Frederick Law Olmsted and Calvert Vaux designed it, but real estate prices made Fort Greene Park the proving ground it is today. Here, you'll find families of every ethnic stripe gutting out the neighborhood's transition from middle-class black enclave to a more mixed slice of brownstone Brooklyn. Parents choose between eyeing one another suspiciously and making friends as their tots unite in conquering the jungle gym. Bigger kids share the common language of scaring the adults to death, bombing their way on scooters and skates down the park's terrifying hill. Over on the lawn that grass forgot, West Indian guys join white ones and their mostly straight girlfriends for co-ed Sunday soccer. And we are seriously talking about the best tennis courts not yet overrun by stock analysts and their rookie lawyer friends. So the place is a tad the worse for wear; the Fort Greene Park Conservancy is working toward a full restoration, from the central Prison Ship Martyrs Monument to the marginal storm drains. This modest patch of contested territory is worth the investment.