digraph dtree {
    node [fontsize=14, shape=box];

    0;
    X0;
    X1;
    Y0 [style=filled, fillcolor=grey89];
    Y1 [style=filled, fillcolor=grey89];
    merge [style=filled, fillcolor=grey49];

    0 -> X0
    0 -> Y0
    X0 -> X1
    Y0 -> X1
    X0 -> Y1
    Y0 -> Y1
    X1 -> merge
    Y1 -> merge
}