digraph {
    rankdir = LR
    dummy [label = "", shape = none]
    0 [shape = circle]
    1 [shape = circle]
    2 [shape = circle]
    3 [shape = circle]
    dummy -> 0
    0 -> 1 [label = "a(x)"]
    0 -> 1 [label = "b(y)"]
    0 -> 0 [label = "c(x)"]
    1 -> 2 [label = "a(x)"]
    1 -> 3 [label = "b(y)"]
    1 -> 2 [label = "c(x)"]
    2 -> 2 [label = "a(y)"]
    2 -> 2 [label = "b(x)"]
    2 -> 2 [label = "c(y)"]
    3 -> 2 [label = "a(x)"]
    3 -> 3 [label = "b(y)"]
    3 -> 2 [label = "c(x)"]
}