edge { arrowstyle: filled; }
graph {
  colorscheme: x11;
  flow: south;
  title: graph;
}
group {
  align: center;
  fill: inherit;
}

[ a ] { border: dotted  black; }
[ b ] { border: dotted  black; }
[ c ] { border: dotted  black; }
[ d ] { border: dotted  black; }

( cluster_me
  [ a ]
  [ b ]
)

[ a ] -- foo --> [ b ]
[ c ] -- foo --> [ d ]