code: private List<String> yield(List<String> labels) {
          labels.add(label);
          for (Tree t : children()) {
              labels.addAll(t.yield());
          }
          return labels;
      }
docs: Returns the list of labels for this node and
  all of its children recursively
  @param labels the labels to add to
  @return the list of labels for this node and
  all of its children recursively