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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 _unset_annotation_to_str(keys: List[str]) -> str: """Return an unset annotation string."""

    if len(keys) == 1:
        return 'UNSET {}'.format(list(keys)[0])

    return 'UNSET {{{}}}'.format(', '.join('{}'.format(key) for key in keys))