Your wiggling friend in ISBN conversions.
@book = Bookworm.new('9780980011111')
@book.as_new
> "9780980011111"
@book.as_used
> "2900980011110"
@book.as_ten
> "0980011116"
@book.type
> 'new'
@book.valid?
> true
Copyright (c) 2011 Tyler Johnston. See LICENSE for details.