%% course_catalog_1.lp -- Chapter 5, Section 5.2
%% Last Modified: 2/12/14
%% Knowledge bases with null values of the form where several 
%% possibilities exist for a given value, but we don't know which one.

prof(mike).  
prof(john).  
prof(bob).

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

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

-teaches(P,C) :- prof(P), course(C),
                 not teaches(P,C).
                 
teaches(mike, prolog) | teaches(john, prolog).