code: function startNode(node) {
              var navNode = emptyNavigationBarNode(node);
              pushChild(parent, navNode);
              // Save the old parent
              parentsStack.push(parent);
              parent = navNode;
          }
docs: Add a new level of NavigationBarNodes.
  This pushes to the stack, so you must call `endNode` when you are done adding to this node.