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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 extras_to_string(extras):
 # type: (Iterable[S]) -> S """Turn a list of extras into a string"""

    if isinstance(extras, six.string_types):
        if extras.startswith("["):
            return extras
        else:
            extras = [extras]
    if not extras:
        return ""
    return "[{0}]".format(",".join(sorted(set(extras))))