code: def accel_prev(self, *args):
          
          if self.get_notebook().get_current_page() == 0:
              self.get_notebook().set_current_page(self.get_notebook().get_n_pages() - 1)
          else:
              self.get_notebook().prev_page()
          return True
docs: Callback to go to the previous tab. Called by the accel key.