CellTree How to render "different" cell types at a given level ?

How to implement this use case, which is also mentioned in Google
documentation ?
"Each node has a Cell of a specific type; usually, all Cells at a
given level are of the same type, but that isn't required." [1]

I am using GWT CellTree.
public static class MyFolderTreeModel implements TreeViewModel

@Override
public <T> NodeInfo<?> getNodeInfo(T value)

in this method, I check each Node value

if( value instance of Folder)
//data provider
//render

return new DefaultNodeInfo<Folder>(dataProvider, cell);

The problem is, each Folder can contain child "Folders" AND "Files".
but I can return "only one" from getNodeInfo.

Does anyone know how I can get around this problem ?

[1] http://code.google.com/webtoolkit/doc/latest/DevGuideUiCellWidgets.html#celltree

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