Calculate the Luhn check digit for ``number``.

    :param number: string
    :param chars: string

    >>> luhn_calc('42')
    '2'

def luhn_calc(number, chars=DIGITS):
    '''
    Calculate the Luhn check digit for ``number``.

    :param number: string
    :param chars: string

    >>> luhn_calc('42')
    '2'

    '''

    checksum = luhn_checksum(str(number) + chars[0], chars)
    return chars[-checksum]