new expression code checked in

[Ben Pfaff]

I checked in the new expression code.  I still plan to do some
more work on it--especially more thorough testing and some
polishing of the code--but it's there now.

Perl is now a build dependency.  Hope that's not a big deal.

I may have no network access for a few days.  I'm flying to LA in
the morning and coming back Thursday night.

Ben
-- 
"I consider that the golden rule requires that if I like a program
 I must share it with other people who like it."
--Richard Stallman