I also noted that the assert section essentially does Design-by-Contract. This is a subset of formal specification that's prime value is in preventing interface errors (vast majority in big apps) and supporting formal verification. Done in design/spec phase in high-assurance since both Edsger Dijkstra and Margaret Hamilton independently discovered technique's value. Done at language-level since Eiffel due to Bertrand Meyer. Good to see that, even if not all techniques, they're doing the 80/20 rule to get most benefit out of what formal specs they're using. Also allow you to easily enable run-time checks if you can accept performance hit. Nice.