The default implementations for `Set` and `Map` should be the `LinkedHashSet` and `LinkedHashMap`

because it offers stable iteration order.

Using the Stream API is ok in general, but it must kept simple. Stream maps inside

maps should be avoided and the inner logic should be refactored into a method.

Every class/interface should have its one file in general.

Inner classes are almost never ok (especially public ones). Every class should have its own file to make it easier to read and explore the code.

We maintain our own small set of Assert util methods. Don't use `Objects.requireNonNull` and others in order

to be consistent.