hylomorphisms/img/merge5.dot

digraph G {
  a[label="", shape=rectangle];
  b[label="merge [4] [2]", shape=rectangle];
  c[label="merge [6] [1]", shape=rectangle];
  {rank=same; b,c}
  d[label="[4]", shape=circle, style=invis];
  e[label="[2]", shape=circle, style=invis];
  f[label="[6]", shape=circle, style=invis];
  g[label="[1]", shape=circle, style=invis];
  {rank=same; d,e,f,g};
  a -> b;
  a -> c;
  b -> d[style=invis];
  b -> e[style=invis];
  c -> f[style=invis];
  c -> g[style=invis];
}