// try running the query exposed(Who, 0615)

kiss("jill",  "james", 0321);
kiss("julian", "jill", 0405);
kiss("jill", "jasper", 0412);
kiss("james", "jenni", 0420);
kiss("julian", "judy", 0508);
kiss("jed",    "judy", 0614);

kissed(Student1, Student2, Date) :- 
    kiss(Student1, Student2, Date);

kissed(Student1, Student2, Date) :- 
    kiss(Student2, Student1, Date); 

infected("julian", 0307);

exposed(Student, Date) :- 
    infected(Student, Date1), 
    >(Date, Date1);

exposed(Student2, Date2) :-
    kissed(Student2, Student1, Date1),
    >(Date2, Date1),
    exposed(Student1, Date1);