code: def _get_single_exp_classes(self, exp_id) -> Optional[Set[int]]:
      b = self._benchmark.stream_definitions[self._stream]
      if (
          not b.is_lazy
          and exp_id not in b.exps_data.targets_field_sequence
      ):
          raise IndexError
      targets = b.exps_data.targets_field_sequence[exp_id]
      if targets is None:
          return None
  
      return set(targets)
  
imports: from typing import Callable, TypeVar, Union, Sequence, Optional, Iterable, List, Set, Tuple, Mapping, overload, TYPE_CHECKING
  import warnings
  from avalanche.benchmarks.scenarios.deprecated import AbstractClassTimelineExperience
  from avalanche.benchmarks.scenarios.deprecated.dataset_scenario import ClassesTimelineCLScenario, FactoryBasedStream, TStreamsUserDict
  from avalanche.benchmarks.utils.dataset_utils import manage_advanced_indexing
label_desc: Use image node_id 7 for calling the _LazyClassesInClassificationExps obj's underlying member method code with example usage: obj._get_single_exp_classes(exp_id) and returns: set, None