0 -> y
y <-> x1
y -> 0
z0 <-> z1
x <-> x1
z1 <-> z2
0 -> w0
w0 <-> w1
0 -> u
0 -> i
i + x <-> ix
ix <-> ixp
u -> 0
i -> 0
ixp -> x
w0 -> 0
w1 -> 0
z0 -> 0
z1 -> 0
z2 <-> 0
0 -> z0
x -> 0