prolog编程错误，网上教程很少

不太懂什么原因

domains
     person, activity = symbol
predicates
     likes(person,activity)
clauses
     likes(ellen,tennis).
     likes(john,football).
     likes(tom,baseball).
     likes(eric,swimming).
     likes(mark,tennis).
     likes(bill,X):-likes(tom,X).
  goal 
     likes(bill, What).