code: function (root, applyFishEyeViewToEachNode) {
      if (root.data("expand") && root._private.data.collapsedChildren != null) {
        // Expand the root and unmark its expand data to specify that it is no more to be expanded
        this.expandNode(root, applyFishEyeViewToEachNode);
        root.removeData("expand");
      }
      // Make a recursive call for children of root
      var children = root.children();
      for (var i = 0; i < children.length; i++) {
        var node = children[i];
        this.expandTopDown(node);
      }
    }
docs: expand the nodes in top down order starting from the root