% from Fung/Kowalski paper

lamp(X)        iff [[X=a]].
battery(X,Y)   iff [[X=b, Y=c]].
faulty_lamp(X) iff [[lamp(X), broken(X)], [power_failure(X), not(backup(X))]].
backup(X)      iff [[battery(X,Y), not(empty(Y))]].

% Try: 
% ?- ciff( 'lamp.alp', [faulty_lamp(X)], Answer).