1. a Xerox Corporation , Webster Research Center, W-114, Rochester, New York, 14644
2. c Dept. of Physics and Astronomy , University of Rochester , Rochester, New York, 14620
3. d IBM Research Laboratory , K461282, 5600 Cottle Rd., San Jose, CA, 95193
4. b Laboratory for Research on the Structure of Matter , University of Pennsylvania , Philadelphia, Pennsylvania, 19104