% adapted from
% Computational Intelligence:
% a logical approach. 

% lit(L) is true if light L is lit.
lit(L) :-
   light(L) ,
   ok(L) ,
   live(L).

% live(W) is true if W is live (i.e., current
% will flow through it if grounded)
live(outside).
live(W) :-
   connectedto(W,W1) ,
   live(W1).

% connectedto(W0,W1) is true if W0 is
% connnected to W1 such that current will
% flow from W1 to W0.

connectedto(l1,w0).
connectedto(w0,w1) :- up(s2).
connectedto(w0,w2) :- down(s2).
connectedto(w1,w3) :- up(s1).
connectedto(w2,w3) :- down(s1).
connectedto(l2,w4).
connectedto(w4,w3) :- up(s3).
connectedto(p1,w3).
connectedto(w3,w5) :- ok(cb1).
connectedto(p2,w6).
connectedto(w6,w5) :- ok(cb2).
connectedto(w5,outside).

% light(L) is true if L is a light
light(l1).
light(l2).

% up(S) is true if switch S is up
% down(S) is true if switch S is down
up(s2).
up(s3).
down(s1).

% ok(G) is true if G is working
% cb= circuit breaker
ok(l1).
ok(l2).
ok(cb1).
ok(cb2).