QComboBox (model based) - provide an empty entry

Hi !

I have a two QComboBoxes using a model based on QAbstractItemModel.
The model is a tree. below is a simplified example:

root
+--user1
...+--doc1
...+--doc2
+--user2
...+--doc1

the first combo shows the user and
the second combo shows the docs for the selected user.
That's working well.

Now I have to provide the possibility to select for the second combo (doc) an
empty entry, which means, an new doc will be created for this user.
But how can I show an empty entry without changing the underlying model ?

best regards,
bernd