*
       |
       |0
       |
       V
       i
       |
 +-----+-----+
 |     |     |      
 |0    |1    |2 
 |     |     |      
 V     V     V
 j     S2    j
 |           |
 |0       +--+--+
 |        |     |
 V        |0    |1
 S1       |     |
          V     V
          k     S4
          |
          |0
          |
          V
          S3