code: public void depthFirst(Closure c) {
          Map<String, Object> options = new ListHashMap<String, Object>();
          options.put("preorder", true);
          depthFirst(options, c);
      }
docs: Provides a collection of all the nodes in the tree
  using a depth-first preorder traversal.
  
  @param c the closure to run for each node (a one or two parameter can be used; if one parameter is given the
  closure will be passed the node, for a two param closure the second parameter will be the level).
  @since 2.5.0