alt Hacker NewsIt was extremely common back in the 1500/1600s, when books were far more rare and valuable to write in them. Almost every copy of these truly old books has something written in them, often by multiple people as the owners changed. That's part of the history of the book.
they also would get all the words scraped off and get a new book written on the pages, often some religious text.
and so now, some ancient math tehcniques can be unearthed by finding all the layers of writing in a book