code: function (callback) {
              return this.walk_down(function (depth, node, output, depth_first) {
                  if (!depth_first) {
                      for (var n = 0; n < depth; n++)
                          output += "    ";
                      output += "\"" + node.name() + "\"";
                      if (typeof callback === "function")
                          output += ": " + callback(node);
                      output += "\n";
                  }
                  return output;
              }, "");
          }
docs: /*  method: dump tree as indented string representation