docs: public void depthFirst(Map<String, Object> options, Closure c) {
          boolean preorder = Boolean.valueOf(options.get("preorder").toString());
          if (preorder) callClosureForNode(c, this, 1);
          depthFirstRest(preorder, 2, c);
          if (!preorder) callClosureForNode(c, this, 1);
      }