Turns on the debugger in zip mode.
zip/0 turns the debugger on and sets it to zip mode. Turning
the debugger on in zip mode means that it will stop at the next
spypoint encountered in the current execution. Until the spypoint is
reached, it does not keep any information of the execution of the
goal, and hence you will not be able to see the ancestors of the goal
when you reach the spypoint.
The effect of this predicate can also be achieved by typing the letter z after a ^C interrupt (see Execution).