digraph DDG {
    node [shape=box];

    6.1 -> 9.1;
    9.1 -> 10.1;
    9.1 -> 11.1;
    11.1 -> 12.1;
    12.1 -> 17.1;
    12.1 -> 18.1;

    6.1 -> 6.2;

    6.2 -> 9.2;
    9.2 -> 10.2;
    10.2 -> 18.1 [dir="back"];
    9.2 -> 11.2;
    11.2 -> 12.2;
    12.2 -> 17.2;
    12.2 -> 18.2;

    6.2 -> 6.3;

    6.3 -> 9.3;
    9.3 -> 10.3;
    9.3 -> 11.3;
    11.3 -> 12.3;
    12.3 -> 17.3;
    12.3 -> 18.3;

    6.3 -> 6.4;

    6.4 -> 9.4;
    9.4 -> 10.4;
    9.4 -> 11.4;
    11.4 -> 12.4;
    12.4 -> 17.4;
    12.4 -> 18.4;

    6.4 -> 6.5;

    6.5 -> 9.5;
    9.5 -> 10.5;
    9.5 -> 11.5;
    11.5 -> 12.5;
    12.5 -> 17.5;
    12.5 -> 18.5;

    6.5 -> 6.6;

    6.6 -> 9.6;
    9.6 -> 10.6;
    9.6 -> 11.6;
    11.6 -> 12.6;
    12.6 -> 17.6;
    12.6 -> 18.6;
}