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"""Iterate over keys within the index.
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:param keys: An iterable providing the keys to be retrieved.
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:return: An iterable of (index, key, reference_lists, value). There is no
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:return: An iterable of (index, key, value, reference_lists). There is no
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defined order for the result iteration - it will be in the most
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efficient order for the index (keys iteration order in this case).