The functionality of this library is to calculate compounded interest... but the real purpose is to test Matt Godbolt's suggestions from "Correct by Construction: APIs That Are Easy to Use and Hard to Misuse - Matt Godbolt - C++ on sea" on a high-level language.
Matt gives us the example of a stocks trading API that is implemented in C++. Because we wanted to test what we saw there on a high-level language, we picked Python.
A lib with a hard to misuse API (based on Matt Godbolt)