code: def set_cycle_mrkr(self, epoch_start, end=False):
          
          if self.rater is None:
              raise IndexError('You need to have at least one rater')
  
          bound = 'start'
          if end:
              bound = 'end'
  
          for one_epoch in self.rater.iterfind('stages/epoch'):
              if int(one_epoch.find('epoch_start').text) == epoch_start:
                  cycles = self.rater.find('cycles')
                  name = 'cyc_' + bound
                  new_bound = SubElement(cycles, name)
                  new_bound.text = str(int(epoch_start))
                  self.save()
                  return
  
          raise KeyError('epoch starting at ' + str(epoch_start) + ' not found')
docs: Mark epoch start as cycle start or end.
  
          Parameters
          ----------
          epoch_start: int
              start time of the epoch, in seconds
          end : bool
              If True, marked as cycle end; otherwise, marks cycle start