Flex: Expand one level in advanceddatagrid
===

I'm using AdvancedDataGrid in flex with the following structure:

     - A
      - AA
         - AAA
         - AAB
      - AB
         - ABA
         - ABB
     - B
      - BA
         - BAA
         - BAB
      - BB
         - BBA
         - BBB

I want to implement a function in actionscript, which expand only one level of the tree. 

E.g.: When I call the function at the following state,

     + A
     + B

I will get this:
 
     - A
      + AA
      + AB
     - B
      + BA
      + BB

When I call it again, I will see the whole tree.

I tried to use the expandItem() method of AdvancedDatagrid, but I have no idea how to list the nodes. Can someone write me a sample code?

Thanks for your help!