I had a lot of fun working with the maintainers of this utility to better support Gaussian units[1]. This is usually done (incorrectly) by multiplying, say, some number of SI coulombs by a dimensionless constant (actually 2_997_924_580) to produce some number of statcoulombs that represents the same charge. It's very subtle, though, that statcoulombs and coulombs have different dimensions (which is the point of Gaussian units) despite representing the same physical quantity[2], so the conversion is much more involved. I learned a lot about the nature of units and software development working with them.

[1]: https://en.wikipedia.org/wiki/Gaussian_units

[2]: A consequence of this is that electromagnetic equations formulated in terms of Gaussian units have goofy things like sqrt(grams) as "base" units. That was the tricky part to handle.