A successful public space, they say, is one where the citizens block the steps. So suggested urbanologist Lewis Mumford nearly a hundred years ago. I'm not certain he would have had San Francisco's busy Union Square in mind, but he may have, even then. What Mumford never envisaged, surely, was the odd and telling assemblage of human beings who make the Square the center of their lives and a Rorschach test for the character of one of America's great cities. I am one of them. For those of us living downtown, it is the front yard.