Friends Prolog example

character(joey).
character(ross).
character(rachel).
character(phoebe).
character(monica).
character(chandler).
likes(ross,rachel). /* This means ross likes rachel */
likes(rachel,ross).
likes(monica,chandler).
likes(chandler,monica).
likes(chandler,phoebe).
likes(phoebe,joey).
likes(monica,joey).
likes(joey,rachel).

dating(X, Y) :- /* These are rules. */
likes(X, Y),
likes(Y, X).

attraction(X, Y) :-
likes(X, Y);
likes(Y, X).