digraph g {
  edge [arrowhead=none];

  node [shape=square,width=0.47,fixedsize=true]; 
  f0 [label=<F<SUB>0</SUB>>,group=d0];
  fk [label=<F<SUB>k</SUB>>,group=dk];

  subgraph cluster_0 {
    node [shape=circle,width=0.55,fixedsize=true]; 
    d0 [label=<D<SUB>0</SUB>>,group=d0];
    dk [label=<D<SUB>k</SUB>>,group=dk];
  }

  edge [weight=10]
  f0->d0;
  fk->dk;
}