It occured to me just today that my 'city buildings' code might already have
such a feature in it. The thing is, I'm not sure(!). I've been adding and
changing the scene code so much-- in an on-going step-by-step way-- that I need
to step back and take an overall look. If I can dis-entangle the pertinent code
(or maybe just the general idea), I'll post it here.
