cheater(Cheater) :-
	saw_each_other(Cheater, Person1),
	saw_each_other(Cheater, Person2),
	Person1 \== Person2,
	\+ saw_each_other(Person1,Person2),
	!.
cheater(gondola_driver).

saw_each_other(Person1,Person2) :- saw(Person1,Person2).
saw_each_other(Person1,Person2) :- saw(Person2,Person1).