code: def update_label(self, old_label, new_label, callback=dummy_progress_cb):
          
          current = 0
          total = self.index.get_nb_docs()
          self.index.start_update_label(old_label, new_label)
          while True:
              (op, doc) = self.index.continue_update_label()
              if op == 'end':
                  break
              callback(current, total, self.LABEL_STEP_UPDATING, doc)
              current += 1
          self.index.end_update_label()
docs: Replace 'old_label' by 'new_label' on all the documents. Takes care of
          updating the index.