How to determine allowable external formats at run time

Hi,

I'm wondering if there is a way to get the list of allowable values of
the :EXTERNAL-FORMAT keyword for OPEN. Is there a way to determine the
possible values at run time? I looked into the source code for
MAKE-FILE-STREAM but I got a little lost ... perhaps someone can point
me in the right direction.

What I'm trying to do is to verify whether a given format is
acceptable. I am thinking that I can inspect a list to see if the
given format is on the list. Another workable solution would be a
function which can report whether the given format is acceptable, if
such a function exists.

Thanks for your help,

Robert Dodier