CellTree - which nodes are open or closed on it?

I have a cellTree with few nodes.
I need to send to the server side which nodes are open on the tree, and which are closed.

I was thinking on two solutions, but i have problem with both.

1. Walk the cellTree and double-check all the nodes if they are open or closed.
My problem with this is that i dont know, how to walk the tree.
I can get it's root element from the CellTree by calling it's tree.getRootTreeNode() method,
but i cant find any way how to get the rootNode's children.
http://google-web-toolkit.googlecode.com/svn/javadoc/2.1/com/google/gwt/user/cellview/client/TreeNode.html
The TreeNode interface does not have a TreeNode getChild(int i) method.

2. Assume, that all children are closed / opened, and catch the close / open node event, and save it's state in a variable.
I do not know if there is any kind of event that can deal with this user activity.

What's your opinion and how to fix this issues?

Regards,
Sandor

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