-a.

a :- x, not b, delay2.

delay1.
delay2 :- delay1.

x :- not y.
y :- not x.


-------------------
({}, {delay2, delay1, x, b, y, a})
({delay2, delay1}, {y, delay2, delay1, x})
({delay2, delay1}, {y, delay2, delay1})
({y, delay2, delay1}, {y, delay2, delay1})
({y, delay2, delay1}, {y, delay2, delay1})