:- dynamic light/1, down/1, up/1, ok/1.
/* house wiring */

light(l1).  light(l2).
down(s1).   down(s2).     up(s3).
ok(l1).     ok(l2).     ok(cb1).   ok(cb2).

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

lit(L) :- light(L), ok(L), live(L).

live(outside).
live(Y) :- connect_to(Y,Z), live(Z).