{#1}\b:overleftarrow}
\HLet\overleftarrow=\:tempc
\NewConfigure{overleftarrow}{2}
-_-_-
<..plain math..>+
\def\:tempc#1{\a:overrightarrow
\o:overrightarrow:{#1}\b:overrightarrow}
\HLet\overrightarrow=\:tempc
\NewConfigure{overrightarrow}{2}
\def\:tempc#1{\a:overleftarrow
\o:overleftarrow:{#1}\b:overleftarrow}
\HLet\overleftarrow=\:tempc
\NewConfigure{overleftarrow}{2}
-_-_-
<..amsmath.sty..>+
\def\:tempc#1{\a:underrightarrow
\o:underrightarrow:{#1}\b:underrightarrow}
\HLet\underrightarrow=\:tempc
\NewConfigure{underrightarrow}{2}
\def\:tempc#1{\a:underleftarrow
\o:underleftarrow:{#1}\b:underleftarrow}
\HLet\underleftarrow=\:tempc
\NewConfigure{underleftarrow}{2}
-_-_-
<..amsmath.sty..>+
\def\:tempc#1{\a:overleftrightarrow
\o:overleftrightarrow:{#1}\b:overleftrightarrow}
\HLet\overleftrightarrow=\:tempc
\NewConfigure{overleftrightarrow}{2}
\def\:tempc#1{\a:underleftrightarrow
\o:underleftrightarrow:{#1}\b:underleftrightarrow}
\HLet\underleftrightarrow=\:tempc
\NewConfigure{underleftrightarrow}{2}
-_-_-
<..plain, fontmath, amstex..>
\def\:temp#1{\a:overbrace \mathop{\hbox{\b:overbrace
$\o:overbrace:{#1}$\c:overbrace}}\limits}
\HLet\overbrace=\:temp
\NewConfigure{overbrace}{3}
\def\:temp#1{\a:underbrace \mathop{\hbox{$\b:underbrace
\o:underbrace:{#1}\c:underbrace$}}\limits}
\HLet\underbrace=\:temp
\NewConfigure{underbrace}{3}
-_-_-