Napkin: Back-of-the-envelope calculations in Python (opens in new tab)

I don't want to knock anyone's project but I fail to see the way in which this resembles back-of-the-envelope calculations (which I take to mean, roughly, estimates RE order of magnitude). This seems more like plain old arithmetic with some built-in units. Am I missing something?

That’s a creative use of a custom 2to3 fixer [1] to transform the calculations [2]. Too bad this will no longer work with the removal of 2to3 in Python 3.13.

[1]: https://docs.python.org/3/library/2to3.html#fixers

[2]: https://github.com/rescrv/napkin/blob/e6a4de2/napkin/substit...

What is this?