[pygtk] Is it possible to make parent items in ComboCoxEntry drop-down list selectable

[Sergey Snitsaruk]

I'm adding ComboBoxEntry and a TreeStore as a model to a window.
TreeStore contains only simple hierarchy of strings.
Example:
str1
str2 --> str3
str2 --> str4

My question is: Is it possible to make parent items in drop-down list
selectable (str2 in example)?
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Kind regards,
Sergey