code: def __getitem__(self, exp_id: int) -> Optional[Set[int]]:
      ...
  
imports: from typing import Iterable, Mapping, Optional, Sequence, Set, Tuple, TypeVar, List, Callable, Union, overload
  import warnings
  from avalanche.benchmarks.scenarios.deprecated.dataset_scenario import ClassesTimelineCLScenario, DatasetScenario, FactoryBasedStream, TStreamsUserDict
  from avalanche.benchmarks.scenarios.deprecated import AbstractClassTimelineExperience
  from avalanche.benchmarks.utils.data import AvalancheDataset
  from avalanche.benchmarks.utils.dataset_utils import manage_advanced_indexing
  from avalanche.benchmarks.utils.detection_dataset import DetectionDataset
label_desc: Use image node_id 3 for calling the _LazyClassesInDetectionExps obj's underlying member method code with example usage: obj.__getitem__(exp_id) without return types