lecturer(Lecturer,Course) :-
		course(Course,Time,Lecturer,Location).

	duration(Course,Length) :-
		course(Course,time(Day,Start,Finish),Lecturer,Location),
		plus(Start,Length,Finish).

	teaches(Lecturer,Day) :-
		course(Course,time(Day,Start,Finish),Lecturer,Location).

	occupied(Room,Day,Time) :-
		course(Course,time(Day,Start,Finish),Lecturer,Location),
		Start =< Time, Time =< Finish.

%	Program 2.4: Course rules