Write a Prolog program with the following knowledge base:

directTrain(union-station, san-bernardino). directTrain(oxnard, union-station). directTrain(burbank, lancaster). directTrain(lancaster, union-station). directTrain(san-bernardino, riverside). directTrain(santa-ana, burbank). directTrain(oceanside, santa-ana).

This knowledge base holds facts about towns you can travel between by taking a direct train, e.g., from union station to san bernardino.

Write a Prolog recursive predicate travelBetween that tells us when we can travel by train between two towns (not necessarily by a single direct train).