TreeView select next

[Konstantin L Kouptsov <klk206 panix com>]

In Gtk::TreeView, if a particular element is selected, and I press an Up 
or Down key, the previous or the next element is selected. How can I 
achieve this from within a program?

That is, for example, I select an element in the TreeView, and catch an 
event that PageUp or PageDown key was pressed, then select another 
element in the TreeView list (I want that choice of the new selection 
depended on the context, so that the number of items the selection moves 
is not fixed).

How to select the next element w/respect to the one currently selected? 
How to select the third next element?

Thanks in advance.
Konstantin.