Department of Computer Science, University of California at Santa Barbara, Santa Barbara, CA 93117-5110, USA. tcan@cs.ucsb.edu|Department of Computer Science, University of California at Santa Barbara, Santa Barbara, CA 93117-5110, USA. tcan@cs.ucsb.edu|Department of Computer Science, University of California at Santa Barbara, Santa Barbara, CA 93117-5110, USA. tcan@cs.ucsb.edu|Department of Computer Science, University of California at Santa Barbara, Santa Barbara, CA 93117-5110, USA. tcan@cs.ucsb.edu