Usually, marked buildings vanish within a day or two. Driving by, you sense an absense, but often can't exactly remember what once stood there. Sometimes the name of the replacement reminds you -- the Bakery Square shopping center, of course, sits where only a few months ago there was a real bakery. Often, though, only a gap remains -- vacant, scraped, but soon overrun with greenery -- a blank slate upon which some new, more flexible idea of a city might land.