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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 is_none(string_, default='raise'):
 """ Check if a string is equivalent to None. Parameters string_ : str default : {'raise', False} Default behaviour if none of the "None" strings is detected. Returns ------- is_none : bool Examples -------- False True """

    none = ['none', 'undefined', 'unknown', 'null', '']
    if string_.lower() in none:
        return True
    elif not default:
        return False
    else:
        raise ValueError('The value \'{}\' cannot be mapped to none.'
                         .format(string_))