Check for other running emacs sessions?

[noreply@u.wasshington.edu]

Hello,

I typically have three different instances of emacs running at the
same time.  I'd like to put something in my .emacs file to check if
another instance is running and if so, not do something
(activate-appt).  I haven't been able to find anything on it, but
perhaps I'm not using the right terms.  Is there a way to do this in
emacs, or should I call a shell script that greps for 'emacs' in ps
output?

Thank you for any thoughts or suggestions.

Brian