At 17.31 +0200 2001-06-24, Mark Steinberger wrote:
>Thus what is needed is an improved \bar macro that adjusts its length
>to the letter it occurs over.

I suspect there is font magic involved in this which prevents you from
doing quite that (maybe with new math fonts it could happen automagically),
but I've recently used the following with acceptable results for most
capitals:

\makeatletter
\newcommand{\cmpl}[1]{%
\sbox\z@{$#1$}%
\dimen@=\wd\z@
\llap{$\overline{\box\tw@}$}%