def MultiListChildren(self, urns, token=None, limit=None, age=NEWEST_TIME):
    checked_subjects = set()

    index_prefix = "index:dir/"
    for subject, values in data_store.DB.MultiResolvePrefix(
        urns, index_prefix, token=token,
        timestamp=Factory.ParseAgeSpecification(age),
        limit=limit):

      checked_subjects.add(subject)

      subject_result = []
      for predicate, _, timestamp in values:
        urn = rdfvalue.RDFURN(subject).Add(predicate[len(index_prefix):])
        urn.age = rdfvalue.RDFDatetime(timestamp)
        subject_result.append(urn)

      yield subject, subject_result

    for subject in set(urns) - checked_subjects:
      yield subject, []
Lists bunch of directories efficiently.
dataset/ETHPy150Open google/grr/grr/lib/aff4.py/Factory.MultiListChildren