[texhax] Centering hack

[Johan Glimming]

Hi

I am trying to center some inference rules inside a center environment:

\begin{center}
RULE GOES HERE
\end{center}

In order to center the rule I first mbox them. However, center adds 
more space over the rule than under it.

I wonder if there is a quick way of computing the length l of some 
expression, the length n remaining of the line, and then add hspace 
before the next character of (n-l)/2, in both math mode and text mode?

Johan Glimming.