@Test
public void
a_node_with_two_levels_generates_two_plans(){

   DFSTree tree = DFSTreeBuilder.aTreeBuilder().withLabel(0)
     .withChild(DFSTreeBuilder.aTreeBuilder().withLabel(1)
       .withChild(DFSTreeBuilder.aTreeBuilder().withLabel(2))
       .withChild(DFSTreeBuilder.aTreeBuilder().withLabel(3))).build();
   assertThat(tree.generateCombinations()).isEqualTo(Lists.newArrayList("0,1,2,3","0,1,3,2"));
}