circuit2(Sw1,Sw2,Sw3,B1,B2,B3,L1,L2,L3) :- 
      VSw3 = VB3, 
      sum(VSw1, VB1, V1), 
      sum(V1,VB3,+), 
      sum(VSw2,VB2,VB3), 
      switch(Sw1,VSw1,C1), 
      bulb(B1,L1,VB1,C1), 
      switch(Sw2,VSw2,C2), 
      bulb(B2,L2,VB2,C2), 
      switch(Sw3,VSw3,CSw3), 
      bulb(B3,L3,VB3,CB3), 
      sum(CSw3,CB3,C3), 
      sum(C2,C3,C1). 
   
  switch(on,0,_). 
  switch(off,_,0). 
   
  bulb(blown,dark,_,0). 
  bulb(ok,light,+,+). 
  bulb(ok,light,-,-). 
  bulb(ok,dark,0,0). 
   
  sum(+,+,+). 
  sum(+,0,+). 
  sum(+,-,_). 
  sum(0,+,+). 
  sum(0,0,0). 
  sum(0,-,-). 
  sum(-,+,_). 
  sum(-,0,-). 
  sum(-,-,-).