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

( cluster_first
  [ a ]
  [ b ]
) { label: first; }

( cluster_second
  [ p ]
  [ q ]
  [ r ]
  [ s ]
) { label: second; }

[ a ] --> [ b ]
[ b ] --> [ q ]
[ q ] --> [ r ]
[ r ] --> { style: bold; } [ a ]
[ r ] --> [ s ]
[ s ] --> [ p ]
[ p ] --> [ q ]