[pygtk] Combobox Inside Treeview for SINGLE cell -- Not Entire Column?

[darethehair]

Hello All!

I am struggling with adding a 'cell-based' (not 'column-based') combobox 
to particular cells (not entire columns) of a treeview.  The normal 
techniques do not apply in this case, since they apply a combobox to 
*every* row in a column.  This example works fine for me in this context:

http://www.mail-archive.com/pygtk@daa.com.au/msg13782.html

After struggling for a long time, I opted to try using a 'cell data 
function' as mentioned in the FAQ:

http://www.pygtk.org/pygtk2tutorial/sec-CellRenderers.html#filelistingfig 
(section 14.4.5)

However, when I stumble to try this technique, it seems to apply the 
'custom' technique (combobox or otherwise) to *every* row in the treestore.

Is it actually possible to do what I want?  Can someone supply an 
extremely *simple* example showing a *cell-based* behavior override to 
what is displayed in a treeview cell -- even better one that inserts a 
'combobox' inside a *single* cell of a column of treeview data?

Thanks!