Computers Can’t Debug
	There are some things computers cannot do … and we can prove it! • No computer program can tell, give another program P, if P loops forever … halting prob • If possible, it would be handy for debugging • In fact, it seems possible … look closely at the program, check the for-statements (and other looping structures we didn’t learn) • Suppose Loop_Check (P, Q) tests pgm P on input Q, answering “yes/no” to loops forever