%% course_catalog_1.lp -- Chapter 5, Section 5.2
%% Last Modified: 2/6/14
%% Knowledge bases with null value 'staff'

prof(mike).  
prof(john).  

course(pascal).  
course(c).  
course(prolog).

teaches(mike,pascal).
teaches(john,c).
teaches(staff,prolog).

-teaches(P,C) :- prof(P), course(C),
                 not ab(d(P,C)), 
                 not teaches(P,C).
                 
ab(d(P,C)) :- prof(P), teaches(staff,C).