code: public List breadthFirst(boolean preorder) {
          List answer = new NodeList();
          if (preorder) answer.add(this);
          answer.addAll(breadthFirstRest(preorder));
          if (!preorder) answer.add(this);
          return answer;
      }
docs: Provides a collection of all the nodes in the tree
  using a breadth-first traversal.
  
  @param preorder if false, a postorder breadth-first traversal will be performed
  @return the list of (breadth-first) ordered nodes
  @since 2.5.0