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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _implementations(cls):
 """Returns all the concrete subclasses of MetadataExtractor. """

        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