[bluGill]

Because some books are more valuable than others. You might enjoy Harry Potter, but if your only copy of a calculus 101 is deteriorating you will erase Harry Potter and copy the calculus text over the top to save the more important calculus book - or maybe you would erase the calculus textbook to copy Harry Potter over the top. This is a statement of relative value when paper is scarce, but it doesn't mean they didn't value the lost text, just that the replacement was even more valuable. If they had plenty of paper they wouldn't have erased the lost text in the first place and probably would have made a new copy.

[hypertele-Xii]

There's a third way.

You combine the best halves of calculus and Harry Potter into one book.

It'll be more difficult to understand, but managing so grants one the best of both worlds.

[kmstout]

  - Harry Potter and the Two-Sided Limit.
  - Harry Potter and the Area Beneath the Curve.
  - Harry Potter and the Osculating Circle.

[williamdclt]

It's probably how "Harry Potter and the Methods of Rationality" (an actual real thing) came about.

[phyrex]

And what an amazing read it is! Www.hpmor.com