thesis2/graph21.dot

   1 digraph graph21 {
   2 rankdir=LR;
   3 1, 2 [shape="circle"];
   4 3, 4 [shape="doublecircle"];
   5 1 -> 2 [label="a"];
   6 2 -> 3 [label="a"];
   7 2 -> 4 [label="b"];
   8 }