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