School of Computer Science and Engineering, The Hebrew University of Jerusalem 91904, Israel. royke@cs.huji.ac.il|School of Computer Science and Engineering, The Hebrew University of Jerusalem 91904, Israel. royke@cs.huji.ac.il|School of Computer Science and Engineering, The Hebrew University of Jerusalem 91904, Israel. royke@cs.huji.ac.il|School of Computer Science and Engineering, The Hebrew University of Jerusalem 91904, Israel. royke@cs.huji.ac.il