x2 + y1 -> x1 + x3
x1 + x3 -> x2 + y1
x3 + y4 -> x2 + y5
x2 + y5 -> x3 + y4
x1 -> y2
y2 -> x1
x1 -> y3
y3 -> x1
x3 -> y6
y6 -> x3