code: def label_total_duration(self, label_list_ids=None):
          
          duration = collections.defaultdict(float)
  
          for label_list in self.label_lists.values():
              if label_list_ids is None or label_list.idx in label_list_ids:
                  for label_value, label_duration in label_list.label_total_duration().items():
                      duration[label_value] += label_duration
  
          return duration
docs: Return a dictionary containing the number of seconds,
          every label-value is occurring in this utterance.
  
          Args:
              label_list_ids (list): If not None, only labels from label-lists
                                     with an id contained in this
                                     list are considered.
  
          Returns:
              dict: A dictionary containing the number of seconds
                    with the label-value as key.