[Gtk-sharp-list] Editable TreeView question

[Wolfgang Mauer]

Hi all,
i have a TreeView with 3 Columns, they all Editable, so if i use the 

CellRendererText rendererText = new CellRendererText();
rendererText.Editable = true;
rendererText.Edited += new EditedHandler(rendererText_Edited);
...

and then the handler

public void rendererText_Edited(object o, EditedArgs args)
{
	TreeView tv = (TreeView)o;
	TreeIter iter = new TreeIter();
	ListStore store = (ListStore)tv.Model;
	store.GetIterFromString(out iter, args.Path);
	store.SetValue(iter, ?????????????????????????????, args.NewText);
}

my primary question: how can i find out witch Column number i'am on.
or may this is a wrong way ?

Thanks for help
Wolfgang