0 -> y1
y1 -> 0
0 -> y2
y2 -> 0
y2 -> y3
y3 -> y2
y3 -> 0
0 -> y4
y4 -> 0
0 -> y5
y5 -> 0
y5 -> y6
y6 -> y5
y6 -> 0
y6 -> y7
y7 -> y6
y7 -> 0