code: def list_available() -> list:
      from matchzoo.engine.base_preprocessor import BasePreprocessor
      from matchzoo.utils import list_recursive_concrete_subclasses
  
      return list_recursive_concrete_subclasses(BasePreprocessor)
  
imports: from .None import units
  from .dssm_preprocessor import DSSMPreprocessor
  from .naive_preprocessor import NaivePreprocessor
  from .basic_preprocessor import BasicPreprocessor
  from .cdssm_preprocessor import CDSSMPreprocessor
  from .diin_preprocessor import DIINPreprocessor
  from .bert_preprocessor import BertPreprocessor
label_desc: Use image node_id 1 for calling a global function with example usage: list_available() and returns: list_recursive_concrete_subclasses