%%% Frank Mittelbach: is there a font?
The following code allows to check, if some font ({\tt tfm} metric)
is available.

\beginverbatim
\batchmode
\font\X=<font name>%
\errorstopmode
 \ifx\X\nullfont
   <font not available>
 \else
   <use the font>
 \fi
\endverbatim

Having \eTeX{} we can switch between modes using 
handy \type{\interactionmode} primitive.
