1. E. Fradkin is in the Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL 61801-3080, USA.
2. S. A. Kivelson is in the Department of Physics, Stanford University, Stanford, CA 93105, USA.
3. V. Oganesyan is in the Department of Physics, Yale University, Post Office Box 208120, New Haven, CT 06520-8120, USA.