You swallow hard when you discover that the old coffee shop is now a chain pharmacy, that the place where you first kissed so-and-so is now a discount electronics retailer, that where you bought this very jacket is now rubble behind a blue plywood fence and a future office building. Damage has been done to your city. You say, ''It happened overnight.'' But of course it didn't. Your pizza parlor, his shoeshine stand, her hat store: when they were here, we neglected them. For all you know, the place closed down moments after the last time you walked out the door. (Ten months ago? Six years? Fifteen? You can't remember, can you?) And there have been five stores in that spot before the travel agency. Five different neighborhoods coming and going between then and now, other people's other cities. Or 15, 25, 100 neighborhoods. Thousands of people pass that storefront every day, each one haunting the streets of his or her own New York, not one of them seeing the same thing.