# File lib/isbn-converter.rb, line 28
def to10(isbn)
        isbn = clean(isbn)
        return nil unless isbn.length == 13 && isbn[0..2] != "979"
        new_isbn = isbn.chop[3..(isbn.length - 1)]
        check = 0
        10.downto(2) { |n| check += new_isbn[(10 - n)..(10 - n)].to_i * n }
        check = 11 - (check % 11)
        check = "X" if check == 10
        new_isbn + check.to_s
end