digraph G {
    a [shape = box, fontname = "monospace", label = "a = x_0\nif (a == 0) goto _L1"];
    b [shape = box, fontname = "monospace", label = "x_1 = 1"];
    c [shape = box, fontname = "monospace", label = "x_2 = 2"];
    d [shape = box, fontname = "monospace", label = "x_3 = phi(x_1, x_2)\ny_0 = x_3 + 1"];
    a -> b;
    a -> c;
    b -> d;
    c -> d;
}