digraph g {
  edge [arrowhead=none];


  node [shape=circle,width=0.5,fixedsize=true];   
  subgraph cluster_b {
    b1 [label=<B<SUB>1</SUB>>,group=b1];
    b0 [label=<B<SUB>0</SUB>>, group=b0];
  }

  subgraph  cluster_c {

    c1 [label=<C<SUB>1</SUB>>,group=c1];
    c0 [label=<C<SUB>0</SUB>>, group=c0];
  }

  subgraph cluster_d {
    di [label=<D<SUB>i</SUB>>, group=di];
    d0 [label=<D<SUB>0</SUB>>,group=d0];
  }

  b0->d0;  
  b0->di;
  b1->d0;  
  b1->di;
  c0->d0;  
  c0->di;
  c1->d0;  
  c1->di;
}