docs: def Backup(self, duration=0):
          '''
          method to use when a backup tag is encountered in musicXML. Moves back in the bar by <duration>
          :param duration:
          :return:
          '''
          total = 0
          duration_total = duration * 4
          children = self.GetChildrenIndexes()
          notes = 0
          for voice in children:
              v = self.GetChild(voice)
              indexes = v.GetChildrenIndexes()
              if len(indexes) > 1:
                  indexes.reverse()
              for index in indexes:
                  notes += 1
                  note = v.GetChild(index)
                  if hasattr(note, "duration"):
                      total += note.duration
                      if total >= duration_total:
                          break
              gap = [
                  v.GetChild(i).duration for i in range(
                      0,
                      self.index -
                      notes) if hasattr(
                      v.GetChild(i),
                      "duration")]
              previous = 0
              for item in gap:
                  if item == previous:
                      self.gap -= previous
                      item = item / 2
                  self.gap += item
                  previous = item
              #self.gap = sum([])
          self.index -= notes