%% cowardly.lp -- Chpter 5, Section 5.1.2
%% Last Modified: 1/29/14
%% Cowardly Students -- Strong and weak exceptions to defaults:
%%   Normally, students are afraid of math.
%%   Mary is not.
%%   Students in the math department are not.
%%   Those in CS may or may not be afraid.

student(john).  
student(mary).  
student(bob).  
student(pat).

dept(english).  
dept(cs).  
dept(math).

in(john,english).
in(mary,cs).
in(bob,cs).
in(pat,math).

-in(S,D1)  :- dept(D1),
              in(S,D2),
              D1 != D2.
              
afraid(S,math) :- student(S),
                  not ab(d(S)),
                  not -afraid(S,math).

-afraid(mary,math).
ab(d(mary)).

-afraid(S,math) :- in(S,math).
ab(d(S)) :- student(S),
            not -in(S,math).
            
ab(d(S)) :- student(S),
            not -in(S,cs).