code: def _implementations(cls):
          
          if cls.__implementations:
              return cls.__implementations
  
          cls.__implementations = {}
          for implementation in all_subclasses(MetadataExtractor):
              try:
                  feature_name = implementation.feature_name()
                  cls.__implementations[feature_name] = implementation
              except NotImplementedError:
                  pass
          return cls.__implementations
docs: Returns all the concrete subclasses of MetadataExtractor.