docs: def MultiListChildren(self, urns):
      """Lists children of a bunch of given urns. Results are cached."""
      result = {}
      not_listed_urns = []
  
      for urn in urns:
        try:
          result[urn] = self._children_lists_cache[urn]
        except KeyError:
          not_listed_urns.append(urn)
  
      if not_listed_urns:
        for urn, children in FACTORY.MultiListChildren(not_listed_urns):
          result[urn] = self._children_lists_cache[urn] = children
  
        for urn in not_listed_urns:
          self._children_lists_cache.setdefault(urn, [])
          result.setdefault(urn, [])
  
      return result