Also, as publishers re-package and re-print public domain books, these also are considered separate products with new ISBNs and new dates.

The listing shows the variable "diff" being calculated on line 6, which we can re-evaluate by supplying the "diff" expression to print as 'print no1 - no2'.

Therefore if a publisher re-issues a book in their backlist after the previous print run has been exhausted (say, a decade later) and with a new introduction, it is considered a different book.