%% s_cars.sp -- SPARC solution to additional exercise for Section 5.5 
%% (Indirect Exceptions to Defaults)
%% Last Modified: 3/16/14
%% Write a program with consistency restoring rules
%% representing the following information:
%% "Cars normally work; in rare cases, a car does not work.
%% Working cars have working engines. 
%% The engine of John's car does not work."
%% Make sure that your program entails that John's car does not work but 
%% Mary's car does.

%%%%%%%%%%%%%%%%%%%%%%%%%%
sorts

 #person = {mary,john}.
 #car = {c1,c2}.
 #engine = e(#car).
 #thing = #car + #engine.
 #default = d(#car).

%%%%%%%%%%%%%%%%%%%%%%%%%%
predicates

 owns(#person,#car).
 works(#thing).
 ab(#default).

%%%%%%%%%%%%%%%%%%%%%%%%%%
rules

works(C) :-
        #car(C),
        not ab(d(C)),
        not -works(C).

-works(C) :+
        #car(C).

works(e(C)) :-
        #car(C),
        works(C).

owns(john,c1).
-works(e(c1)).
owns(mary,c2).