digraph {
  rankdir="BT";
  node[shape="circle"];
  {
  node[fillcolor="black", color="black", style=filled, label="", rank="sink"];
  a, b, c, d;
  }
  {
  node[fillcolor="white",  fontcolor="white" style=filled, label="", rank="source"];
  e, f, g;
  }
  a -> {e, f};
  b -> {f, g};
  c -> {e};
  d -> {e, f};
  
}