First Author:   Yuri Takhteyev UC Berkeley School of Information yuri@ischool.berkeley.edu   102 South Hall, 102 South Hall Berkeley, 94720-4600   650-281-7360(Phone) 650-281-7360(Fax) Second Author: Michael Schwarz Yahoo! Research schwarz.m@gmail.com   Yahoo! Research, 2397 Shattuck Ave Berkeley, 94704, USA