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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 optional(validator):
 """ A validator that makes an attribute optional. An optional attribute is one which can be set to ``None`` in addition to satisfying the requirements of the sub-validator. :param validator: A validator (or a list of validators) that is used for non-``None`` values. :type validator: callable or :class:`list` of callables. .. versionadded:: 15.1.0 .. versionchanged:: 17.1.0 *validator* can be a list of validators. """

    if isinstance(validator, list):
        return _OptionalValidator(_AndValidator(validator))
    return _OptionalValidator(validator)