code: function (callback, context) {
          var _children = this._children;
          for(var i = 0, len = _children.length; i < len; i++) {
              var child = _children[i];
              callback.call(context, child, i);
          }
      }
docs: Traverse all children nodes.
  
  **WARN** DON'T do `add`, `remove` operation in the callback during iteration.
  
  @param {Function} callback
  @param {Node} [context]