int CompareTitles (Gtk.TreeModel model, Gtk.TreeIter a, Gtk.TreeIter b)
{
string title_a = model.GetValue (a, 1 /* title */) as string;
string title_b = model.GetValue (b, 1 /* title */) as string;
if (title_a == null || title_b == null)
return -1;
return title_a.CompareTo (title_b);
}