digraph {
        label="Successors";
        r -> a;
        r -> b;
        a -> c;
        b -> c;
        b -> d;
        c -> d;
        d -> b;
        d -> e;
        d -> f;
        e -> g;
        f -> g;
        { rank=same; r }
        { rank=same; a; b }
        { rank=same; c; d }
        { rank=same; e; f }
        { rank=same; g }
}