true.

?- city(Which).
Which = eureka ;;
Which = arcata ;;
Which = fortuna.

?- city(which).
false.

?- city(Pie).
Pie = eureka ;;
Pie = arcata ;;
Pie = fortuna.

?- city(Houston).
Houston = eureka ;;
Houston = arcata ;;
Houston = fortuna.

?- halt.