def isbn_gendigit (numStr):
    """
    (string)-->(string + 1-digit)
    Generates the 10th digit in a given 9-digit isbn string. Multiplies the values of individual digits within the given 9-digit string to determine the 10th digit.
    Prints the original string with its additional 10th digit. Prints X for the 10th digit if the value of isbn_mathremainder is 10.
    ('123456788')-->1234567881
    ('123456789')-->123456789X
    """
    
    if (len(numStr)) == 9:
        isbn_math = sum([(int(numStr[i])*(i+1)) for i in range(0,9)])
        isbn_mathremainder = (isbn_math % 11)
        if isbn_mathremainder == 10:
            print ('ISBN 10-digit: ', str(numStr) + 'X')
            return (str(numStr) + 'X')
        else:
            print ('ISBN 10-digit: ', str(numStr) + str(isbn_mathremainder))
            return (str(numStr) + str(isbn_mathremainder))
    else:
        print ('unacceptable')
        return None