docs: def _get_labels(self, frequency, subject_id=None, visit_id=None):
          """
          Returns the labels for the XNAT subject and sessions given
          the frequency and provided IDs.
          """
          if frequency == 'per_session':
              subj_label = '{}_{}'.format(self.project_id,
                                          subject_id)
              sess_label = '{}_{}_{}'.format(self.project_id,
                                             subject_id,
                                             visit_id)
          elif frequency == 'per_subject':
              subj_label = '{}_{}'.format(self.project_id,
                                          subject_id)
              sess_label = '{}_{}_{}'.format(self.project_id,
                                             subject_id,
                                             self.SUMMARY_NAME)
          elif frequency == 'per_visit':
              subj_label = '{}_{}'.format(self.project_id,
                                          self.SUMMARY_NAME)
              sess_label = '{}_{}_{}'.format(self.project_id,
                                             self.SUMMARY_NAME,
                                             visit_id)
          elif frequency == 'per_study':
              subj_label = '{}_{}'.format(self.project_id,
                                          self.SUMMARY_NAME)
              sess_label = '{}_{}_{}'.format(self.project_id,
                                             self.SUMMARY_NAME,
                                             self.SUMMARY_NAME)
          else:
              assert False
          return (subj_label, sess_label)