Department of Computer Science, Ben-Gurion University, Beer Sheva, Israel 84105. dotna@cs.bgu.ac.il|Department of Computer Science, Ben-Gurion University, Beer Sheva, Israel 84105. dotna@cs.bgu.ac.il|Department of Computer Science, Ben-Gurion University, Beer Sheva, Israel 84105. dotna@cs.bgu.ac.il