1. Department of Computer Science University of Southern California Los Angeles, CA 90089-0781
2. Corresponding author: Department of Computer Science MC 0781, University of Southern California, 941 W. 37th Place, Los Angeles, CA 90089-0781. Phone: 213-740-0218; Fax: 213-821-5696; E-mail: gaurav@usc.edu
3. Department of Biological Sciences University of Southern California Los Angeles, CA 90089-0371