I have to say the API seems to be very pleasant and simple at a glance. Kudos to the author!

> Being able to use Rust functions directly as systems might feel like magic, but I promise it's not! You may have noticed that we do this when registering systems in our App (...)

> (...) This works because we implement the IntoQuerySystem trait for all functions that match a certain set of function signatures.

This pattern is a great example of how Rust achieves to be both very ergonomic or dynamic (in the sense of expressiveness) and type safe at the same time.