code: def find_preservation_criteria(self, backups_by_frequency):
          
          backups_to_preserve = collections.defaultdict(list)
          for frequency, delta in ORDERED_FREQUENCIES:
              for period in backups_by_frequency[frequency].values():
                  for backup in period:
                      backups_to_preserve[backup].append(frequency)
          return backups_to_preserve
docs: Collect the criteria used to decide which backups to preserve.
  
          :param backups_by_frequency: A :class:`dict` in the format generated by
                                       :func:`group_backups()` which has been
                                       processed by :func:`apply_rotation_scheme()`.
          :returns: A :class:`dict` with :class:`Backup` objects as keys and
                    :class:`list` objects containing strings (rotation
                    frequencies) as values.